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SPATIAL VARIATION IN GRAZING INTENSITY OVER HEATHER
MOORLAND IN ENGLAND
A. E. Riding, D. S. Allen and M.J.W. Burke
ADAS Woodthorne, Wergs Road, Wolverhampton, WV6 8TQ, U.K.
Heather moorland is an internationally valuable habitat that has suffered a decline
in extent and condition in recent years largely because of increased stocking rates.
As a consequence of this decline the British government has used various policy
instruments to encourage farmers to reduce sheep stocking rates on vulnerable
moorland. However, reductions in stocking do not take into account spatial
variation in grazing intensity, which can result in localised suppression of heather
growth. A geostatistical approach was used to investigate grazing intensity on
heather by sheep over a five-year period on five grazing units under reduced stock
densities within the North Peak Environmentally Sensitive Areas Scheme. Spatial
analysis showed that in most of cases studied, grazing intensity was randomly
distributed. However, for four grazing units, evidence of large-scale patchiness in
grazing intensity was found. The implication of this spatial variation in grazing
intensity for the conservation of heather moorland is discussed.
Keywords: moorland, Calluna, sheep grazing, geostatistics
Running title: Spatial variation in sheep grazing
Author to whom correspondence should be addressed: Dr Alison Riding, ADAS Woodthorne, Wergs Rd, Wolverhampton, WV6 8TQ, U.K. Tel: +44 (0)1902 693129, Fax: +44 (0)1902 693166E-Mail: [email protected]
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Introduction
The plant and animal communities associated with heather moorland make it a
habitat of outstanding conservation value (Thompson et al., 1995). Human influence
has been integral to both the creation and maintenance of this habitat; in particular,
management by grazing and burning has a long history of practice in the uplands
(Sydes and Miller, 1988). Whilst grazing and burning can act to maintain heather
moorland by preventing succession to woodland, these same practices can also
contribute to degradation of moorland vegetation. The implementation of
appropriate grazing and burning management is seen as vital to the long-term
maintenance and enhancement of heather moorland (Phillips and Watson, 1995).
Losses of heather (Calluna vulgaris) from heather moorland have been widely
reported. In the Peak District, an estimated 36% of heather moorland was lost
between 1914 and 1979 (Anderson and Yalden, 1981) and a similar proportion of
Calluna was lost from Cumbria between 1940 and 1980 (Felton and Marsden, 1990).
Overall, the net loss of heather moorland in England and Wales between 1947 and
1980 has been estimated at 20% (Thompson et al., 1995). A similar degree of loss
was recorded in Scotland between the late 1940s and the early 1970s (Tudor and
Mackey, 1995). A range of processes have been identified as contributing to such
losses, including the wholesale conversion of moorland to other land uses (e.g. to
productive agricultural grassland or to commercial forestry), overgrazing by
livestock, bracken encroachment, poor burning management and accidental fires.
Various initiatives aimed at reducing and/or reversing the trend of loss of heather
have been introduced by UK government departments and agencies over recent 2
years. These initiatives include the Environmentally Sensitive Area (ESA) scheme
which was launched by the UK Ministry of Agriculture, Fisheries and Food (MAFF)
in 1987 (Anon., 1989). The ESA scheme offers management agreements to
landowners and farmers who receive a payment in return for following a series of
management prescriptions. A tiered system of payments operates within the scheme
with more restrictive practices attracting higher payments. A total of 22 ESAs have
been designated in England, amongst which are several that contain substantial areas
of heather moorland. The principal mechanism by which moorland ESAs attempt to
reduce Calluna loss is through the control of stocking rates.
The intensity of grazing by livestock is an important determinant of the floristic
composition of moorland vegetation, particularly the balance between ericacious and
graminoid species (Welch, 1984; Welch and Scott, 1995). Grazing animals exhibit
clear preferences for particular plant species and this, combined with the varying
capacity of plant species to withstand grazing, exerts a strong influence on the
pattern and dynamics of moorland vegetation. The distribution of preferred
vegetation types, alongside factors such as the availability of shelter, will influence
the spatial distribution of grazing by livestock (e.g. Clarke et al., 1995a). Spatial
variation in grazing intensity may have serious implications for schemes that aim to
enhance Calluna cover or reduce Calluna loss by imposing restrictions on overall
stocking densities. A non-random pattern of grazing will lead to effective stocking
densities that exceed overall stocking density and such localised concentrations of
grazing intensity may have profound effects on vegetation.
3
The effect on moorland vegetation dynamics of spatial variation in grazing intensity
has been little studied. That the spatial configuration and degree of fragmentation of
vegetation types within Calluna/grass mosaics influences the distribution of grazing
by livestock on Calluna was demonstrated within small (2.5ha) experimental plots
by Clarke et al. (1995a). Most moorland however is managed at a much larger scale
than this, with grazing units frequently in excess of several hundred hectares. An
understanding of the spatial distribution of grazing livestock at this larger scale is
necessary in order to determine whether overall reductions in stocking rates can
produce the maintenance or enhancement of heather moorland that is the stated
objective of ESA and other agri-environment schemes.
In this paper we report the findings of an annual programme of monitoring on
heather moorland in the North Peak ESA. The specific objectives of the study were:
(1) to quantify the intensity of grazing on Calluna in the ESA; (2) to investigate the
spatial pattern of grazing of Calluna by sheep using a geostatistical approach and (3)
to identify if there were consistent year-by-year patterns of high or low grazed areas.
METHODS
STUDY AREA
The North Peak ESA occupies c. 55,000 ha of land at the southern end of the
Pennines in the U.K. and c.80% of the ESA is open moorland at altitudes of up to
636 m. Blanket mire communities, principally Eriophorum vaginatum blanket mire
(M20, after Rodwell, 1991) but also, more locally, Calluna vulgaris-Eriophorum
vaginatum blanket mire (M19), predominate on the highest ground and in areas are
subject to severe erosion. At lower altitudes these plant communities grade into
drier heathland communities (Calluna vulgaris-Deschampsia flexuosa heath, H9;
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Calluna vulgaris-Vaccinium myrtillus heath, H12; and Vaccinium myrtillus-
Deschampsia flexuosa heath, H18). At the lower edge of the moors, where the open
moor meets the enclosed land below, heathland communities frequently form
mosaics with calcifugous grassland communities (Festuca ovina-Agrostis capillaris-
Galium saxatile grassland, U4 (after Rodwell, 1992); Nardus stricta-Galium saxatile
grassland, U5; and Deschampsia flexuosa grassland, U2); in some areas these
grassland communities, as well as areas dominated by Molinia caerulea, are more
extensive.
The main agricultural use of the open moorland is for grazing of sheep. In addition,
many moors have a shooting interest and Calluna is managed for red grouse
(Lagopus lagopus scoticus). Increases in sheep numbers (Anderson and Yalden,
1981) and the threat that overgrazing posed to the conservation value of the area led
to the designation of the ESA in 1988. By 1996, 74% of moorland was managed
under an ESA agreement (ADAS, 1997a). The significance of the area in landscape
and conservation terms is recognised in its status as a National Park and a Site of
Special Scientific Interest.
At the time of the study, the North Peak ESA offered two basic tiers of management
under which moorland could be managed. Tier 1 covers all moorland and farmland
and contains two sub-divisions that apply to moorland; Tier 1A is the basic entry-
level tier and restricts cultivation and agricultural inputs and Tier 1C dictates limits
on stocking density. Tier 2 requires a greater degree of change to management
practices and contains two sub-divisions; Tier 2A provides for still lower stocking
rates and off-wintering of stock, and Tier 2B is concerned with exclosure of
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moorland to encourage regeneration. The grazing units included in this study were
managed under tiers 1C and 2A.
SAMPLING STRATEGY
The annual programme of monitoring in the North Peak ESA formed part of an
extensive programme established to assess the effectiveness of the ESA scheme in
maintaining and enhancing the wildlife conservation, landscape and historical value
of the area (Hooper, 1992). Whilst the sampling strategy was not designed
specifically to provide data for spatial analysis, the approach gave sufficient
coverage over each grazing unit to enable an investigation of the nature of spatial
pattern in grazing intensity.
A grazing unit is an area of moorland that is managed as a single entity, with a
specified number of livestock. This was used as the primary sampling unit. Only
those grazing units with > 25 ha of Calluna, as shown on a land-cover map derived
from aerial photographs (ADAS, 1997b), were included within the sampling frame.
A random sample of five grazing units was then selected from a population of 34
units.
On each grazing unit estimates were made of the grazing intensity on Calluna.
Sampling within grazing units followed a stratified random sampling procedure
using 100 quadrats. Forty quadrats were allocated initially to each of two strata -
dominant Calluna (>75% cover) and sub-dominant Calluna (<75% cover) - with the
remaining 20 quadrats distributed proportionately in relation to the areas of the two
Calluna cover classes. Initial cover of Calluna was determined from land cover
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maps and aerial photography. This resulted in the sampling plan outlined in Table 1.
Random co-ordinates within the stratified areas were plotted for each grazing unit on
1:10,000 Ordnance Survey maps, until the required number of points were obtained.
These points were located in the field from the maps using compass bearings and by
pacing. Quadrats were not fixed, an independent sample being obtained in each year
of monitoring. The grazing units were sampled first in the period late March-late
May in 1993 and annually within the same time frame until 1997.
ESTIMATION OF GRAZING INTENSITY
Grazing intensity was estimated by assessing the proportion of the current year’s
Calluna shoots that had been grazed by sheep. The technique has two stages: a field
stage involving the collection of Calluna and a laboratory stage to calculate a
grazing index. By conducting the assessment of grazing index in the laboratory,
observer bias was minimised.
The field technique involved the collection of a sample of Calluna stems. At each
sample point, four Calluna stems were collected from the corners of a 1 m 0.5 m
quadrat. If there was no Calluna under the corner of the quadrat, the stem nearest to
the corner was chosen. If Calluna was entirely absent then the quadrat was moved
to the nearest area of Calluna within a radius of 10 m and the above procedure was
repeated. The sample bags were opened in the laboratory and each stem cut a
distance of 4 cm from the crown of the stem to yield a sample of shoots. All shoots
less than 1 cm in length were discarded. Each of the remaining shoots was then
assessed for sheep grazing and any shoot with a terminal zone intact was classified as
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being “ungrazed”. The number of shoots defined as “grazed” or “ungrazed” was
used to calculate the following index:
Grazing Index (GI) = Grazed Shoots / Total shoots
ANALYSIS OF VARIANCE
The effects of the main survey factors (grazing unit, ESA agreement status, Calluna
dominance and year) on GI were determined by Analysis of variance (ANOVA).
The ANOVA was a mixed model treating Grazing Unit as a random factor nested
within Agreement Status, with Agreement Status, Year and Calluna Dominance as
fixed factors. Grazing units were allocated Agreement Status codes 1 or 2,
corresponding to ESA tiers 1C and 2A respectively. GI was arcsine-square root
transformed to improve the distribution and satisfy assumptions of normality and
homogeneity of variance.
GEOSTATISTICAL ANALYSIS
Geostatistical analysis (e.g. Burrough, 1987; Rossi et al., 1992) was used to quantify
the nature of spatial variation present within each data set. Geostatistics provide
quantitative tools for the description and unbiased prediction of spatially distributed
variables. The spatial correlation structure is described by the semivariogram, which
quantifies spatial dependence by measuring the variation among samples
(semivariance) separated by the same distance (lag classes). Models fitted to the
semivariograms provide a quantitative representation of the spatial variation
exhibited in the field.
8
Where distinct spatial variation occurs, the values of semivariance rise gradually to a
point known as ‘the range’ at which it levels off. The range parameter (usually
denoted a) indicates the scale of the spatial dependence. Values of a variable
separated in space by distances less than the range are spatially autocorrelated, or
predictable. The value of semivariance at the point where a plateau occurs is known
as the sill, and is theoretically equal to the variance of the data. Theoretically, the
semivariogram should pass through the origin, because differences in values of a
variable at a point in space separated by zero distance should be zero. However,
semivariograms often appear to intersect the y-axis at a positive value of
semivariance. This value is known as the nugget variance (denoted CO) and has two
components: first, sampling error, and, secondly, unmeasured variation below the
smallest sampling distance (Isaaks and Srivastava, 1989).
Patterns of spatial variation in grazing intensity for each year surveyed were
examined using geostatistical analyses with GEO-EAS (Englund and Sparks, 1991)
and S-Plus (Mathsoft, 1995). Within the geostatistical analysis, each lag class was
represented by at least 40 pairs of points, and the maximum lag class was half of the
total distance measured in the field. Semivariograms were calculated at 50 metre
lags. Where distinct spatial structure was identified, spherical semivariogram
models (Isaaks and Srivastava, 1989) were fitted by weighted least squares (Cressie,
1985). The proportion of the nugget variance (CO) in relation to the overall
structural variance, or sill (CO+C1) was used as a measure of unexplained variation at
distances smaller than the smallest distance class, and any other sampling error for
each data set.
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COMPARISONS BETWEEN YEARS
Since the location of surveyed points was not consistent over time, comparison
between years was carried out using interpolated values of GI at fixed locations over
each grazing unit. Interpolation and comparisons between years were only carried
out for those sites and years where spatial dependence was identified. The
semivariograms constructed were used with a kriging approach to interpolate values
of GI onto regular 100m×100m grids superimposed upon the grazing units. Sets of
interpolated values of GI for each year were then plotted against each other to assess
the extent to which patterns were repeated between years.
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Results
LEVELS OF GRAZING INDEX
A total of 2,461 quadrats were sampled over 5 years on the 5 grazing units. 47% of
these quadrats had GI values below 0.2, with 93% having values below 0.6 The
overall mean GI across years was 0.26. Grazing Units 1, 2 and 3 all had overall
mean GI's of 0.28. Grazing Units 4 and 5 had mean GI's of 0.25 and 0.20
respectively.
Year (F(4,16)=24.8; p<0.001), Grazing Unit (F(4,2411)=18.9; p<0.001) and their
interaction (F(16,2411)=2.9; p<0.001) were all significant factors affecting changes in
GI (Fig 1). All grazing units showed broadly similar trends in GI between 1993 and
1997. This trend is one of a large decrease in GI between 1993 and 1994, followed
by a slight increase in 1995 and then a decrease in 1996, with similar intensities in
1997. The significant grazing-unit-by-year interaction is represented by
considerable variation between grazing units in changes in the relative increase in GI
in 1995. Grazing Units 1 and 2 showed greater increases than 3-5.
GEOSTATISTICAL ANALYSIS
The results of the geostatistical analysis are summarised in Table 2. Spatial
dependence in GI was seen in at least one of the survey years for four out of the five
grazing units studied, the exception being Grazing Unit 5. Semivariograms for
Grazing Unit 1 are shown in Figure 2 and typify the results found in the analysis. In
many cases, no evidence of spatial structure in GI was observed, and the
semivariogram form was flat (e.g. years 1993 - 1995 in Figure 2). This ‘pure
nugget’ semivariogram form is indicative of random spatial behaviour, as there is no
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similarity between values of GI either at locations near to each other or those further
apart.
Where distinct spatial variation occurs in GI over the grazing unit, semivariance is
lower at small lag distances and the value of semivariance then increases with
increasing lag distance until it reaches a plateau at the range distance (e.g. years 1996
and 1997 in Figure 2). Figure 3 shows the spatial variation in GI in 1996 and 1997
for Grazing Unit 1. These kriged maps give a visual representation of the spatial
behaviour described by the semivariogram models. A more homogeneous
distribution is seen for the year where a longer range of spatial dependence was seen
(Figure 3b) compared with the spatial variation observed where a shorter range of
spatial dependence exists (Figure 3a).
The range of spatial dependence observed varied between grazing units and years
(Table 2). These values should be interpreted with reference to the overall area of
the grazing unit surveyed (Table 1) to gain an idea of the degree to which they
represent heterogeneous conditions. Spatial dependence over a small distance
relative to the size of the grazing unit indicates heterogeneity in grazing levels. This
was the case for Grazing Units 2, 3 and 4, and Grazing Unit 1 in 1996. In contrast,
spatial dependence over a longer distance relative to the overall size of the grazing
unit was seen for Grazing Unit 1 in 1997 which indicates less overall heterogeneity.
The nugget variance expressed as a proportion of the overall semivariogram
structural variance (Table 2) varied from 9.5 to 61.1%. These values indicate that
where it was possible to characterise spatial variation with the semivariogram, there
12
was considerable spatial variation at smaller distances not adequately detected by the
sampling approach. Since for most sites information was not collected on spatial
variations below 150 m, it is likely that much of this unexplained variation relates to
spatial variation over this distance.
Temporal change
Evidence of distinct spatial distribution in levels of grazing intensity was observed in
more than one year for three grazing units. These were 1996 and 1997 for Grazing
Unit 1; 1993 and 1996 for Grazing Unit 3; and 1994 and 1996 for Grazing Unit 4.
For these sites, the similarity between the years was seen by comparison of data
interpolated onto a common grid using scatter plots (Figure 4). Varying degrees of
similarity between years were seen for the three grazing units
For Grazing Unit 1 (Figure 4a), the scatterplot indicates broadly similar values in GI
at points between 1996 and 1997, and there was no real change in mean levels of GI
(0.254% in 1996 and 0.264% in 1997). For some points on the scatterplot, low
values in 1996 showed higher corresponding values of GI in 1997. The locations of
these points over the grazing unit were found in spatially distinct areas and can be
identified in Figure 3a as areas of low grazing intensity. There are also points on the
scatterplot where high values of GI were observed in 1996 which were then lower in
1997 which has much the same effect in making the distribution of GI more
homogeneous over space. The increased range of spatial dependence in levels of GI
from 495m in 1996 to 879m in 1997 (Table 2) quantifies this shift towards a more
homogeneous distribution of GI over the grazing unit.
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The scatterplot for Grazing Unit 3 (Figure 4b) indicates considerable change in
values of GI at the same points from 1993 to 1996. A decrease in the mean value of
GI was seen, from 0.446 in 1993 to 0.224 in 1996. The reduction in values of GI did
not occur in a spatially consistent manner over the grazing unit. At sites where low
values of GI were recorded in 1993, these sites were also low in 1996. In contrast,
where high values of GI were recorded in 1993 these sites were, for the main part,
much lower when monitored in 1996.
For Grazing Unit 4 (Figure 4c) the scatterplot shows a slight increase in the levels of
GI from 1994 to 1996, with mean levels of GI changing from 0.169 in 1994 to 0.215
in 1996. A map of differences between the two years was constructed which
indicated that the temporal change in levels of GI was represented by a ‘blanket’
increase in GI over a large area of the moor and one region of size 25 ha that showed
a major increase in GI.
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Discussion
In this study we have identified the existence of large-scale spatial variation in the
distribution of grazing of calluna by sheep. For four of the five grazing units
examined, spatial heterogeneity in grazing intensity was observed in at least one
year. This heterogeneity was evident in the form of spatially distinct areas of
uniformly high or low grazing intensity. The geostatistical approach used made it
possible 1) to examine temporal change in the spatial pattern of grazing intensity, 2)
to quantify changes to the scale of heterogeneity and 3) to identify change from a
heterogeneous distribution to a random distribution or vice versa.
Spatial variation in grazing pressure may result from factors such as social behaviour
of the grazing animal, the distribution of preferred vegetation types, availability of
shelter and provision of supplementary feed. These factors may be interrelated and
it may be difficult to identify causes of observed distributions of grazing pressure.
Furthermore, temporal variation in grazing patterns may make understanding
underlying causes more difficult. Considerable temporal change in grazing pattern
was identified in this study. Changes to the spatial distribution of grazing intensity
may be the result of a broad range of influences including management and climatic
effects. There were no major changes in stocking densities between the surveys on
the grazing units in this study as the ESA agreements had already been in place for
some time. However, changes in stock management and/or burning practices could
have been responsible for changes in grazing patterns. From field visits to Grazing
Unit 1, it became clear that the main localised area of high grazing intensity
corresponded with an area where sheep were regularly turned out, resulting in this
area receiving a higher density of sheep on a regular basis.
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The existence of variation in the distribution of grazing sheep means that in some
areas effective stocking densities will exceed the overall stocking density for a
grazing unit; in other areas effective densities will be lower than the grazing unit
average. This has implications for the effective conservation of heather moorland.
A reduction of stocking levels alone would appear not to be a wholly effective
control against overgrazing. The occurrence of patches of localised high grazing
intensity may affect the competitive balance of moorland plant species by reducing
the vigour of Calluna, allowing invasion by other species (Welch, 1984; Welch and
Scott, 1995).
The specific locations where overgrazing occurs in relation to the distribution of
vegetation communities may have a considerable effect on the nature of any
vegetation change. Increased grazing intensity on Calluna at Calluna/grass
interfaces (Clarke et al., 1995a) will have a more marked effect on vegetation
change than in the middle of extensive monospecific areas of Calluna. Continued
incidence of high grazing intensity in particular areas (e.g. as seen in Grazing Unit 1)
may result in erosion of Calluna cover at the edges of Calluna stands in the same
way as that predicted by Clarke et al., (1995b) at smaller scales.
Uneven grazing pressure across moorland is not a problem per se. Localised high
grazing pressure is only undesirable if vegetation cannot sustain this level of grazing
and is likely to lead to a reduction in cover or condition of a valued resource, such as
calluna. Indeed, in some areas it may be beneficial to encourage heavier grazing in
order to reduce dominance by competitive grasses such as nardus stricta and molinia
caerulea or to prevent succession to woodland. Decisions on the manipulation or 16
control of spatial variation in grazing will depend on the overall management aims
for a moor. Spatial variation in grazing pressure may provide benefits if it provides
a diverse range of habitats for invertebrates (e.g. Gardner et al., 1997) and other
fauna (e.g. Fuller, 1996). Conversely, deleterious effects of uneven grazing are
likely to be most evident where patches of heavily grazed moorland vegetation
occur, especially where this results in the decline in condition and cover of plant
species which are of conservation importance (e.g. Calluna). Therefore where the
prime objective of a moorland management plan is the conservation of calluna, the
spatial pattern of grazing intensity on this species needs to be monitored and
managed.
In the absence of intervention by land managers, it seems that even under reduced
stocking levels sheep continue to congregate in favoured areas, rendering any
calluna located there vulnerable to grazing. Attracting sheep away from such areas
should be a key objective of any management plan that aims to conserve heather
moorland. There are several methods that can be used to redistribute grazing
intensity, either indirectly through changing the spatial pattern of vegetation through
burning and cutting (phillips and watson, 1995), or directly through amendment of
shepherding practices, such as the selective positioning of feed blocks (davies and
griffiths, 2000). Routine evaluation of practices such as the location of
supplementary feeding should be made by land managers to prevent overgrazing
hotspots in the same location year on year.
Whilst the monitoring and analysis techniques described in this paper are too labour
intensive to be used for moorland management in practice, alternative simple
assessment measures could be employed. Localised overgrazing of calluna and 17
other signs of heavy grazing pressure (e.g. Dunging) can be relatively easily
identified (e.g. Through recognition of growth forms typical of overgrazing
(macdonald, 1990)). Regular assessment of moors in respect of the occurrence and
distribution of such features would provide adequate warning of potentially
deleterious effects. Managers could then amend shepherding or feeding practices
appropriately to maximise the potential for the maintenance or enhancement of
moorland under their stewardship.
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Acknowledgements
The financial support of the Ministry of Agriculture Fisheries and Food for the
monitoring of the ESA programme is gratefully acknowledged. We thank the
landowners for access and all that have assisted in the field and laboratory work.
The authors are grateful to Sarah Gardner and Francis Kirkham for helpful
comments on the work presented. This paper was written with financial assistance
from MAFF under project BD1217.
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Tudor, G.J. and Mackey, E.C. (1995). Upland land cover change in post-war Scotland. In Heaths and Moorland: Cultural Landscapes, (D.B.A. Thompson, A.J. Hester and M.B. Usher, eds) pp. 28-42. HMSO, Edinburgh.
21
Welch, D. (1984). Studies in the grazing of heather moorland in north-east Scotland. III. Floristics. Journal of Applied Ecology 21, 209-225.
Welch, D. and Scott, D., (1995). Studies in the grazing of heather moorland in north-east Scotland. VI. 20-year trends in botanical composition. Journal of Applied Ecology 32, 596-611.
22
Table 1. Areas of total moorland, dominant and sub-dominant heather and the consequential number of quadrats in each stratum for each grazing unit.
Grazing Unit Total area (ha) Moorland area surveyed (ha) Number of quadrats
Dominant heather Sub-dominant heather Dominant heather Sub-dominant heather
1 866 155 265 47 53
2 885 406 173 54 46
3 662 332 83 56 44
4 649 28 162 43 57
5 1,489 793 531 52 48
23
Table 2. Summary of geostatistical analysis for those years and grazing units where
spatial dependence was identified. In all other cases, variation in grazing intensity was
randomly distributed. Parameters refer to a spherical semivariogram model where a
is the range parameter indicating the distance over which spatial dependence occurs,
CO is the intercept on the y axis and (CO + C1) is the value of semivariance at which
the semivariogram form plateaus. The nugget variance expressed as a proportion of
the overall structural variance (CO/(CO+C1)) is a measure of error and unexplained
variation at distances smaller than the smallest lag class.
Grazing Unit Year Semivariogram model parameters % CO/(CO+C1)a CO C1
1 1996 495.2 0.019 0.023 45.23
1 1997 878.8 0.024 0.016 60.00
2 1994 549.6 0.015 0.014 51.72
3 1993 303.4 0.0045 0.043 9.47
3 1996 470.5 0.01 0.022 31.25
4 1994 533.8 0.011 0.007 61.11
4 1996 549.5 0.014 0.015 48.27
24
List of Figures
Figure 1. Changes in mean grazing index for individual grazing units between 1993
and 1997.
Figure 2. Semivariograms of grazing index for Grazing Unit 1 for 1993-1997. Lines
are spherical semivariogram models fitted by weighted normal least squares. The
semivariogram model parameters are given in Table 2.
Figure 3. Kriged maps of spatial variation in grazing index over Grazing Unit 1 in
1996 (a) and 1997 (b). These maps were produced using the semivariograms shown
in Figure 2. Axis units are in metres.
Figure 4. Scatter plots with 1:1 reference lines showing the relationship between
interpolated values of grazing index for years in which spatial dependence was
detected. a) Grazing Unit 1: Grazing Index in 1996 and 1997; b) Grazing Unit 3:
Grazing Index in 1993 and 1996; c) Grazing Unit 4: Grazing Index in 1994 and 1996.
25
Characterisation of the response of over-grazed upland heath to the
implementation of reduced grazing levels
Gardner, Sarah M.1, Allen, D.1, Harvey, S.2 & Wildig, J.3
1ADAS Woodthorne, Wergs Road, Wolverhampton WV6 8TQ, 2Scottish Power
Technology, 45-47 Hawbank Road, College Milton North, East Kilbride G74 5EG;
3ADAS Pwllpeiran, Cwmystwyth, Aberystwyth, Dyfed SY23 4AB; Fax: +44 1902
693166: E-mail [email protected]
Abstract. The response of seven different vegetation types, three dominated by
calluna vulgaris and four dominated by grasses, to reduced stocking levels is
reported. Stocking levels on three of the four sites were consistent with those
used in environmentally sensitive areas (esas), whilst on the remaining site, levels
approximated those of normal farming practice. Changes were assessed at the
community and species level over a seven year period. Calluna cover was
maintained by esa stocking levels of 1.4-1.5 sheep/ha but declined when stocking
levels were reduced below this level. Greatest differences were observed in grass
dominated communities with molinia increasing significantly in the first two
years. Change in plant community composition was slow, even though it was
evident that changes had occurred at the level of individual species.
Keywords: Environmentally Sensitive Areas; Reduced stocking rates; Sheep
grazing.
Nomenclature: Stace (1991) and Rodwell, (1991)
26
Introduction
Heather moorland is recognised as a habitat of international conservation significance
(Thompson et al. 1995) and one that has recently been in decline (Bardgett et al.
1995; Tudor & Mackay 1995). In the UK, over-grazing by sheep, following the
increase in stocking levels during the 1970’s and ‘80’s, is seen as an important factor
contributing to the loss of heather (Calluna vulgaris) (Anderson et al. 1981; Welch
1984a; b; Barr 1997). Reducing grazing pressure is thus an important goal of current
moorland management policies. A key tool for achieving this on moorland in the UK
has been via agri-environment schemes such as Environmentally Sensitive Areas
(ESAs) and Countryside Stewardship. These schemes include specific environmental
aims to maintain and enhance the cover of dwarf-shrubs such as Calluna on upland
moor (ADAS 1997a). These aims are supported by prescriptions regarding stocking
levels and stock management and subsequent monitoring of the moorland vegetation
response.
There have been several studies to assess the impact of grazing on moorland
vegetation. Many of these have been undertaken in small plots (for example Clarke et
al.1995 on 2.5 ha plots) to assess the response of individual plants or species (Hartley
& Amos 1999). Rather fewer studies have been conducted at a larger scale (10 ha or
more), where grazers can roam freely throughout the year and demonstrate their
forage preferences. Under these circumstances, vegetation change is influenced by
the encounter rate between the grazer and the different vegetation types present and
also by the nutritional quality of each vegetation type. This study presents an
evaluation of the variation in response of over-grazed moorland vegetation over a
seven year period to a reduction of sheep stocking levels associated with ESA
27
stocking prescriptions within four large-scale (15-50 ha) moorland management units.
The assessment of the vegetation response is undertaken at two different scales,
namely for the vegetation type or plant assemblage and for the species. The results
are discussed with respect to the future management and large-scale monitoring of
moorland vegetation in England and Wales.
Methods
Experimental set-up
Two moorland sites at two different hill farms, ADAS Redesdale in the
Northumberland National Park and ADAS Pwllpeiran in the Cambrian Mountains
ESA, were used for the study.
At ADAS Redesdale, 117 ha of over-grazed Calluna/Molinia moor were divided into
two study areas of similar size and balance of vegetation types. On Area 1 (55.6 ha),
a stocking level of 1.5 sheep/ha was established and maintained throughout the period
September 1989 - September 1997. This level is equivalent to Tier 1 of the North
Peak ESA scheme operating nearest to the site. On Area 2 (47.9 ha), a stocking level
of 2.1 sheep/ha, comparable to ‘normal’ farming practice in 1990, was established.
This stocking level was discontinued after March 1995.
Two 14 ha study areas of over-grazed Calluna/Vaccinium/Nardus moor were used for
the study at ADAS Pwllpeiran. Two stocking levels of 1.4-1.5 sheep/ha and 1.0-1.1
sheep/ha were assigned to Areas 3 and 4 respectively, and maintained throughout the
period September 1989 - September 1997. These stocking levels were equivalent to
those set as part of the Cambrian Mountains ESA management prescriptions.
28
Measurement of plant community composition
Vegetation composition on each study area was assessed at 1-2 year intervals
during july-august.
At ADAS Redesdale, a 75 x 75 m grid was established across both study areas and a
single 1m2 permanent quadrat established at the cross-points between each grid cell.
A total of 167 permanent quadrats were obtained, 93 on Area 1 (1.5 sheep/ha) and 74
on Area 2 (2.1 sheep/ha). At ADAS Pwllpeiran, a similar grid was set up in each
study area to establish 18 permanent 1m2 quadrats in Area 3 (1.4-1.5 sheep/ha) and 27
permanent 1m2 quadrats in Area 4 (1.0-1.1 sheep/ha).
The permanent quadrats were divided into one hundred 10 x 10 cm cells and the
composition of the plant community, present in each quadrat, assessed on the
basis of first-hit scores obtained for each of these 100 cells. First-hits were
determined using a square sighter (dimensions 10 x 10 cm) with a pair of cross-
wires fixed at the base. The first species to touch the cross-wires was recorded as
the first-hit of a cell. One hundred first-hit scores were obtained for each
quadrat to give an estimate of the frequency of occurrence of plant species
present in each quadrat.
Characterisation of moorland vegetation types
The ability to identify different vegetation types within each study area was
considered important, since each type might be expected to respond differently to the
same stocking treatment. Variation in plant community composition was therefore
characterised using the Detrended Correspondence Analysis (DCA) routine within
CANOCO (ter Braak 1987) and fuzzy clustering (following the approach of Equihua
1990) of data from the baseline sampling year - 1990. A separate analysis was
29
undertaken for the two moorland sites - ADAS Redesdale (Areas 1 and 2) and ADAS
Pwllpeiran (Areas 3 and 4). Fuzzy clustering analysis fitted 2-6 clusters for each
moorland dataset using a fixed fuzziness criterion (2) and a fixed convergence
criterion (0.001) in each analysis (Equihua, 1990). The resulting plant community
groups were interpreted with reference to the National Vegetation Classification
(Rodwell 1991).
Assessment of change in plant community composition
Since replication at the level of the study area was not feasible, due to the large
area required to support free-ranging sheep throughout the year, the data from
each area were analysed separately as case studies.
Quadrat data for the period 1991-1997 (1991-94 for Area 2) were ordinated passively
within the baseline (1990 data) plant community ordination. This approach allows
the initial composition (1990 data) of each quadrat to be fixed in ordination space,
and their composition in subsequent years to be tracked, thus allowing shifts in
community composition to be identified. To determine the significance of any
observed shifts, repeated measures analysis-of-variance (ANOVA) of the DCA scores
was undertaken for each ordination axis. Where more than one plant community
group (identified from the fuzzy clustering) was present on a study area, plant
community group was included as a fixed factor within the ANOVA. Quadrats were
allocated to the plant community group for which they had the largest membership
value. Where appropriate, between year differences for individual plant community
groups were tested subsequently using Tukey HSD tests.
30
Analysis of change in the cover of key species
Changes in the occurrence of Calluna vulgaris, Vaccinium myrtillis, Nardus stricta
and Molinia caerulea (subsequently referred to as Calluna, Vaccinium, Nardus and
Molinia) were assessed for each study area in which they occurred. The frequency
data for each species were arcsine-square root transformed to satisfy assumpations of
normality and homogeneity of variance associated repeated measures ANOVA, which
was used fo the analyses. Where more than one plant community group was present
on a study area, plant community group was included as a fixed factor in the
ANOVA.
Results
CHARACTERISATION OF PLANT COMMUNITIES
Figure 1 presents the ordination for quadrat samples from Areas 1 and 2 at ADAS
Redesdale for the baseline year of 1990; the position of several key species is marked
to assist interpretation. Samples dominated by grasses were clearly separated from
those dominated by dwarf-shrubs along Axis 1 (eigenvalue 0.54). This axis also
reflected a soil moisture gradient separating species typical of wet heath (e.g.
Eriophorum vaginatum) from those of drier grassy heath (e.g. Agrostis sp., Potentilla
erecta). No clear biological interpretation could be made for Axis 2 (eigenvalue
0.23). A fuzzy analysis using five clusters (partition coefficient 0.4934) gave the
most clearly defined plant community groups (Table 1). These included two groups
dominated by Calluna (Table 1: groups 1 and 4) and three groups dominated by
grasses: Molinia (group 2), Nardus (group 3) and mixed grassland (group 5).
Quadrats were distinguished according to those that had a large membership value for
a specific group (value >0.66, maximum value 1), and those that were intermediate
31
between several groups. The latter are marked with an open symbol on Figure 1 and
were assigned to the group for which they had the largest membership value.
A fuzzy analysi of the vegetation on Areas 3 and 4 at ADAS Pwllpeiran identified
two distinct plant groups, one dominated by Vaccinium and Nardus (Table 1: group
6) and a second dominated by Calluna, Vaccinium and Nardus (Table 1: group 7).
Response of different plant communities to reduced grazing
Significant temporal differences in ordination axis 1 quadrat scores were
recorded for both areas 1 (1990 - 1997) and 2 (1990-1995) at adas redesdale
(f5,290= 5.22, p<0.001 and f5,245= 5.085, p<0.001 for areas 1and 2 respectively),
suggesting an overall change in vegetation composition on both study areas. In
both cases, there was also a significant interaction between plant community
group and year (f20,290=2.91, p<0.001 and f20,245= 1.713, p<0.001 for areas 1 and 2
respectively), suggesting that plant groups differed in their response to the
stocking treatments imposed on each area.
Analysis of between-year differences for the five plant community groups on Area 1
(stocking level 1.5 sheep/ha) revealed no significant changes in community
composition for the Calluna-dominated (group 1), Molinia-dominated (group 2) or
Calluna/Eriophorum dominated (group 4) vegetation. There was, however, a
significant decrease in the Axis 1 scores for Nardus-dominated vegetation (group 3),
suggesting a shift in the composition of this group towards Molinia-dominated
vegetation (Tukey HSD, p<0.01). A change was also detected in the mean Axis 1
scores for mixed grassland (group 5). These increased significantly for the period
1990-1994 (Tukey HSD, p<0.04) indicating an increased dominance in rushes,
particularly Juncus effusus, within the mixed grassland vegetation (group 5). for No
32
significant shifts in community compostion were detected for any of the plant
community groups on Area 2 (stocking level 2.1 sheep/ha), despite the detection of an
overall change in Axis 1 ordination scores.
At ADAS Pwllpeiran, a significant change in community composition was observed
for Calluna/Vaccinium/Nardus (group 7) vegetation on Area 3 (1.4-1.5 sheep/ha),
suggesting a shift towards Calluna-dominated vegetation. No significant change was
observed in either Vaccinium/Nardus (group 6) or Calluna/Vaccinium/Nardus (group
7) vegetation on Area 4 (1.0-1.1 sheep/ha), although the analysis confirmed the
occurrence of two distinct communities on this site.
Response of different species to reduced grazing
No significant between-year differences in Calluna cover were observed for any of
the plant community groups on Area 1 (1.5 sheep/ha) at ADAS Redesdale, although
the ANOVA analyses confirmed that the five plant community groups did differ from
each other in terms of Calluna cover (F4,58= 25.929, p<0.0001). There was a
significant increase in Calluna cover on Area 2 (2.1 sheep/ha, F5,245=2.831, p<0.02),
but this was confined to Calluna/Eriophorum vegetation (group 4) where a significant
increase was observed four years after the start of the experiment (p<0.018). Calluna
cover in Calluna/Vaccinium/Nardus (group 7) vegetation remained unchanged on
Area 3 (1.4-1.5 sheep/ha) at ADAS Pwllpeiran and showed a marked decrease on
Area 4 (1.0-1.1 sheep/ha) after seven years (Tukey HSD test, p<0.01).
Molinia increased rapidly on Area 1 (1.5 sheep/ha), over the first two years of the
study, in Calluna- (group 1, Figure 2c), Molinia- (group 2, Figure 2a) and Nardus-
dominated (group 3, Figure 2b) vegetation. Although no further change was
observed after this initial period, the increased cover of Molinia persisted in both the
33
Nardus- and Molinia- dominated vegetation (Figure 2b&a) but did not persist in
Calluna-dominated vegetation. Molinia cover in Calluna-dominated vegetation in
1997 did not differ significantly from that in 1990. A significant increase in Molinia
cover also occurred across Area 2 (F5, 245=16.688, p<0.0012) but there was no
consistent directional change within any of the five plant groups. There was
insufficient Molinia present on either of the two study areas at ADAS Pwllpeiran to
enable formal analysis.
Vaccinium cover in Calluna/Vaccinium/Nardus (group 7) vegetation decreased after
four years on Area 3 (1.4-1.5 sheep/ha) at ADAS Pwllpeiran (F4,68= 2.8302, p<0.03),
but no consistent change in Vaccinium cover was observed on Area 4 (1.0-1.1
sheep/ha) in either this plant community group or in Vaccinium/Nardus (group 6)
vegetation. No significant change in the cover of Nardus was observed in either plant
groups 6 or 7 (Table 1) on either area.
34
Discussion
The response of seven different vegetation types, three dominated by Calluna and
four dominated by grasses, to reduced stocking levels is reported. Stocking levels on
three of the sites were consistent with those used in ESAs, whilst on the remaining
site, levels approximated those of normal farming practice.
Calluna cover was maintained on both sites with stocking levels of 1.4-1.5 sheep/ha
(equivalent to normal ESA Tier 1 stocking prescriptions – ADAS 1997b), but
declined by 14% on the lowest stocking level (1.0-1.1 sheep/ha - normally equivalent
to higher Tier 2 ESA prescriptions). A small but significant increase in Calluna was
recorded for Calluna /Eriophorum vegetation at the higher stocking rate (2.1
sheep/ha) but no other consistent changes in vegetation composition were recorded at
this site. Community analyses suggested enhancement of the dwarf shrub community
in only one plant group, Calluna/Vaccinium/Nardus, but this occurred as a result of a
decrease in the cover of Vaccinium rather than from an enhancement of Calluna.
These data indicate, therefore that whilst a reduction in stocking levels was effective
in maintaining the existing cover of Calluna, it was insufficient for stimulating a
significant enhancement in its abundance.
Greater differences were seen in the grass dominated communities. The rapid
increase in Molinia observed within both Molinia and Nardus-dominated
communities is commonly seen in overgrazed upland heath in response to a reduction
or removal of grazing (Hill et al. 1992). The different response reflects differences in
the physiognomy of grasses and dwarf shrubs with respect to grazing (Grime et al.
1988) The overall impact of reducing grazing on moorland vegetation change will,
therefore, depend on the respective competitive vigour of Calluna and the grass
35
species present. Where Calluna is vigorous, then it will out-compete grasses by over-
shading (Alonso et al., 1998). If, as is common on formerly over-grazed sites in
ESAs, Calluna growth is woody and old, then grasses such as Molinia and Nardus
can easily encroach into areas formerly dominated by dwarf-shrub (Watt, 1955;
Gimingham, 1972).
For all seven communities, it was evident that change in community composition was
very slow. This is unsurprising for established moorland vegetation, where expansion
of each species occurs as a result of vegetative growth rather than from the
germination and establishment of seed . In this circumstance, Calluna can only
increase in extent at the interface with other vegetation types. Significant seed
germination and establishment will only occur on burns or similar areas of bare
ground (Gimingham 1972) and occurs more slowly than vegetative regrowth
(Gardner et al., 1993).
Change was most readily detected at the level of the individual species rather
than at the community level. Change in the latter may be detectable only after
shifts have occurred among several species and at a larger scale. Focusing on
species that are characteristic of a community type or environmental variable
(critchley, 2000) may provide more reliable data on the impacts of management
rather than reliance on shifts in community composition. However, the initial
characterisation of the variation in vegetation composition present on a moor is
important, since as seen in this study, communities differ in their response to
changes in management.
Results from this study suggest that for formerly over-grazed moorland, the reduction
in stocking levels is beneficial in maintaining the existing cover of Calluna but does
36
little to enhance its extent. The latter is likely to require more pro-active management
measures such as burning, seeding and cutting. In each case, careful consideration of
the composition and condition of the moorland vegetation, particularly Calluna, will
be needed to ensure that additional management will be effective in enhancing the
cover of dwarf shrubs rather than that of aggressive less-preferred grass species that
tend to coexist with them.
Acknowledgements. The authors are grateful for comments and assistance from
ADAS staff, particularly Dr Sarah Hetherington, in the preparation of this
manuscript. This research was funded by the UK Ministry of Agriculture,
Fisheries and Food under contracts BD0101, 0106 and 1217.
37
ReferencesADAS. 1997a. Biological Monitoring of Moorland in the North Peak ESA 1988-
1996. Unpublished report to MAFF, April 1997.
ADAS. 1997b. Environmental Monitoring IN the Exmoor ESA 1993-1996. Unpublished report to MAFF, April 1997.
Alonso, I. & Hartley, S.E. 1998. Effects of nutrient supply, light availability and herbivory on the growth of heather and three competing grass species. Plant Ecology 137: 203-212.
Anderson, P. & Yalden, D.W. 1991. Increased sheep numbers and the loss of heather moorland in the Peak District, England. Biological Conserv. 20: 195-213.
Bardgett, R.D., Marsden, J.H. & Howard, D.C. The extent and condition of heather moorland in the uplands of England and Wales. Biological Conserv. 71: 155-161.
Barr, C.J. (Ed.) 1997. Current status and prospects of key habitats in England: Upland landscapes. Department of the Environment, Transport and Regions, London.
Clarke, J.L., Welch, D. & Gordon, I.J. 1995. The influence of vegetation patterrn on the grzing of heather moorland by red deer and sheep. II. The impact on heather. J. Appl. Ecol. 32: 177-186.
Critchley, C.N.R. 2000. Ecological assessment of plant communities by reference to species traits and habitat preferences. Biodiversity and Conservation 9: 87-105.
Equihua, M. 1990. Fuzzy clustering of ecological data. J. Ecol. 78: 519-534.
Gardner, S.M., Leipert, C. & Rees, S. 1993. Managing heather moorland: Impacts of heather burning on Calluna regeneration. Journal Environmental Planning and Management 36: 283-293.
Gimingham, C.H. 1972 Ecology of Heathlands. Chapman and Hall, London.
Grime, J.P., Hodgson, J.G. & Hunt, R 1988. Comparative Plant Ecology: a Functional approach to Common British Species. Unwin Hyman, London.
Hartley, S.E. & Amos, L. 1999. Competitive interactions between Nardus stricta L. and Calluna vulgaris (L.) Hull. The effects of fertilizer and defoliation on above- and below-ground performance. J. Ecol. 87: 330-340.
Hill, M.O., Evans, D.F. & Bell, S.A., 1992. Long term effects of exluding sheep from hill pastures in North Wales. J. Ecol. 80: 1-13.
Rodwell J.S. 1991. British Plant Communities: Volume 2. Mires and Heaths. Cambridge University Press, Cambridge.
Stace, C. 1991. New Flora of the British Isles. Cambridge University Press, Cambridge.
ter Braak, C.J.F. 1987. Ordination. In (eds) Jongman, R.H.G. ter Braak, C.J.F. & van Tongeren O.F.R. Data analysis in community and landscape ecology. PUDOC Wageningen, The Netherlands.
Thompson, D.B.A., Macdonald, A.J. Marsden, J.H. & Galbraith, C. A. (1995) Upland heather moorland in Great Britain, A review of international importance,
38
vegetation change and some objective for nature conservation. Biol. Conserv. 71: 163-178.
Tudor, G.J. & Mackey, E.C. 1995. Upland land cover change in post-war Scotland. In (eds.) Thompson, D.B.A, Hester, A.J. & Usher, M.B. Heaths and moorlands, pp. 28-42. HMSO Edinburgh.
Watt, A.S. 1995. Bracken vs heather, a study in plant sociology. J. Ecol. 35: 105-112.
Welch, D. 1984a Studies in the grazing of heather moorland in the north-east of Scotland I. Site descriptions and patterns of utilization. J, Appl. Ecol. 21: 179-195
Welch, D. 1984b Studies in the grazing of heather moorland in the north-east of Scotland II Response of heather J, Appl. Ecol. 21: 197-207.
39
Table 1. Characteristics of the different plant groups identified on Areas 1 and 2 at ADAS Redesdale (groups 1-5) and on Areas 3 and 4 at ADAS Pwllpeiran (groups 6 & 7).
GROUP CHARACTERISTICS
1. Calluna-dominated heath
Calluna-dominated vegetation with constant Carex nigra and frequent Molinia and Eriophorum vaginatum but at relatively low covers. Closest NVC community is M15, Scirpus cespitosus – Erica tetralix wet heath
2. Molinia - dominated heath
Dominant Molinia with frequent Calluna and C. nigra. Closest NVC community is M15, Scirpus cespitosus - Erica tetralix wet heath
3. Nardus - dominated heath
Predominantly grassy vegetation dominated by Nardus and characterised by species typical of dry grassy vegetation such as Agrostis spp., Anthoxanthum odoratum, Festuca ovina and Potentilla erecta. There is frequent Molinia but at low cover. Closest NVC community is U5, Nardus stricta - Galium saxatile grassland.
4. Calluna / Eriophorum heath
A Calluna-dominated group characterised by species typical of wet conditions such as E. vaginatum, E. angustifolium and Sphagnum spp. This community has affinities with M15 and M19, Calluna vulgaris-Eriophorum vaginatum blanket mire.
5. Mixed grassland No single dominant species although C. nigra is more common than other species. The composition is similar to group 3 but Nardus is not dominant, merely frequent at low covers. Juncus effusus frequency is also high but again at low covers. Closest NVC community is U5, with strong affinity to M15 and M23 - Juncus efusus/acutiflorus-Galium palustre rush pasture.
6. Vaccinium / Nardus heath
Dominated by grasses and constant Vaccinium myrtillis. Nardus stricta, Deschampsia flexuosa, Festuca sp. and Galium saxatile all have high constancy values
7. Calluna/Nardus /Vaccinium heath
Dominated by Calluna, Vaccinium and Nardus and including species from both wet (e.g. Scirpus cespitosus) and dry (e.g. D. flexuosa) heath vegetation.
40
Figure legends
Fig. 1. Detrended Correspondence Analysis of quadrat samples for Areas 1 (1.5
sheep /ha) and 2 (2.1 sheep/ha) at ADAS Redesdale. Eigenvalues for axes 1 and 2 are
0.54 and 0.23 respectively. The positions of species that are characteristic for each
plant community group are marked with a solid square and the species name. The
positions of the five plant groups identified from fuzzy clustering are marked with a
solid diamond and the group number (1-5). 1 is the Calluna-dominated heath group, 2
- the Molinia-dominated heath group, 3 - the Nardus-dominated heath group, 4 -
Calluna-Eriophorum dominated heath and 5 - mixed grassland.
Fig. 2. Mean percentage frequency of Calluna and Molinia in a) Calluna-dominated
heath, b) Molinia-dominated heath and c) Nardus-dominated heath on Area 1 (1.5
sheep/ha) at ADAS Redesdale. P-values indicate change in Molinia frequency
(Tukey HSD tests) between the year, above which the P-value is positioned, and the
baseline year of 1990. Results for Calluna were all not significant
41
Fig.1.
43
0
10
20
30
40
50
60
70
90 92 94 97Year
% fr
eque
ncy
Calluna Molinia
p<0.01ns
ns
a)
ns
Fig.2
44
0
10
20
30
40
50
60
70
90 92 94 97Year
% fr
eque
ncy
Calluna Molinia p<0.01 p<0.01p<0.01
b)
Fig.2.
45
0
10
20
30
40
50
60
70
90 92 94 97Year
% fr
eque
ncy
Calluna Molinia
p<0.01 p<0.01 p<0.01
c)
Fig.2
46
Techniques for the control of Molinia caerulea on wet heath after burning
S Y ROSS, S HARVEY, H F ADAMSON and A E MOON
ADAS Redesdale, Rochester, Otterburn, Newcastle-upon-Tyne NE19 1SB, UK
SUMMARY
The effectiveness of three treatments to control the competitive vigour of Molinia caerulea were compared on a Calluna vulgaris -
Molinia dominated wet heathland. The treatments were burning, burning with cutting or burning with herbicide application, along with
an unburned control. The effect of the treatment was dependent on the dominant species present in each of the vegetation types sampled.
Where Molinia was the dominant species, the burning and herbicide treatment reduced Molinia cover in the short term, but the effect
were not sustained. Other treatments showed little effect. Burning stimulated seedling germination of Calluna where it was a prominent
component of the vegetation, and additional cutting or herbicide application increased germination in the mixed heathland vegetation
only. This effect was, however, also short-lived. Repeated post-burn herbicide application may have produced more sustained responses
in Molinia and in the regeneration of Calluna.
INTRODUCTION
47
There has been a reduction in the amount and quality of heather moorland across the UK over the last 50 years. The associated decrease
in Calluna and other dwarf shrubs has often been accompanied by increase in competitive grass species such as Molinia caerulea. In
response to these changes a key objective of the Ministry of Agriculture, Fisheries and Food (MAFF) agri-environment schemes is to
reduce grazing levels and to encourage the maintenance and enhancement of heather on upland moors. Burning management is often
included within these schemes to rejuvenate Calluna, but burning may also increase the competitive advantage of M. caerulea. This
experiment aims to investigate the effectiveness of different management methods in reducing the cover and competitive vigour of
Molinia whilst maintaining, and potentially enhancing, the cover of Calluna.
METHODS
Replicated plots were established in an area of wet heath dominated by Calluna and Molinia, under a stocking rate of 1.5 ewes/ha within
ESA Tier 1 stocking levels. Each plot allocated a different treatment, as follows: (i) burning and cutting to a height of 8 cm, (ii) burning
only, (iii) burning and ‘Fusilade’ herbicide application, and (iv) a control.
In July 1995 five 1m2 permanent quadrats were established in each plot prior to treatment application. Each quadrat was divided into 100
10cm2 cells and three baseline floristic assessments carried out: (i) first hits by cross-wires (ADAS, 1999), (ii) frequency of Molinia and
Calluna, and (iii) dominant plant species. These assessments were repeated in July 1997, 1998 and 1999, after treatments had been
applied. Post-burn regeneration of Calluna was assessed by counting the number of seedlings in 20 10 cm2 cells per quadrat, twice yearly
between June 1996 (before cutting or herbicide treatments) and May 1998.
48
RESULTS
Changes in species composition
DCA ordination and fuzzy clustering analysis (Equhia 1990) highlighted three main vegetation groups from the 1995 baseline vegetation
data (Fig. 1). Group 1 showed a higher proportion of grass species, dominated by Molinia, group 2 was dominated by Calluna, while
group 3 showed a mixture of heathland species. The response of these vegetation groups to the treatments applied was assessed from
1997 to 1999 using the change in DCA ordination axis 1 scores. Overall, there was a significant difference in axis 1 scores between the
four treatments (F(3) = 5.775; P = 0.0443), and trends could be identified within each vegetation group. In 1997 for all treatments, except
the control, the Calluna-dominated group 2 quadrats moved toward the Molinia and Deschampsia flexuosa ordination points. The mixed
heath quadrats (group 3) showed a similar but smaller shift. In all treatments the Molinia-dominated vegetation (group 1) showed very
little change in ordination space. In 1998 and 1999 there was some indication that vegetation groups 2 and 3 moved slightly to the right
of the ordination suggesting a change back toward Calluna-dominated vegetation.
Change in frequency and dominance of Calluna and Molinia
There was no significant difference in the frequency of Molinia between the four treatments (F(3) = 0.804; P = 0.5430), with this species
recorded in at least 70 of the 100 cells. There was, however, some indication of a decline in Molinia in the Molinia-dominated vegetation
type directly after herbicide application (Fig. 2 treatment 3 1997), although this was not statistically significant. There was an overall
49
significant reduction in Calluna between the treatments (F(5) = 63.566; P = 0.0002). This decline was seen in the three treatments after
burning for both the Calluna-dominated and the mixed heath vegetation types (Fig. 3).
Overall, there were few significant changes in species dominance within the three vegetation types. A significant change from Calluna to
Deschampsia dominance was observed in the Calluna-dominated vegetation group, between 1995 and 1998 (H(22) = 8.729; P = 0.0331).
A significant change from Calluna to Carex nigra was seen for the same year for the mixed heath vegetation type (H(18) = 8.826; P =
0.0317). There was some change in dominance from Calluna to Molinia in the mixed heath vegetation, but this was not statistically
significant (H(28) = 7.465; P = 0.0585).
Regeneration of Calluna
Regeneration was greatest in the Calluna and mixed heath vegetation types, while the Molinia-dominated vegetation group showed
reduced seedling germination overall. Within the Calluna vegetation type regeneration was highest after burning and herbicide
application (treatment 3) in the few months directly after treatment application, although this difference was not statistically significant.
For the mixed heath vegetation type regeneration under burning plus cutting, and burning plus herbicide treatments were significantly
greater than for burning alone (H(120) = 7.080; P = 0.0290) (Fig. 4).
DISCUSSION
50
The Calluna canopy was reduced in all three treatments of burning, or burning with additional cutting or herbicide application, as much
of the aboveground biomass was removed by the burn. The trend was similar for all three treatments indicating burning had the greatest
impact on Calluna with little additional effect of cutting or herbicide treatments.
Molinia was not significantly reduced in any of the treatments over the five years. There was, however, some indication that burning
with herbicide application reduced Molinia in the Molinia-dominated vegetation, but effect was not sustained suggesting that repeated
application of the herbicide may need to be considered for longer term control.
Burning has been shown to increase the competitiveness of M. caerulea in heathland due to nutrient release and reducing competition for
light (Heil & Bruggink, 1987; Alonsi & Hartley, 1998). Our results suggest burning alone did not increase the frequency of Molinia.
There was, however, some increase in its dominance in the mixed heath vegetation for two years after treatment application, but this was
not sustained. Species composition data also suggested that although Calluna-dominated and mixed heath areas initially become more
similar to Molinia-dominated areas after treatments, they begin to move back toward a more Calluna-rich species composition after three
years.
Heather regeneration after burning is typically highly variable, and dependant on many factors including plant age, site characteristics
and intensity of the burn. Burning stimulated seedling germination where Calluna was initially present as more than 40 % of the
vegetation canopy. Calluna is not expected to achieve dominance until several years after a burn (Gimingham, 1958; McFerran,
McAdam & Montgomery, 1995), and as Calluna was still widely found in the sward giving potential for re-establishment as a dominant.
Burning with cutting or herbicide application increased the numbers of seedlings, with the greatest effect being seen for the mixed heath
vegetation. The effect was short term, and all treatments showed lower seedling survival in the second year (1997). This may be 51
attributed to seedling die back over the winter between October 1996 and May 1997, or increased grazing/trampling on the Calluna
seedlings during the winter months (Welch, 1984). Where Calluna cover was low, and Molinia dominated the vegetation, seedling
germination was much reduced under all treatments, and it is unlikely that Calluna would re-establish as a dominant.
ACKNOWLEDGEMENTS
The authors would like to thank Jeff Byrne, Anna Gundry, Fiona Kennedy and David Oatway for help collecting data, and Dr Sarah
Gardner for comments on the manuscript. The work was funded by MAFF, as part of project BD1218.
REFERENCES
ADAS 1999. STATISTICAL ASSESSMENT OF THE TECHNIQUES FOR MONITORING SPECIES COMPOSITION IN
UPLAND PLANT COMMUNITIES. UNPUBLISHED REPORT TO MAFF. 46PP.
ALONSI I, HARTLEY S E. 1998. EFFECTS OF NUTRIENT SUPPLY, LIGHT AVAILABILITY AND HERBIVORY ON THE
GROWTH OF HEATHER AND THREE COMPETING GRASS SPECIES. PLANT ECOLOGY 137: 203-212.
DIEMONT W H, LINTHORST HOMAN H D M. 1989. RE-ESTABLISHMENT OF DOMINANCE BY DWARF SHRUBS ON
GRASS HEATHS. VEGETATIO 85: 13-19.
EQUHIA M. 1990. FUZZY CLUSTERING OF ECOLOGICAL DATA. JOURNAL OF ECOLOGY 78: 519-534.
52
GIMINGHAM C H. 1958. BIOLOGICAL FLORA OF THE BRITISH ISLES: CALLUNA VULGARIS (L.) HULL. JOURNAL OF
ECOLOGY 48: 455-483.
HEIL G W, BRUGGINK M. 1987. COMPETITION FOR NUTRIENTS BETWEEN CALLUNA VULGARIS AND MOLINIA
CAERULEA L. OECOLOGIA (BERLIN) 73: 105-107.
MCFERRAN D M, MCADAM J H, MONTGOMERY W I.. 1995. THE IMPACT OF BURNING AND GRAZING OF
HEATHLAND PLANTS AND INVERTEBRATES IN COUNTY ANTRIM. BIOLOGY AND ENVIRONMENT - PROCEEDINGS
OF THE ROYAL IRISH ACADEMY 95B: 1-17.
WELCH D. 1984. STUDIES IN THE GRAZING OF HEATHER MOORLAND IN NORTH-EAST SCOTLAND. I. SITE
DESCRIPTIONS AND PATTERNS OF UTILISATION. JOURNAL OF APPLIED ECOLOGY 21: 179-195.
Fig. 1 Detrended Correspondence Analysis ordination plot for pre-treatment (1995) 1st hits data, showing the three vegetation groups
identified through fuzzy clustering techniques in relation to plant species ordination. The partition coefficient is 0.4983. Species as
follows; Cv - Calluna vulgaris, Et - Erica tetralix, Df - Deschampsia flexuosa, Ns - Nardus stricta, Mc - Molinia caerulea, Vm -
Vaccinium myrtillus, Cn - Carex nigra
Fig. 2 The mean (+/- SE) frequency of Molinia for the Molinia-dominated vegetation group, over time for each treatment as follows; 1 -
burning and cutting, 2 - burning only, 3 - burning and herbicide application, 4 - control.
53
Fig. 3 The mean (+/- SE) frequency of Calluna for (A) the Calluna-dominated and (B) the mixed heath vegetation groups, over time for
each treatment, as follows; 1 - burning and cutting, 2 - burning only, 3 - burning and herbicide application, 4 - control.
Fig. 4 The mean (+/- SE) germination of Calluna for the mixed heath vegetation group over time for each treatment as follows; 1 -
burning and cutting, 2 - burning only, 3 - burning and herbicide application.
FIG 1
54
FIG 2
55
FIG 3
56
FIG 4
57
TECHNIQUES FOR THE CONTROL OF PURPLE MOOR-GRASS
S.Y. ROSS AND H.F. ADAMSON
ADAS Redesdale, Otterburn, Newcastle-upon-Tyne, NE19 1SB, UK
INTRODUCTION
There has been a reduction in UK heather moorland over the last 50 years (Thompson et al., 1995), which includes both a loss of area,
and a decline in quality (Bardgett et al., 1995). The decrease in heather (Calluna vulgaris) can be accompanied by increases in purple
moor-grass (Molinia caerulea), particularly after burning (e.g. Welch & Scott, 1995). Such a change in the balance of species affects
both heathland grazing quality and its biodiversity. This experiment investigates the effectiveness of three management techniques in
reducing the frequency and dominance of Molinia on heathland, under two stocking rates that fall within Environmentally Sensitive Area
(ESA) prescriptions for upland areas.
MATERIALS AND METHODS
Experimental plots were established under stocking rates of 1.5 ewes/ha (Area 1: ESA Tier 1 prescription), and 0.66 ewes/ha (Area 2:
ESA Tier 2 prescription). On each area, three replicated blocks were split into four 10 m 2 plots, with a 2 m discard, and each plot
randomly allocated a different treatment. The treatments were (1) burning + cutting to 8 cm high, (2) burning, (3) burning + herbicide,
and (4) a control receiving no treatment. Burning was carried out in April 1996, and additional cutting was applied to plot 1, and
58
fluazifop-P-butyl herbicide (‘Fusilade’ 250EW, 1.5 litres/ha) applied to plot 3 in July 1996. In July 1995 five 1m 2 permanent quadrats
were established in each plot prior to treatment application. They encompassed three vegetation types; Molinia-dominated, Calluna-
dominated and mixed Calluna/Molinia vegetation. Quadrats were divided into 100 10 cm2 cells, and baseline data of Molinia frequency
(presence/absence) and dominance were recorded in each cell. These assessments were repeated annually from 1996 to 1999 after
treatments had been applied. Frequency data were arcsine transformed and analysed using repeated measures ANOVA, with vegetation
group entered as a covariate, while dominance data were analysed separately for each vegetation group using Kruskal-Wallis tests.
RESULTS
Changes in Molinia frequency
For all three vegetation types there was no significant difference in Molinia frequency between the treatments (P = 0.5430) for Area 1
(1.5 ewes/ha). There was, however, a decline in Molinia in the Molinia-dominated vegetation type directly after herbicide application in
1997, although this was not statistically significant. For the lower stocking rate (Area 2: 0.66 ewes/ha), there was no significant
difference in the frequency of Molinia between treatments (P = 0.8896), and no indication of a 1997 decline in Molinia under the
burning plus herbicide treatment.
Changes in Molinia dominance
59
Overall, Molinia dominance across all three vegetation types was not significantly different (P = 0.2927) between treatments, for higher
stocking rates (Area 1). There was, however, some reduction in Molinia dominance in the Molinia-dominated vegetation, and this
reduction was sustained until 1999 (Fig. 1). Under lower stocking rates (Area 2) there was a significant increase in Molinia dominance
compared with the control (P = 0.0017), and this was particularly evident for the Molinia-dominated vegetation.
60
Fig. 1. Change in the mean dominance (+/-SEM) of Molinia over time, in the Molinia-dominated vegetation, for Area 1 (1.5
ewes/ha). Treatments are 1 – burning and cutting; 2 – burning only; 3 – burning and herbicide; 4 – control.
0.00
20.00
40.00
60.00
80.00
100.00
1 2 3 4
Treatment
% d
omin
ance
of M
olinia
1995
1998
1999
61
DISCUSSION AND CONCLUSIONS
The management treatments showed little effect on reducing Molinia frequency in either
Area 1 (1.5 ewes/ha) or Area 2 (0.66 ewes/ha). Most management techniques increased
Molinia dominance under both stocking rates, particularly burning alone. There was,
however, some indication that under 1.5 ewes/ha, burning followed by herbicide
application reduced both Molinia dominance and frequency in Molinia-dominated
vegetation. The reduction in frequency was only sustained for the year after herbicide
application, although dominance remained lower until 1999. This suggests that repeated
application of herbicide would need to be considered for longer-term control of Molinia,
although long-term effects on heathland are not known and require further research. The
effectiveness of this management technique strongly depends on both the dominance of
Molinia in the vegetation prior to treatment, and stocking rates applied after treatment.
REFERENCES
Bardgett R.D, Marsden J.H, and Howard J.H. (1995) The extent and condition of heather on moorland in the uplands of England and Wales. Biological Conservation, 71, 155-161.Thompson D.B.A., MacDonald A.J., Marsden J.H., and Galbraith C.A. (1995) Upland heather moorland in Great Britain: a review of international importance, vegetation change and some objectives for nature conservation. Biological Conservation, 71, 163-178.Welch D. and Scott D. (1995) Studies in the grazing of heather moorland in north-east Scotland. VI. 20-year trends in botanical composition. Journal of Applied Ecology, 32, 596-611.
62
The effects of summer-only grazing on the post-burn recovery of a wet heath,
Northumberland, England.
Sarah Ross, Andrew Nolan
ADAS Redesdale, Rochester, Otterburn, Newcastle-upon-Tyne, NE19 1SB. Macaulay
Land Use Research Institute, Aberdeen, AB15 8QH.
Corresponding author: Sarah Ross, ADAS Redesdale, Otterburn, Newcastle-upon-Tyne,
NE19 1SB. Fax: (+44) 01830 520 608, Email: [email protected].
Word count (incl. Tables and Figure legends) = 5,559
63
Summary1. Molinia caerulea is a common, typically sub-dominant species of wet heath
vegetation. Inappropriate burning and grazing regimes have encouraged this
invasive grass to become dominant at the expense of Calluna on some heathland
areas. However, Calluna typically requires some management to prevent it from
becoming degenerate, and heather burning is typically the management option
employed. This study aims to assess the effectiveness of summer-only grazing on
enhancing Calluna post-burn re-establishment while simultaneously reducing the
regrowth and vigour of Molinia.
2. Two stands of wet heath, dominated by either mature (22 year old) or building (8
year old) Calluna, were burnt in April 1996. Sheep were subsequently grazed on
plots from June to August, 1996 to 1999, at two grazing levels along with an
ungrazed control.
3. Results indicated that while the ungrazed control showed the greatest increase in
Calluna cover, there was also a significant increase in the cover and dominance of
Molinia in the absence of grazing. The high summer grazing level effectively
controlled Molinia but showed reduced Calluna re-establishment. The low
summer-only grazing levels provided the best balance between encouraging
Calluna recovery, while also controlling Molinia regrowth.
4. The response of Calluna cover to grazing treatments was significantly affected by
the age of Calluna at burning, with the younger age stand showing greater
increases in Calluna cover. In terms of Calluna regeneration, seedling counts
were not significantly affected by stand age, but vegetative regrowth was. This
suggests there was a viable seed bank under both age stands of Calluna that is able
to germinate after fire. On burning the younger Calluna stand was, however, also
able to regenerate effectively from vegetative regrowth, enabling Calluna to re-
establish more rapidly. On the stand of mature Calluna aged 22 years at burning,
vegetative regeneration was reduced. Change in Molinia cover in response to the
grazing treatments was not significantly affected by the age of Calluna at burning.
5. Therefore, for areas of wet heath where invasive Molinia may become dominant
after heather burning, a shorter burning rotation with low levels of post-burn
64
summer-only grazing may be the most appropriate management technique for the
re-establishment of Calluna.
65
Key-wordsCalluna vulgaris, heathland management, Molinia caerulea, secondary succession.
66
IntroductionWet heathland vegetation is a major component of the vegetation of the uplands of
northern Britain, and the current lack of knowledge regarding grazing and burning
management of these habitats has recently been highlighted (Thompson & Miles 1995;
Shaw et al. 1996). The loss of large areas of heathland through land use change is well
known (Tudor & Mackey 1995), but the decline of heather Calluna vulgaris (referred to
as Calluna throughout) due to inappropriate grazing and burning is also thought to be
significant (Thompson et al. 1995). The Ministry and Agriculture, Fisheries and Food’s
(MAFF) suite of agri-environment schemes, such as the Environmentally Sensitive Area
(ESA) and Countryside Stewardship (CS) schemes, attempt to redress dwarf shrub
species loss by recommending more appropriate management prescriptions for heathland
areas. These schemes have attracted a high uptake amongst landowners and farmers in
the uplands, but there is little experimental evidence upon which to base management
prescriptions for wet heath vegetation types, particularly for management through heather
burning.
Heather burning is the traditional form of management applied to heathland vegetation to
rejuvenate Calluna, and is used as a management tool on wet as well as dry heaths to
prevent Calluna from becoming over-mature and degenerate. There is, however, a
greater likelihood that species such as Molinia (Welch & Scott 1995) may gain
dominance in preference to Calluna in post-burn secondary succession on wet heath.
This is because wet heaths typically include a number of co-dominant or sub-dominant
species along with Calluna, including Vaccinium myrtillus, Erica tetralix, Trichophorum
cespitosum and Eriophorum vaginatum. Therefore, wet heaths may require more careful
management in order to maintain Calluna as a component species and to prevent the
invasion of grass and sedge species, particularly competitive species such as Molinia
caerulea (referred to as Molinia throughout). Previous work on applying a range of
stocking densities to regenerating (post-burn) wet heath vegetation (Nolan et al. in prep)
indicated that with year-round stocking regimes, particularly on stands that were of
mature Calluna on burning, Molinia showed a greater relative increase in cover compared
to Calluna. With year-round grazing at lower stocking rates, as often applied under agri-
environment schemes, herbivores may be unable to graze Molinia at a level that affects its
cover during maximum growth rates. In addition, even a lower level of winter grazing is
67
typically concentrated on Calluna even if present at relatively low cover (Grant et al.
1976). In addition, sheep prefer many grass and sedge species to Calluna as a grazing
resource in the summer months (Welch 1984). These seasonal grazing preferences may
provide a management tool for enhancing Calluna re-establishment after burning, while
controlling the regrowth and competitive vigour of associated competitive grass species,
in particular Molinia. This paper presents the results of a study aimed at exploring
management strategies to enhance the recovery and regeneration of post-burn Calluna on
wet heath. The management strategies applied were two levels of post-burn summer-only
grazing along with an ungrazed control. The strategies were carried out on two age
stands of heather, mature and building, to investigate the effect of age of Calluna at
burning on regeneration under the grazing regimes. The grazing treatments were chosen
on the basis of previous work (Grant et al. 1996) that showed that Molinia was sensitive
to higher levels of defoliation. Nomenclature follows Stace (1992) for higher plants, and
Watson (1988) for bryophytes.
68
Materials and methodsThe experimental site was located on an upland area of Trichophorum cespitosum - Erica
tetralix wet heath (National Vegetation Classification M15; Rodwell 1991) at ADAS
Redesdale, Northumberland, England. The vegetation was dominated by Calluna vulgaris
with abundant Molinia caerulea, Eriophorum vaginatum and Carex nigra. Ten 0.12 ha
plots were established in 1995, five plots on an area dominated by 22 year old mature
heather, and five on an area dominated by younger 8 year old building heather. Baseline
vegetation data were collected in June 1995, and all plots were then burnt in April 1996.
Three summer-only (01 June to 31 August) grazing treatments were applied from 1996 to
1999. The grazing treatments were a high and low summer-only grazing regime,
replicated within two different age stands of Calluna (building and mature), and an
ungrazed control.
The post-burn summer-only grazing treatments were based on work by Grant et al.
(1996). The grazing treatments were (i) low grazing level, with Molinia lamina lengths
controlled at 10-12 cm approximating to 25-33 % off-take of Molinia annual growth, (ii)
a high grazing level, with Molinia lamina lengths controlled at 4-6 cm approximating to
50-66% off-take of Molinia annual growth, and (iii) an ungrazed control. Molinia lamina
lengths were assessed weekly across each plot using 20 random measurements, and sheep
numbers on each plot were adjusted accordingly to retain the correct Molinia off-take
rates. The sheep were present on the plots between 01 June and 31 August only, from
1996 through to 1999.
In each plot, six 1 m2 permanent quadrats were established in 1995. Each quadrat was
gridded to give 100, 10 x 10 cm cells. Post-burn Calluna regeneration was assessed in
the two central columns (20, 10 x 10 cm cells) of each quadrat. Counts of the number of
Calluna seedlings and stems of vegetative regrowth were used to assess regeneration
within the different age stands of heather. These counts were carried out seven times
between June 1996 and April 1998 inclusive. Statistical analysis was carried out on
square-root transformed counts of both seedling and vegetative regeneration, using a
three-way analysis of variance (ANOVA) incorporating time as a repeated measures
factor. The percent cover of all plant species present was estimated for each quadrat,
along with the dominant canopy and ground layer species within the central two columns
69
(20 10 x 10 cm cells) of each quadrat. These assessments were repeated annually in July,
from 1995 (pre-burn) and 1996 to 1999 (post-burn). Changes in the percent cover of key
species, and the regeneration of Calluna, were assessed to determine the post-burn
recovery of Calluna, and the control of Molinia under the different grazing treatments.
Statistical analysis of cover data was carried out using three-way ANOVA incorporating
time as a repeated measure, with the 1995 pre-burn data entered as a covariate. Changes
in the dominant canopy and ground layer species over time under the grazing treatments
were also assessed. These data were not normally distributed and were analysed using
non-parametric Friedman rank ANOVA, which incorporates time as a repeated measure.
Statistical analyses were carried out in Statistica 99 (StatSoft Inc., US) on the replicated
high and low grazing treatments, but did not include the unreplicated control plot.
70
ResultsREGENERATION OF CALLUNA BY SEEDLING GERMINATION
Calluna seedling germination counts were highly variable within the different treatments
and heather age stands, with mean counts ranging from two to 40 seedlings for the 22
year old heather stand on burning, and from two to 65 for the 8 year old heather stand on
burning. There was no significant difference in counts of seedling germination between
the 22 and 8 year old heather stands for the two grazing levels (Table 1). There was also
no significant difference between grazing treatments, although the low summer grazing
showing similar seedling germination counts to the high summer grazing control, for both
heather stands (Fig. 1). The Calluna seedling germination in the ungrazed control was
greater than either grazing treatment the over much of the experimental period, but this
difference could not be statistically tested due to incomplete replication of the control
plot. The change in seedling germination over time was significant (Table 1) for the high
and low grazing treatments. As Figure 1 illustrates, there were significant increases in
germination counts from June to August 1996 and also August to October 1996, and
subsequent (but not statistically significant) declines from May 1997 onwards. For the
ungrazed control, seedling counts continued to decline to April 1998 in the 22 year old
heather stand (Fig. 1a), while the low grazing treatment recovers its seedling count. A
similar trend is observed for the 8 year old heather stand (Fig. 1b). Under the high
grazing treatment, both age stands of heather continued to show low numbers of seedlings
(typically < 20 counts per quadrat) throughout the experiment.
REGENERATION OF CALLUNA BY VEGETATIVE REGROWTH
Regeneration counts for Calluna vegetative regrowth were consistently lower than
seedling regeneration counts for all sites and grazing treatments, showing a maximum of
20 counts (Fig. 2), compared to up to 65 seedling counts. The regeneration of Calluna by
vegetative regrowth showed a significant difference between the different aged stands of
heather for the two grazing levels (Table 1). The 8 year old heather stand showed
significantly higher vegetative regrowth counts than the mature heather stand. This
difference was echoed by the ungrazed control (Fig. 2), although this could not be
statistically tested due to incomplete replication of the control plot. There was, however,
71
no statistically significant difference between the two grazing treatments (Table 2).
Vegetative regeneration counts of Calluna did change significantly over time, although
the interaction between age of heather and time indicated that the direction of change was
dependant on the age of the heather stand at burning.
CHANGES IN THE PERCENT COVER OF KEY SPECIES
Changes in the estimated percent cover of Calluna for both 8 and 22 year old heather
stands are presented in Figure 3. There was a significant difference in the percent cover
of Calluna between the heather stands for the high and low grazing treatments (Table 2)
with the 8 year old stands showing a lower initial (1995) Calluna cover (approximately
40%), but an increased post-treatment Calluna recovery (1998 - 1999) compared to the
22 year old heather stand. There was no significant difference between grazing treatment
on Calluna cover. There was, however, a significant difference in Calluna cover over
time, with the 1998 and 1999 Calluna cover being significantly greater than the 1996 –
1997 Calluna cover. There was also a significant interaction between time and grazing
treatment, indicating that changes in Calluna cover over time were significantly affected
by the grazing treatment applied.
Changes in the estimated percent cover of Molinia in both the 8 and 22 year old heather
stands are presented in Figure 4. No significant difference was detected in the cover of
Molinia between the two different aged stands (Table 2), indicating that overall, Molinia
was not more or less prevalent after burning in any one age stand. There was also no
significant effect of grazing treatment, although the ungrazed control showing greater
Molinia recovery after burning than either of the summer-only grazing treatments. This
could not be statistically tested. There was no significant interaction between age of
heather stand and grazing treatment, indicating the effect of the grazing treatment on
Molinia cover was not significantly different between the 8 and 22 year old heather. The
Molinia cover did vary significantly over time, with 1998 and 1999 being significantly
greater than 1996, and 1999 cover also being greater than 1997.
72
CHANGES IN THE DOMINANT SPECIES OF THE CANOPY LAYER
The most commonly occurring dominant canopy species in the wet heath vegetation were
found to be Calluna, Molinia, Carex nigra, Deschampsia flexuosa, Eriophorum
vaginatum and Vaccinium myrtillus. For both 22 year old and 8 year old heather stands
there were significant changes in the dominance of these main canopy species over time,
under all of the three grazing treatments. In particular there was a significant decline in
the dominance of Calluna due to the effects of burning in spring 1996, with subsequent
increases in the dominance of associated grass and sedge species. There was, however,
no significant change in the canopy dominance of Vaccinium myrtillus from 1995 to 1999
in any of the grazing treatments. This indicated that neither the burning nor the post-burn
grazing treatments significantly increased or decreased the dominance of V. myrtillus in
the wet heath vegetation, with mean counts of dominance remaining low throughout the
experiment.
Assessing the effect of each grazing treatment on these six canopy species for each year
enabled further extrapolation of responses of secondary succession to grazing level. There
was no difference in Calluna dominance between treatments for either the 22 or 8 year
old heather stand (Fig. 5a,b). Both age stands of heather and all grazing treatments
showing similar trends in Calluna dominance throughout the experiment. However, for
Molinia dominance the ungrazed control showing a higher Molinia dominance than the
summer-grazing treatments (Fig. 5c). For the 8 year old stand the trend was similar (Fig.
5d). Under summer grazing Carex nigra showed a trend toward rapidly increasing
dominance from the first summer after burning (1996 - 1999) on the 22 year old stand
(Fig. 5e). In contrast, the 8 year old stand showed no similar increases, despite both
stands showing similar pre-burn (1995) levels of Carex nigra, at less than three dominant
cells out of 20. The response of Eriophorum vaginatum showed few identifiable trends in
dominance, although under the 8 year old stand both high and low grazing treatments
showed increased E. vaginatum as compared to the ungrazed control. Both Deschampsia
flexuosa and Vaccinium myrtillus dominance remained low throughout the experiment
(less than five cells being dominant), showing little change in response to either burning
or grazing treatments.
73
CHANGES IN THE DOMINANT SPECIES OF THE GROUND LAYER
The ground layer was dominated by litter in all treatments, with a mixture of associated
bryophyte species including Polytrichum spp. and Sphagnum spp. Although litter
dominance declined somewhat in both age stands of Calluna after burning, this still
remained the dominant component of the ground layer throughout the experimental
period (Fig. 6a,b). The main bryophyte species of the ground layer was Polytrichum
commune. Polytrichum commune showed a small increase in dominance under the high
grazing treatment for both age stands of Calluna (Fig. 6c,d). Sphagnum spp. also showed
some trends toward changes in dominance, with the ungrazed control showing a small
increase in dominance compared to the two grazing treatments, although numbers remain
low (<10 cells dominant). Other ground layer species, such as Hypnum jutlandicum,
Pleurozium schreberi and Aulacomnium palustre were not dominant to any great extent
(<5 cells dominant).
74
DiscussionResearch has shown that sheep will graze preferentially on different heathland species at
different times of the year. Typically, they will graze Molinia caerulea and
Trichophorum cespitosum in the summer months, switching to Calluna vulgaris and
Eriophorum vaginatum during the autumn and winter (Welch 1984; Grant, Bolton &
Torvell 1985; Grant et al. 1987). The preferred species in any one season is grazed
heavily, even if it is present as a relatively small proportion of the overall sward (Grant et
al. 1976). This has been shown to be the case for Molinia during the summer, although
the build-up of fibrous Molinia tussocks can discourage grazing over time (Grant et al.
1985). This experiment was devised to exploit the preference of sheep to graze Molinia
in the summer months, so as to exert control on Molinia regrowth after heather burning,
and alter the balance of post-burn secondary succession in favour of Calluna re-
establishment. The early summer start to grazing was intended to control the new
Molinia growth during its period of peak productivity. This also helped to retain the
sheep preference for Molinia in subsequent years by ensuring there was minimal build-up
of Molinia litter over each season.
Calluna regeneration was assessed from counts of seedling and vegetative regrowth,
and the contribution of each regeneration strategy was assessed for each age stand
of heather. There was no significant difference in seedling counts between the 22
and 8 year old heather stands for the two grazing treatments, with both stands
showing high but variable seedling counts. Calluna regeneration from the seedbank
is typically highly variable (gardner et al. 1993), but this study shows that both
stands of heather had a viable seedbank from which to recruit seedlings, providing
conditions for seedling establishment were met. Ground disturbance in the form of
heavy grazing is a significant factor in calluna seedling death (bridges 1985, gardner
et al. 1993). There was a general trend for the high grazing level to show reduced
numbers of seedling germination in both age stands of heather, while the low
grazing level and ungrazed plots showed very similar levels of germination,
indicating the effect of higher grazing and trampling on reducing calluna seedling
establishment. However, seedling dynamics over time appeared to be unrelated to
grazing management or age of heather at burning. All management treatments,
including the ungrazed control, showed an increase in seedling numbers from
75
heather burning in spring 1996 through to the mid-summer 1997, peaking at 50 – 60
seedlings per 20 10 x 10 cm cells. In the final assessment 21 months after the fire
(april 1998) the seedling counts had reduced to around 25 young plants, similar to
the july 1996 numbers immediately after the fire. These trends may be linked to
additional effects such as competition from associated species, surface moisture
content or climate.
Vegetative regeneration appeared to be unaffected by the grazing treatment
imposed, but significantly affected by the age of the stand at burning. There was a
significant increase in the number of vegetative regeneration shoots in the 8 year old
stand compared to the 22 year old stand. Although not statistically tested, the
ungrazed control plot within each age stand showed more vegetative counts than
either of the grazing treatments for the first year, while second year counts were
similarly low for all treatments. Despite overall vegetative regeneration counts
under the grazing treatments were rather low throughout the experimental period,
they accounted for a larger proportion of the calluna cover than the seedling
regeneration counts. Greater numbers of vegetative regeneration are, therefore,
likely to contribute the greater recovery of calluna cover in the 8 year old stand
compared to the 22 year old stand. In addition to general changes in the physiology
of the heather plants as they age leading to a lower number of dormant buds on the
remaining woody stem after burning, hobbs and gimingham (1987) suggest that the
greater amount of woody material present in mature stands of calluna leads to
hotter fires. Both these physiological and environmental conditions may result in
poor calluna regeneration through vegetative regrowth. This appears to be the case
for our experiment, where the proportion of calluna regeneration through regrowth
was greater for the building (8 year old) heather stand compared to the mature (22
year old) heather stand.
The heather burning reduced the overall cover and dominance of Calluna in the canopy
layer, as the burning removed the majority of the above ground biomass. Calluna
recovery after burning is often greater when grazing is absent (McFerran, McAdam &
Montgomery 1995), and heavy grazing has been shown to be deleterious to Calluna
regeneration (Welch 1984; Sydes & Miller 1988). Our results agree with these findings,
as excluding grazing resulted in increased Calluna cover compared to the two grazing
76
levels. The Calluna dominance was not, however, fully regained over the three-year
post-burn period. This slow recovery of Calluna dominance after burning has been
reported by other researchers (e.g. McFerran, McAdam & Montgomery 1995). This is
likely to be a result of the low relative growth rate of Calluna, particularly at upland sites,
as demonstrated by Grime, Hodgson and Hunt (1988). In addition, on such wet heath
vegetation Calluna is typically a co-dominant species along with a number of grass and
sedge species (Rodwell 1991). Therefore, the time period for Calluna to regain its
former cover is likely to be greater compared to Calluna of dry heath communities,
because of increased competition from associated species.
Molinia was the main species replacing Calluna dominance in all grazing treatments.
This replacement was particularly evident in the ungrazed plots, with Molinia cover and
dominance significantly increasing over time indicating vigorous regrowth. Therefore,
removing all grazing after heather burning did enable successful Calluna re-
establishment, but did not reduce the competitive ability of Molinia, or control the
increase in Molinia cover in the vegetation. In the longer term this increase in Molinia
cover and dominance may exclude Calluna from regaining dominance within the
heathland vegetation, through the shading of Calluna seedlings and vegetative regrowth.
Under low summer-only grazing levels, equivalent to 33% off-take of Molinia annual
growth, Molinia cover and dominance were reduced. Calluna regeneration was greater
under the lower grazing treatment than under the high grazing treatment, although this
was not statistically significant. There was still a negative effect of the low summer
grazing treatment on Calluna re-establishment, as cover did not attain the levels found on
the ungrazed control plot. In the longer term, however, this may be compensated for by
the reduction in growth and competitive vigour of Molinia, a species known to be
sensitive to higher levels of defoliation (Grant et al. 1996).
Under the high summer-only grazing treatment, equivalent to 66% off-take of Molina’s
annual growth, Molinia cover and dominance was also reduced. Despite the effective
control of Molinia regrowth, Calluna recovery was less successful under this grazing
management regime, and both Calluna cover and regeneration counts were reduced. This
indicates that although the high summer-only grazing level effectively controlled Molinia
regrowth after burning, this management was not beneficial to post-burn Calluna re-
77
establishment. Calluna re-establishment is likely to be negatively affected by the
increased grazing and trampling on the regeneration seedlings and vegetative regrowth
under these higher grazing levels. Ground disturbance is a significant factor in Calluna
seedling death (Bridges 1985; Gardner, Liepert & Rees 1993) and grazing and trampling
is the main contributor to ground disturbance during the experiment. In association with
this increased disturbance, there was some indication that Carex nigra increased in
dominance under higher grazing treatments, particularly in the 22 year old mature heather
stand.
The aim of the summer grazing treatments was to reduce the regrowth of Molinia, while
simultaneously enhancing the recovery of Calluna after burning. The summer-only
grazing management treatments were able to significantly change the post-burn secondary
succession on the wet heath, moving the emphasis of species dominance away from
Molinia in favour of Calluna re-establishment. The secondary succession of other
species was not as closely linked to grazing treatments as those of Molinia and Calluna.
For example, summer-only grazing management appeared to favour Eriophorum
vaginatum re-establishment after burning, particularly in the 8 year old heather stand. As
Eriophorum vaginatum is more typically grazed in autumn and winter months (Grany et
al.1985; Welch 1985; Grant et al. 1987), and this may be due to a selection against
grazing on Eriophorum vaginatum by sheep in summer in favour of relatively more
palatable grass species. Deschampsia flexuosa cover and dominance often increases after
burning on heathland (Pakeman & Marshall 1997), but this trend was not seen at this site.
This may be related to low occurrence of Deschampsia flexuosa in the vegetation prior to
burning (Britton 1998), or its upland situation as previous studies on secondary
succession dynamics of this grass have been based on lowland heathland areas (e.g.
Britton et al. 2000).
The most appropriate post-burn management treatment employed in this trial was the low
summer grazing treatment with Molinia lamina length grazed to approximately 6 cm
height. This is approximately equivalent to 33% off-take of annual Molinia growth
(Grant et al. 1996). This treatment successfully controlled Molinia regrowth after
burning, and also allowed Calluna cover to increase significantly. Both the building and
mature stands of heather showed good regeneration of Calluna from seed (Nolan et al. in
78
prep), indicating a viable seed bank. However, because the mature heather stand was
almost entirely reliant on seedling germination for post-burn re-establishment of Calluna,
with little contribution from vegetative regrowth, the effect of the summer-only grazing
reduced the success of Calluna re-establishment. In the building heather stand a higher
proportion of Calluna re-establishment was through vegetative regrowth, and so less
susceptible to ground disturbance and microclimate conditions. Calluna re-establishment
was therefore improved under the building heather stand. This indicated that at this site a
burning rotation which maintains building heather stands (<10 years) combined with low
levels of post-burn summer-only grazing was the most appropriate management technique
to enhance Calluna re-establishment while also controlling Molinia regrowth.
79
AcknowledgementsThe authors would like to thank Gordon Common, Anna Gundrey, Saya Harvey,
David Henderson, Anne Moon and Jim Small for help collecting data, and Sarah
Gardner for comments on the manuscript. The work was carried out with funding
from the MAFF and SERAD.
80
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Grant, S.A., Torvell, L., Common, T.G., Sim, E.M. & Small, J.L. (1996) Controlled
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Grant, S.A., Torvell, L., Smith, H.K., Suckling, D.E., Forbes, T.D.A. & Hodgson, J.
(1987) Comparative studies of diet selection by sheep and cattle: blanket bog and heather
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Grime, J.P., Hodgson, J.G. & Hunt, R. (1988) Comparative Plant Ecology. A functional
approach to common British species. Unwin Hyman, London.
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McFerran, D.M., McAdam, J.H. & Montgomery, W.I. (1995) The impact of burning and
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Rodwell, J. 1991. National Vegetation Classification. Volume 2 Heaths and Moorlands.
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Shaw, S.C., Wheeler, B.D., Kirby, P., Philipson, P. & Edmunds, R. (1996) Literature
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82
Table 1 F-values and significance levels of the main effects of Calluna age, grazing level
and time and interactions on the post-burn regeneration of Calluna by seedling
germination and vegetative regrowth 1996-1998. Data square-root transformed for
analysis.
Factor d.f. Seedling counts Vegetative counts
Age (A) 1 0.0NS 9.5*
Grazing (G) 1 2.0NS 0.1 NS
Time (T) 6 6.2*** 6.8***
AxG 1 0.1 NS 1.2 NS
AxT 6 1.4 NS 8.8***
GxT 6 1.8 NS 0.4 NS
AxGxT 6 0.2 NS 0.9 NS
ANOVA * P < 0.05, ** P < 0.01, *** P < 0.001; NS = not significant
83
Table 2 F-values and significance levels of the main effects of Calluna age, grazing level
and time and interactions on the post-burn percent cover of Calluna and Molinia 1996-
1999, with 1995 pre-burn data included as a covariate.
Factor d.f.Calluna Molinia
Age (A) 1 11.5* 0.2 NS
Grazing (G) 1 6.2NS 0.1 NS
Time (T) 3 23.1*** 9.4***
AxG 1 3.9 NS 0.8 NS
AxT 3 1.4 NS 2.0 NS
GxT 3 9.1 ** 0.1 NS
AxGxT 3 1.3 NS 0.1 NS
ANOVA * P < 0.05, ** P < 0.01, *** P < 0.001; NS = not significant
84
Figure 1. Mean (+/-SEM) counts of post-burn Calluna seedling germination over time
for (a) 22 year old heather stand and (b) 8 year old heather stand under grazing levels as
follows; low (triangle), high (square) and an ungrazed control (circle).
Figure 2. Mean (+/-SEM) counts of post-burn Calluna vegetative regrowth over time for
(a) 22 year old heather stand and (b) 8 year old heather stand under grazing levels as
follows; low (triangle), high (square) and an ungrazed control (circle).
Figure 3. Change in mean (+/-SEM) Calluna percent cover over time for (a) 22 year old
heather stand and (b) 8 year old heather stand under grazing levels as follows; low
(triangle), high (square) and an ungrazed control (circle). Burning took place in spring
1996, post-burn recovery 1996-1999.
Figure 4. Change in mean (+/-SEM) Molinia percent cover over time for (a) 22 year old
heather stand and (b) 8 year old heather stand under grazing levels as follows; low
(triangle), high (square) and an ungrazed control (circle). Burning took place in spring
1996, post-burn recovery 1996-1999.
Figure 5. Change in the mean (+/-SEM) dominance of three main species (Calluna,
Molinia and Carex nigra) found within the canopy; (a), (c), (e) - 22 year old heather
stand and (b), (d) and (e) - 8 year old heather stand; each grazing level as follows; low
(triangle), high (square) and an ungrazed control (triangle). Burning took place in spring
1996, post-burn recovery 1996-1999.
Figure 6. Change in the mean (+/-SEM) dominance of litter and Polytrichum commune
found within the canopy; (a), (c) - 22 year old heather stand and (b), (d) - 8 year old
heather stand; each grazing level as follows; low (triangle), high (square) and an ungrazed
control (triangle). Burning took place in spring 1996, post-burn recovery 1996-1999.
85
Fig 1
86
Fig 2
87
Fig 3
88
89
Fig 4
90
Fig 5
91
92
Fig 6
93
94
FIELD TECHNIQUES FOR MONITORING GRAZING
IMPACTS ON HEATHER MOORLAND
Burke, M.J.W., Allen, D.S., Welch, D.* and Gardner, S.M.ADAS Consulting Ltd, Wergs Road, Wolverhampton, UK, WV6 8TQ.
* Centre for Ecology and Hydrology,
The effectiveness of thirteen techniques for monitoring change in vegetation condition and composition on upland heather moor is evaluated. The evaluation is based on standardised criteria assessing the type of information collected, the objectivity and reliability of the measurements, and the resource requirements for each technique. Two main approaches are currently used for monitoring change in moorland vegetation, namely, the assessment of grazing pressure or of botanical composition. Techniques measuring grazing pressure provide information on the impact of one of the drivers of change on upland moor but little information on the response of the vegetation. Techniques measuring botanical composition provide information on the vegetation response, but few clues to pinpoint the key drivers of change. Ideally, monitoring programmes should therefore aim to include both approaches. Techniques using a proportionate measure of grazed shoots and a frequency count of species presence within fixed sampling unit, were seen to provide the most objective and reliable measurements of vegetation change. These two criteria are of particular importance in large-scale and/or long-term monitoring programmes that employ several field staff and aim to assess the effectiveness of management and/or policy initiatives to conserve upland moor.Keywords: heather moorland, ecological monitoring, grazing pressure, environmentally sensitive areas
95
IntroductionUpland heather moorland is a UK habitat of international conservation importance (Thompson, et al.,
1995), which has declined in area and condition since 1945 (Bardgett et al., 1995; Tudor and Mackey,
1995). A primary reason for the decline has been overgrazing, particularly by sheep (Anderson & Yalden,
1981; Sydes and Miller, 1988; Thompson et al. 1995). Consequently, the habitat has become the subject
of a large number of conservation initiatives, both local and national. These include the implementation of
national agri-environment schemes, such as the Environmentally Sensitive Areas (ESAs) scheme, arising
from EC regulation 2078/92, which aim to promote environmentally sustainable farming practices.
In these schemes and other moorland restoration projects there is increasing pressure on policy makers,
land managers and conservation agencies to use survey and monitoring techniques to ensure correct
management plans are put in place, and to measure the extent to which conservation objectives are being
met. Here we use the precise definitions of Hellawell (1992) for survey and monitoring, in which the key
difference between the two is that monitoring is undertaken with the aim of determining the extent to
which a value/ range of values deviates from a predetermined norm. Survey simply involves the
measurement of values without any preconception of what the findings will be.
Monitoring and survey techniques for heather moorland have evolved quite rapidly since the 1960s when
productivity of vegetation was a major research topic as evidenced by the International Biological
Programme (IBP). Then studies concentrated on heather as a source of forage and measured standing crop
and the differences between grazed and ungrazed plots (Moss, 1969; Grant, 1971). Subsequent grazing
studies focused on individual heather shoots and the proportion of shoot biomass removed by grazing
(Grant et al., 1978; Grant et al., 1981). Succession from heather moorland to acid grassland was becoming
increasingly obvious in the 1970s and 80s (Bunce, 1989; Felton and Marsden, 1990), and hence there was
a need to determine the extent to which the condition of heather was declining and graminoids were
spreading. In response to this, since 1990, there has been a proliferation of techniques for monitoring
trends in heather condition and extent. These include methods for monitoring end of winter grazing
pressure and utilisation (e.g. Welch, 1984a; Henderson et al., 1996), methods based on heather form or
structure (e.g. Gill and Scott, 1995; Rae, 1995) and monitoring of heather cover and species composition
(e.g. Welch, 1984b; Welch and Scott, 1995; English Nature, 1996). In addition to this there has been
development of site ‘condition assessment’ or ‘indicator’ methodologies which aim to obtain an holistic
96
assessment of moorland condition from a single, relatively rapid survey (Macdonald et al., 1997; Jerram
and Drewitt, 1997).
The aim of this paper is to evaluate the range of techniques available to conservation practitioners, policy
makers and researchers in order to inform the development of monitoring activities within upland
restoration projects. Previously no overall review of upland monitoring techniques has been undertaken,
although there has been a comparison of methods for estimating levels of grazing on heather (Armstrong
and Macdonald, 1992). There have also been studies comparing different methods for estimating botanical
composition in other habitats (e.g. Friedel and Shaw, 1987; Everson and Clarke, 1987; Brakenhielm and
Quighong, 1995) but none have addressed heather moorland. This paper will address these issues by a)
ranking the wide range of techniques available against standardised evaluation criteria and b) testing the
sensitivity of methods for estimating botanical composition in a replicated using data collected from field
experiments. In doing so we hope to provide scientists and policy makers with the information they need
to identify the methods most appropriate for use in large-scale heather monitoring programmes or field
trials.
97
Methodology
The study was conducted in three stages. Firstly, information was collated on current field techniques and
the main aspects of their application summarised. The techniques were then evaluated using a ranking
procedure against a number of clearly defined criteria. Finally, a subset of methods for estimating species
composition were compared statistically with respect to their ability to detect change in species
abundance using data from existing field experiments.
Review of techniques
Information on field techniques was extracted from the scientific literature and relevant research and
monitoring project reports. The review was concerned with all field survey techniques that could
practicably be applied at the scale of the moorland management unit (area 30-300 ha). Hence detailed
physiological measurements and other more labour intensive methods were excluded, as were remote
sensing techniques. The key aspects to each methodology are summarised, including the variables
measured, typical size of area assessed, and the size of the individual sampling units used (e.g. quadrats,
plots, management unit etc.).
Evaluation of techniques
As several of the techniques measure different aspects of the moorland vegetation, there was no
experimental way of directly comparing the techniques. A ranking procedure was therefore devised to
enable the comparison of techniques. This procedure included a number of pre-defined and standardised
criteria to enable provision of a transparent and robust evaluation of the different techniques.
Each technique was ranked according to the pre-defined criteria (see Table 1). These included the plant
attributes measured, the precision (and hence to some degree, sensitivity) of the data collected, the
objectivity of the measurements, the amount of training/quality control required to ensure consistent and
accurate use by field staff, and the time required for a single measurement.
In any large-scale monitoring exercise the availability of resources is a key factor influencing the choice
of methods adopted. Therefore, in order for judgements to be made as to the cost-effectiveness of the
techniques, a standardised field ‘scenario’ was developed which allowed judgements to be made as to the
quantity of information collectable within a defined time period. The scenario used in this exercise was
based on a 500 ha management unit of 50:50 heather/grass covered moor, with moderate grazing pressure.
98
For each technique, a random sampling process was assumed and a maximum of two staff days (16 hours
survey time) was permitted.
The ranks for each criterion were not summed in order to identify a single technique as the best. Rather
the ranking system provided information against specific criteria so that once the most important criteria
for a particular study have been identified, it would be possible to select the appropriate technique.
Statistical comparison of techniques for measuring species composition
Existing data from a grazing experiment conducted in the English and Welsh Uplands (Rushton et al.,
1996; ADAS, 1998) were used to compare four different methods that measure change in botanical
composition. The methods were applied to permanent quadrats placed in three contrasting vegetation
types situated within different management units. Each quadrat was sampled in 1995 and again in 1997.
The vegetation types. sampled were Calluna-Molinia vegetation, similar to the M15 Scirpus cespitosus-
Erica tetralix wet heath of the National Vegetation Classification (NVC, Rodwell, 1992), at ADAS
Redesdale (30 quadrats) in the Northumberland National Park, Calluna-Nardus vegetation (NVC H12
Calluna-Vaccinium heath) at ADAS Pwllpeiran (30 quadrats) in the Cambrian Mountains ESA, and
Festuca-Nardus-Vaccinium vegetation, similar to NVC U4e - the Vaccinium-Deschampsia sub-
community of Festuca-Agrostis-Galium grassland and U5 - Nardus-Galium grassland, also at Pwllpeiran
(24 quadrats). Quadrats were 4m², each comprising four 1 m x1 m quadrants. Each quadrant was further
subdivided into a cellular grid of one hundred 100 cm² cells. Each sampling technique tested, utilised the
cellular grid in each quadrant and data were amalgamated to the 4 m² scale. All analyses were conducted
at this scale. The four sampling techniques tested are described below:
i) Frequency of key species: The presence (rooted or canopy) or absence of above-ground vegetation for
each of seven key species was recorded in each 100 cm² cell. For this study, the key species chosen were:
Calluna vulgaris, Empetrum nigrum, Erica tetralix, Vaccinium myrtillis, V. oxycoccus, Molinia caerulea
and Nardus stricta. Values for each 4 m² quadrat were calculated by summing the counts from each 1 m²
quadrant.
ii) Percentage cover using Domin scale: The percentage cover of each species within each 100 cm² cell
was estimated and aggregated to derive cover values for each species at the 1 m² scale. The cover values
for the four quadrants were averaged to derive the mean cover for each species at the scale of the 4 m 2
quadrat. This mean was then used to derive a Domin score for each quadrat (ADAS, 1999).
99
iii) First hit using cross-wires: A square sighter (10 cm x 10 cm) with a pair of wires, crossed at right
angles to each other and fixed to the base, was lowered vertically into the centre of each 100 cm² cell. The
first species to touch the intersection of the wires was recorded. The frequency of each species was
obtained by summing all touches (out of a maximum of 400) within the quadrat.
iv) Dominant species in a gridded quadrat: Within each 100 cm² cell, the species considered to be the
dominant occupant (based on an estimate of cover and biomass) was recorded. The overall estimate of
species dominance at the level of the 4 m2 quadrat, was derived from summing all the dominance records
for each species recorded within the quadrat.
Data Analysis
The analysis of the botanical data was done separately for each site/vegetation type and species
combination. The four different techniques were compared with respect to a) their ability to predict values
against a standard reference dataset and b) their ability to detect changes in species abundance, as this is a
key requirement of large-scale field monitoring programmes such as ESAs.
The key species technique was identified as the standard reference dataset for the first stage of the
analysis. It was based on a structured search for the seven species listed and derived a true frequency
count for each quadrat. Other methods were either more subjective (cover estimates and dominant
species) or were expected to be biased towards dominant species (first hit and dominant species). Linear
regression models were used to determine the extent to which the tested methods predicted the standard
reference data for each species in each vegetation type. Quadrats that did not contain a particular species
were excluded from the analysis, as they would have contributed to producing spuriously high correlation
coefficients.
In order to test the ability of the methods to detect changes in species abundance, data arising from each
technique were also analysed separately using repeated measures ANOVA (Analysis of Variance) and the
outputs of the analyses compared with respect to the frequency (across species and vegetation types) by
which change between years was detected, and to the consistency in direction of change detected by the
techniques.
100
RESULTS
REVIEW OF TECHNIQUES
Three broad groups of techniques were identified from the review. These were a) techniques that assess
utilisation or grazing pressure on Calluna vulgaris, b) those that assess growth form, age or structure of
Calluna vulgaris and c) techniques that assess cover or abundance of heather and other plant species
(species composition). Table 2 provides a full list of all the techniques included within the review with
references to original published sources. Codes have been used to rapidly identify the different techniques
and these are also given in Table 2.
Techniques for monitoring end of winter utilisation of heather (U1-U4)
These methods all aim to estimate the grazing pressure on heather during late winter/early spring
following the season when the grazing of heather by sheep is most intense, due to a lack of fresh grass
growth available. Methods U1 and U2 (Table 2) both concentrate on estimating the proportion of shoots
grazed, typically within 1 m² quadrats. Estimates are made by eye. U2 also combined this grazing index
with estimates of the proportion of the shoot length that had been lost. This was also estimated by eye.
This was combined with the proportion of shoots grazed to derive a condition index.
Technique U3 also estimates the proportion of grazed heather shoots, but based on a random sample of
heather stems collected in the field from 0.5m² quadrats. Samples are returned to the laboratory and stems
sorted into grazed and ungrazed categories. From this a proportion of grazed stems can be derived.
Method U4 measures the offtake of heather by herbivores more directly by establishing a series of caged
areas protected from grazing, and comparing these with similar grazed areas. Each spring and autumn all
species within grazed and ungrazed areas are harvested and the biomass production compared. This
provides a measure of offtake across all species.
TECHNIQUES BASED ON HEATHER FORM OR STRUCTURE (F1-3)
Technique F1 has been developed by Scottish Natural Heritage (SNH) as a method for assessing moorland
condition. It uses the fact that heather growth form can be strongly influenced by grazing pressure. Three
growth forms are commonly recognised as being indicative of high or moderate grazing. These are the
‘carpet’, ‘drumstick’ and ‘topiary’ forms. The prevalence of these growth forms is used to derive a
grazing assessment. Assessors walk 25 ha areas within management units and make estimates of the
abundance of each growth form. The method stipulates that the three forms should be widespread (>20%
101
cover) if the area is to be classed ‘heavily grazed’. The forms should be localised and infrequent (<20%
but >0%) if ‘moderately grazed’ and absent if ‘lightly grazed’.
Bardgett et al (1995) developed technique F2 which assessed heather cover (3 classes, <25%, 25-50%,
>50%), height categories (<15 cm, 15-30 cm, >30 cm) and a further assessment of ‘damaged’ heather
assessed from growth form in a similar approach to F1. These surveys were conducted within 100-ha
squares, in which the distribution and extent of the heather cover classes were mapped.
The Game Conservancy Trust have developed a survey technique (F3) which relates primarily to the
quality of the heather moorland habitat for grouse management. Transects are established within 100 ha
areas of moorland, along which one hundred 5 cm radius circles are sampled. The number of young (<1
year old) and old shoots (>1 year old) are counted within each circle. The total number of shoots and the
ratio of young to old shoots are used as indices of moorland condition.
TECHNIQUES INVOLVING SURVEY OF HEATHER AND OTHER SPECIES
A wide range of methods (GCH1) have been employed on moorlands to estimate botanical composition.
Permanent quadrats are often employed in order to minimise variation between years of survey. Within
such quadrats estimates of cover by eye are often employed, but subdivided quadrats and frequency of
presence within grid cells are used in some cases. Cover estimation by eye is subjective and subject to
observer error, although the use of cover categories such as the DOMIN scale can reduce this. More
objective cover estimation methods include the use of point quadrats or recording of species presence in
quadrats or circular units. The use of nested quadrats allows the measurement of species at a range of
spatial scales. This theoretically maximises the potential to detect change in abundance across a wider
range of species (Critchley and Poulton, 1998). Five different techniques, i) percentage cover classes
using Domin scale, ii) percentage cover, iii) first-hits within a gridded quadrat, iv) first touches for single
points distributed throughout a site and v) species presence/absence in nested quadrats (Critchley &
Poulton, 1998), were evaluated in this review and a more detailed statistical assessment of the
effectiveness of methods i) and iii) for estimating the botanical composition of moorland vegetation is
also provided in this paper.
English Nature (1996) have developed a scoring procedure (GF1) for moorland management units. This
‘Grazing Index’ assesses condition on a 13-point scale. Scores 0-2 are in good condition with light grazing
pressure, 3-8 intermediate condition and grazing pressure, 9-12 in poor condition with heavy grazing
102
pressure. Each score is derived from summing scores from three components. These are: 1) ratio of grass
to heather, 2) cover of heather within heather dominated areas and 3) cover of shaped heather (drumstick,
topiary and carpet growth forms). Assessments are made by walking 50 ha areas and making a single
estimate by eye for each of these components.
EVALUATION OF TECHNIQUES
Table 3 presents the results of the ranking of the monitoring techniques for objectivity, resource
requirements, precision, training and QA requirements. The interpretation of the ranking system is given
in Table 1. No single technique achieves maximum scores for all criteria, and comparing the techniques’
relative merits depends to a large degree on the importance that is assigned to particular criteria. The F3
technique gains the maximum score for objectivity and precision and for ease of training. Under the
standardised field scenario, it also allows for a relatively large sample size, maximising representativeness
and potentially statistical power. Similarly, the U4 utilisation technique scores well for the same criteria,
but the method is labour intensive and not amenable to large sample sizes. However, in both cases the
plant attributes measured are quite restricted and limited to only one species (Calluna vulgaris).
Techniques based on recording botanical composition (GCH1) scored relatively well across the majority
of criteria, but are labour intensive and require sound identification skills for the species of interest.. They
do however, have the advantage of providing information on a wide range of species within the
vegetation. Methods F1, F2 and GF1, based primarily on growth form, were subjective and imprecise but
were meant to be a site assessment for the whole management unit. Hence a trade-off exists between the
precision and objectivity of measurements collected and the area or number of samples that can be taken.
Although there were a few exceptions, training and quality assurance rankings were similar across all
techniques.
STATISTICAL COMPARISON OF TECHNIQUES FOR ESTIMATING BOTANICAL
COMPOSITION
Table 4 summarises the results of the statistical analysis to compare the effectiveness of three techniques
in assessing the variation in the abundance of seven moorland species when compared to a standard
reference dataset (frequency of key species). Overall the first hit method accounted for most variation
across the widest range of species and vegetation types, followed by the Domin method and the dominant
species method. Both Calluna vulgaris and Nardus stricta were accurately assessed by all techniques
103
tested, across all vegetation types in which they were found. However, this was not the case for the other
species. Molinia caerulea was poorly predicted by all techniques, but slightly better by the first hit
method. Species that were sub-dominant such as Empetrum nigrum were most consistently assessed by the
first hit and Domin methods, presumably because the dominant species method biased records against
rarer species.
Table 5 shows the results from the analysis of the ability of techniques to detect change in species
abundance. Change in species cover was detected most frequently by the first hit technique (64% of
occasions). The Domin technique detected change on 50% of occasions, and the dominant species
technique on 43% of occasions. Change was detected most frequently for the grass species Nardus stricta
and Molinia caerulea and less frequently for Calluna vulgaris and Vaccinium myrtillus. In general the p-
values for the first hit and Domin techniques were slightly lower than for the other techniques, suggesting
that they are more sensitive to the detection of change when subjected to statistical analysis. In the
Nardus-Vaccinium vegetation, there was broad consistency of results between the different techniques.
However, for the Calluna-Nardus vegetation some contradictory results were obtained (e.g. for Calluna
vulgaris table 5). In general there was good consistency of results between the first hit and dominant
species methods, but less agreement between first hit and Domin methods. This reflects the fact that the
first hit and dominant species methods both record species ‘dominance’, whilst the Domin method
provides a more balanced measure of composition.
DISCUSSION
The approach employed in any monitoring study should be driven by the objectives for that study
(Hellawell, 1992). The techniques presented here address different aspects of moorland condition and
hence are capable of addressing different objectives. These can be broadly categorised into techniques
that measure grazing pressure on heather moorland and techniques that assess botanical composition or
heather cover. The following discussion aims not to identify the ‘best’ technique for each, but simply aims
to draw out the main strengths and weaknesses of the key methods.
Grazing pressure
Using measures of grazing pressure on heather to evaluate moorland condition, (including those based on
growth form) makes the assumption that if grazing pressure can be maintained at a sustainable level then
104
moorland condition should be maintained or enhanced. In circumstances where grazing is known to be the
main factor influencing condition, this assumption is justified. Objectives for monitoring in these cases
relate to reducing grazing pressure only, rather than enhancing botanical composition per se, although this
of course may be a desirable consequence of such management. The two most objective and precise
methods for estimating grazing pressure were U3 and U4. U3 (ADAS, 1997a) is a relatively rapid method
that can estimate the proportion of grazed heather shoots from a randomly taken sample. Large samples
can be taken in the field and hence a good ‘representative’ sample obtained in relatively short time
periods. U4 (Moss, 1969) is a more time consuming technique, based on the use of caged and uncaged
areas to estimate heather offtake, although potential exists to estimate offtake across all species present.
Due to resource requirements, a much smaller sample is often only possible with this technique, and
hence representativeness is perhaps compromised. It is however, very well suited to small sites or
experimental studies where detailed, precise information is required, and large geographical coverage is
not a priority. The other techniques related to grazing are based on growth form. They are generally more
subjective and often less precise and hence may not be as sensitive to small changes in condition as the
more objective methods. With such techniques special care has to be given to training, as more judgement
is involved in taking measurements. However, these techniques are well suited to conducting site
surveys/assessments of large management units, particularly as a means of collecting information prior to
the development of management plans. They also allow the assessment of a large number of sites in a
relatively small time period, which may be amenable to national surveys of large numbers of sites for
example. This is the context in which English Nature (1996) and Bardgett et al. (1995) used their methods
(GF1 and F2 respectively).
Botanical composition
A contrasting approach is to base objectives on the botanical composition of sites, and this is the
characteristic of moorlands that ultimately defines their botanical conservation value. Generally, the
abundance of dwarf shrub species, particularly Calluna vulgaris defines the condition or quality of a site,
and hence increasing the cover of one or more of these species is often a primary conservation objective.
GF1 and F2 are two rapid methods that incorporate estimates of heather cover as well as other measures.
Both methods are able to conduct whole site surveys relatively quickly, but they do not utilise fixed
105
sampling units, and are based on more subjective estimates of cover. Hence they are not well suited to
monitoring studies in which changes in cover may be small. These techniques are best suited to the kind
of site assessment work mentioned above.
There are a number of methods available under the general technique GCH1 for estimating species
abundance. These have been reviewed in numerous textbooks e.g. Causton (1988) and Grieg-Smith
(1983). Estimates of species abundance may be based on frequency or absolute measures such as biomass,
cover, or area. The evaluation found these methods to be reasonably objective and precise, primarily
because they are based on the structured assessment of small fixed units rather than the subjective
assessment of large moorland areas. Estimates of cover by eye as opposed to use of point analysis or
frequency measures are known to be subject to observer error and may not be ideal for vegetation
monitoring where changes in cover over time may be small (Goldsmith, 1992). These and other methods
that involve species identification do demand a high degree of taxonomic expertise from the assessor,
hence training and QA requirements may be high. These methods can also be time consuming, although
their increased objectivity and precision can be offset against less intensive, but more subjective methods,
and their efficiency of use can be increased via stratified sampling. Two recorders can normally assess
approximately 50 quadrats in one day, depending on the exact method and the species richness of the site.
These methods also have the advantage that they measure changes in the status of the full range of
species, and not just Calluna vulgaris or other dwarf shrubs. This also allows sites to be classified with
respect to vegetation type or community type, which can provide useful contextual information with
which to evaluate vegetation change.
Of the techniques tested in the statistical evaluation, the first hit technique was found to be the most
accurate, with respect to the assessment of species abundance, compared with cover estimates and the
dominant species techniques. This is likely to be because it was the most objective method tested, and
hence was less susceptible to the introduction of variation attributable to observer effects. Perhaps
surprisingly, the first hit method also performed well in the assessment of sub-dominant species within
each of the vegetation types. One might expect this technique to be biased against rarer species or
structurally sub-dominant species, but evidence for this could not be found in this study. It is likely,
however, that the large number of points (400) recorded in each quadrat increased the probability of sub-
dominant species being recorded.
106
In the comparison of the ability of the three techniques to detect change in species abundance, the first hit
technique again performed the most consistently across vegetation types and species. This is also likely to
be explained by its robustness to observer error.
Conclusions
The purpose of this review has been to examine the strengths and weaknesses of a broad range of
techniques available for assessing the composition and condition of heather moorland. The selection of a
specific technique is ultimately determined by the objective of the study. Where rapid assessments of the
condition of a site are required, perhaps in order to prioritise sites for management and/or for further
study, then techniques such as the English Nature scoring assessment (GF1) are particularly appropriate.
This provides information on both grazing pressure and heather cover, which can assist decision-making
on the establishment of realistic conservation objectives and management practices.
For programmes requiring an evaluation of the effectiveness of management prescriptions in maintaining
and/or enhancing heather moorland, two approaches are possible. The first is to measure the size of the
direct impact of the agent of change. A principal agent for heather moor is grazing livestock and
measurements normally involve an assessment of grazing pressure and/or loss of plant biomass. Whilst
these provide information on one of the driving forces behind moorland vegetation change, they do not
provide information on the conservation value of the vegetation, or on how it changes in response to
grazing. A second approach is to measure the response of the vegetation to management by monitoring
either the botanical composition of the vegetation or the cover of heather within the moorland vegetation.
This information may indicate changes in the quality and extent of heather moorland, but the driving
forces behind that change will be poorly understood, and hence judgements about the effectiveness of
management cannot be made. Ideally therefore, monitoring should aim to monitor both driving forces and
their impacts to best understand the changes observed.
Some monitoring techniques identified in this review do combine these two approaches, for example GF1
in which heather cover and grazing pressure are both estimated. However, in this technique the
measurement of heather cover lacks sufficient precision and objectivity to enable reliable detection of
vegetation change. A more informed approach would be to use two methods in parallel and to set
frequency of resurvey appropriately to take into account expected rate of change in the parameters
measured. Changes in grazing pressure may occur from year to year and season to season, whilst plant
107
community impacts will take longer to emerge due to the slow growing nature of the component species.
Therefore grazing pressure should ideally be monitored frequently (perhaps yearly) to ensure management
and stocking levels are appropriate. Measurements of botanical composition need not be taken so
frequently, and depending on the nature of the restoration or re-creation practices employed, monitoring
could be restricted to a three or five yearly cycle. Strategic planning of monitoring programmes in terms
of monitoring techniques and frequency of resurvey in this manner provides the opportunity to maximise
the cost-effectiveness of monitoring activities which is an important consideration when resources for
long–term or large-scale monitoring programmes are limited.
A third consideration in the selection of techniques for use in large-scale monitoring programmes, is their
ease of use and robustness when used by different observers. For such programmes, the use of techniques
that include a high degree of objectivity is important for maximising consistency between observers.
Although no field trials were undertaken of the 13 techniques evaluated in this study, techniques U3, U4
and F3, which measure grazing pressure and technique GCH1 iii, iv and v, which measure botanical
composition were agreed as being the most objective.
There are many potential objectives in moorland monitoring. These may include one or more of the
following:
a) providing information which can inform the development of site objectives and management plans
b) assessing the effectiveness of policy, legislation or conservation initiatives at the local, regional or
national scale,
c) providing an early warning system on high quality sites able to detect incipient, detrimental change.
In choosing the most appropriate field technique for a survey or monitoring programme, scientists are
faced with decisions regarding resources available, the level of objectivity, precision and degree of
sensitivity required and the type of moorland attributes they are most interested in. This study has
evaluated thirteen of the most widely used techniques, using these criteria in order to enable such
decisions to be made in an informed manner.
Acknowledgements
The authors would like to thank colleagues within their respective organisations who have provided
information for this report. Particular thanks to colleagues at ADAS Redesdale and ADAS Pwllpeiran for
108
the collation of data used in the statistical analysis. This work was funded under MAFF contracts
BD0114, 0117 and 1217.
109
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Table 1 Criteria used for assessing heather monitoring techniques
Criterion Definition Ranking system
Plant attribute The feature of the plant that is measured e.g. height, cover, form, utilisation
N / A
Season Time of year when technique should be applied N / A
Area Area over which individual measurements are taken e.g. 1m2 quadrat, individual points / plants / shoots
N / A
Time Time needed to record one measurement 1
2
3
> 30 min
5-30 min
< 5 min
Precision of information
The type of information measured:
1 Nominal exclusive categories e.g. presence/absence, male/female
2 Ordinal - a rank order with unequal intervals between the scale e.g. DAFOR, DOMIN
3 Discontinuous interval e.g. % cover
4 Continuous interval e.g. height
1
2
3
4
categorical
poor sensitivity
quite sensitive
most sensitivea
Objectivity 1. An estimate requiring an observer judgement of quantity e.g. % cover
2. A quantitative measurement based on a standardised & universal scale e.g. individual counts, height in cm
1
2
less objective
more objective
Training requirement
Time taken to learn techniqueb 1
2
3
> 2 days
1-2 days
< 1 day
Quality Assurance
Time needed to verify consistency and standards of operation between observersb.
1
2
3
> 2 days
1-2 days
< 1 daya Sensitivity to quantitative change in abundance or extentb Rank estimates based on authors’ experience
114
Table 2 Techniques of assessing heather moorland condition and examples of use
Code Attribute Season Area Reference
Methods measuring grazing pressure/utilisation of heather
U1 % shoots grazed End of winter
1m² Welch and Scott (1995)*, Welch et al. (1996)*
U2 % shoots grazed + index of biomass lost
End of winter
1m² Henderson et al. (1996)*
U3 % shoots grazed + biomass utilisation
End of winter
0.5m² ADAS (1997a)*
U4 Annual off-take Summer & winter
0.25m² Moss (1969), Moss and Miller (1976)
Methods measuring heather growth form, age or structure
F1 Growth form in response to grazing
Typically Summer
25ha Gill and Scott (1995), Rae (1995)
F2 Cover and height index Typically Summer
100ha Bardgett et al. (1995)
F3 Old/young shoot counts Typically Summer
<0.1m² Smith et al. (1997)*
Monitoring botanical composition
GCH1 Cover, frequency or biomass-based estimates of botanical composition
Summer Typically 0.5-4m²
Welch and Scott (1995)*, Welch et al. (1996)*, ADAS (1997b), Critchley and Poulton (1998)
GF1 % cover and growth form
Summer 4m² English Nature (1996), FRCA (1997)
* authors also use other monitoring techniques
115
Table 3 Results of ranking assessment of heather monitoring techniques (see Table 1 for interpretation of the ranking system)
Method Time No. of measures
Precision Objectivity Training QA
U1 3 100 3 1 3 2
U2 2 60 3 1 3 2
U3 2 100 3 2 2 2
U4 1 40 4 2 3 2
F1 1 1 survey 2 1 2 2
F2 1 1 survey 2 1 2 1
F3 2 100 4 2 3 2
GCH1 (i)b 2 50 2 1 3 2
GCH1 (ii) 2 50 3 1 3 2
GCH1 (iii) 2 50 3 2 3 3
GCH1 (iv) 3 200 points 1 2 3 3
GCH1 (v) 1 1 3 2 3 3
GF1 1 1 survey 2 1 2 1a Area refers to the size of the sampling unit not the total area over which the technique may be appliedb GCH1 includes several options and each is evaluated in turn, i) percentage cover - Domin scale, ii) percentage cover, iii) frequency - first hits in a gridded quadrat, iv) frequency of point touches, v) presence or absence in nested quadrats or ADAS stand.
116
Table 4 Summary of the effectiveness of each assessment technique in predicting the variation in the standard datasets for each species. Variation accounted for is represented as *** >80%, **>70%, *>60%, ---- <50%
Species Vegetation type First Hit Domin Dominant species
Calluna vulgaris C. vulgaris-N. stricta
C. vulgaris-M. caerulea
***
**
***
***
***
***
Empetrum nigrum C. vulgaris-N. stricta
C. vulgaris-M. caerulea
**
***
----
**
----
*
Erica tetralix C. vulgaris-N. stricta
C. vulgaris-M. caerulea
-----
***
----
**
----
***
Molinia caerulea C. vulgaris-N. stricta
C. vulgaris-M. caerulea
N. stricta-V. myrtillis
*
*
---
----
*
----
----
----
----
Nardus stricta C. vulgaris-N. stricta
C. vulgaris-M. caerulea
N. stricta-V. myrtillis
***
***
**
*
**
***
**
**
**
Vaccinium myrtillis C. vulgaris-N. stricta
C. vulgaris-M. caerulea
N. stricta-V. myrtillis
----
*
*
*
***
***
----
----
----
V. oxycoocus C. vulgaris-M. caerulea *** ---- ***
No. of species with > 60% variation accounted for in each vegetation type
C. vulgaris-N. stricta
C. vulgaris-M. caerulea
N. stricta-V. myrtillis
4/6
7/7
2/3
3/6
6/7
2/3
2/6
5/7
1/3
Overall 13/16 11/16 8/16
117
Table 5. Summary of results of repeated measures ANOVA for different species within contrasting vegetation types using three different field methods. CM – Calluna-Molinia vegetation, CN – Calluna-Nardus vegetation, NV-Nardus-Vaccinium vegetation. Figures are P values. ns-non-significant at P<0.05. n/a – data not available. ‘Sensitivity’ relates to proportion of significant results from all tests for each method. Vegetation Type
Species Plot First Hit Domin Dominant Species
CM Calluna vulgaris 1 0.012 n/a ns2 ns n/a 0.015
Molinia caerulea 1 ns n/a ns2 0.005 n/a 0.010
CN Calluna vulgaris 1 ns 0.001 0.0032 0.001 ns ns
Nardus stricta 1 0.014 ns 0.0082 0.001 ns ns
Vaccinium myrtillus 1 ns 0.001 ns2 0.002 ns ns
NV Nardus stricta 1 0.006 0.003 0.0072 0.001 0.001 0.001
Vaccinium myrtillus 1 0.042 0.001 ns2 ns ns ns
Sensitivity 64% 50% 43%
118
