Application of multivariate methods to a study of community composition and structure in an escarpment woodland in northeast Ireland*

Alan Cooper** School of Environmental Sciences, Ulster Polytechnic, Jordanstown, Northern Ireland

Keywords: Basalt, Deciduous woodland, Diversity, Reciprocal averaging, Size structure, Stability

Abstract

Multivariate models based independently on the floristic and environmental attributes of sample quadrats f rom an escarpment woodland, were constructed. They were used to interpret the ecology of plant species distribution and to investigate the size structure of tree populations. Site instability, soil water status, and past management and grazing pressure were found to exert major influences on the composit ion and structure of the woodland.

It is suggested that early mortality associated with site instability, plays an important role in maintaining the floristic diversity of vascular plants in the woodland ground flora, by increasing the frequency of tree replacement. This may have the effect of preventing canopy closure and any associated, detrimental effects of low light intensities. The relevance of the work of the conservation management of escarpment woods in Northern Ireland is discussed.

Introduct ion

Throughout northeast Ireland, deciduous wood- land, thicket or scrub, often dominated by Corylus avellana, is a common feature of the rock outcrops, scree and stony drift of basaltic escarpments. Al- though historical studies are difficult to carry out in Ireland, where archives are generally poor, most escarpment woods are probably ancient relicts of native woodland. They almost certainly owe their survival to the habitats which they occupy, being either too steep, rocky, wet or inaccessible for any other land use (Kelly, 1975). The ancient nature of

* Nomenclature follows Webb (1977) for vascular plants and Watson (1968) for bryophytes. ** The work was supported by grant-aid from the British Eco- logical Society and the Ulster Polytechnic, and field work was carried out by Stephen D. H. Williamson. The author thanks Jennifer Larkin, Eithne Moran, lan Alexander and Ian Moran for help with preparation of the manuscript and Mrs E. McCrum for the development of computer programs.

Vegetatio 55, 93-104 (1984). © Dr W. Junk Publishers, The Hague. Printed in the Netherlands.

the woods and their content of regionally and na- tionally rare species (Praeger, 1974), enhances their conservation value (Rackham, 1980).

Few published ecological studies on Irish woods exist. In particular, site description has been grossly neglected. Kelly (1975) classified woodland on free- ly draining, base-rich soils in western Ireland, as a Corylo-Fraxinetum association. Woodland and thicket dominated by Corylus avellana on basaltic escarpments in northeast Ireland seem to corres- pond to this association. Related communities exist in western Scotland. Details of their ecology are given by Jermy & Crabbe (1978), Birks (1973), McVean (1964) and McVean & Ratcliffe (1962). A brief, preliminary description of the floristic com- position of similar communities in northern Ireland is contained in a report by Steele et aL (1968). M ore detailed species lists and habitat descriptions of two of these sites are available in reports by Harron (1977) and the Depar tment of the Environment for northern Ireland (1978).

94

It is apparent from these studies that the floristic diversity of escarpment woods is high, despite their usually small area. Little is known, however, of the factors determining the distribution of herbaceous species, the relationship between canopy structure and ground flora composition, or the dynamics of tree replacement. Since the woods are generally small (Graham, 1981) and in view of recent changes in traditional land use, involving the cessation of traditional management and the virtual elimination of populations of the rabbit, Lepus cuniculus in northeast Ireland since about 1954, an understand- ing of their ecology is essential for any conservation management strategy.

The results of a comparative study of some Cotswold beechwoods using multiple-discriminant analysis, showed that ground flora variation was primarily within-wood rather than between-wood (Norris & Barkham, 1970). An intensive, as op- posed to an extensive approach was, therefore,

adopted in the present investigation. The objectives were to describe the variation in species composi- tion, of the woodland ground flora at a single es- carpment site, determine the distribution and size structure of the tree species present and to investi- gate species distribution in relation to physiogra- phic factors and soil physicochemical characteris- tics. A site at Knockagh was chosen for study (O.S. grid ref. J 372875) because it contains most of the geomorphological elements characteristic of basal- tic escarpments (Wilson & Manning, 1978) and shows a considerable diversity of ground flora as- pect, slope, stability, elevation and soils.

The study area

The total area of woodland investigated at Knockagh is about 11.5 ha (Fig. 1). Above the wood, there are two main sections of cliff, bisected

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O 5 0 1 0 0 M e t r e s . . . . . . " ' - ' - ;" " [ I

~ Ungrazed Woodland - - Road ~ Cliff

Grazed Woodland ,.2OZ. Footpath ~ ~ Steep slope

' E~J Clearing . . . . Contours (m) ' Field Boundary

Fig. 1. M a p of the s t udy area. N o r t h e r n I re land is s h o w n as an inset.

by a gully, filled with glacial drift and rocks. Wood- land extends to about 250 m up this feature, but is mainly confined to partially stabilised, angular, ta- lus scree slopes and to the soils at lower elevations, in which boulders predominate. Here, altitude var- ies between 150 m and 210 m. The wood receives debris and rocks f rom the cliffs above, when mate- rial is loosened by weathering processes. Most trees have curved stems indicating talus and soil creep, on slopes which may be up to 54 ° .

With the exception of 3.3 ha, the woodland sampled, has been fenced f rom farm animals for at least the past 50 yr. Before the first outbreak of myxomatosis , the local population of the rabbit, Lepus cuniculus was extremely abundant, but these animals are rarely seen at the present time. There is no history of regular felling, but traditionally, small amounts of t imber have been cut for firewood, on an occasional basis, by local farmers. No wide- spread fires have been reported.

The earliest record of woodland at Knockagh ('hill of white thorn') is given by McSkimin (1909), as 1728, with the description, 'a considerable quan- tity of that plant (white thorn) growing beneath its (Knockagh) brow'. It is likely, therefore, that there has been a continuous woodland or scrub cover at Knockagh and that the steepness and inaccessibility of the site has contributed to its survival.

Methods

The method used to sample and describe the vegetation and habitat characteristics is given by Bunce & Shaw (1973) but with the following modi- fications. The number of samples (45) was chosen as the number of points of intersection of a square 50 m grid system, positioned onto a 1:10 000 Ord- nance Survey map of the area. A square, 25 m 2 quadrat was selected after trials to determine an estimate of community minimal area in a range of communities, representing the variation encoun- tered during preliminary field survey (Mueller- Dombois, 1974).

All species of vascular plant were recorded, but Circaea intermedia was not distinguished from Cir- caea lutetiana. Elements of the vernal flora, namely Orchis mascula and Ranunculus ficaria, which could have been missed, were not included in the analysis of the data. Since small trees (<5 m) domi-

95

nated the study area and since their density was normally high, the same square, 25 m 2 quadrat, as was used for the herbaceous ground fo ra , was chosen to estimate the tree species composit ion of the wood. The diameter of all stems taller than 1.3 m was recorded at this height. The density of all individuals not reaching 1.3 m was recorded separ- ately, along with the total number of seedlings per plot, up to a maximum count of 25. The number of dead stems per coppiced stool was also recorded.

Stems of Corylus avellana were aged by counting the annual rings of 32 felled, random samples, throughout the stands examined. Allowing six years for a stem to reach a height of 1.3 m (Merton, 1970), there was a highly significant correlation (0.84, p =0.001), between stem diameter at 1.3 m and age.

The following environmental data were recorded: slope, aspect, presence of rock outcrops (<5 m), subjective assessment of the percentage cover of stones (<5 cm), rocks (5-50cm) and boulders (>50 cm) on the soil surface, the percentage cover of bare mineral soil and the depth of accumulation of rocky debris behind tree bases.

Within a one-hour period on either side of mid- day, under clear skies, the intensity of photosyn- thetically active radiation (PAR) at the centre and corners of each quadrat, was recorded with a Lambda LI-185 photometer. Also at these points, the depth of mineral soil was measured and soil samples f rom 0-5 cm of the profile were taken dur- ing a two-day period, three days after heavy rain- fall. Soil water content was measured gravimetri- cally (Allen, 1974). The stone fraction of the soil was recorded as that fraction of the dry weight of the sample which did not pass through a 2 mm sieve. Soil pH was determined using a 5:1, water: soil suspension (Allen, 1974).

A reciprocal averaging ordination (Hill, 1973) was used to summarise species autecological rela- tionships and stand similarity, using the O R D I - FLEX program of Gauch (1977). Tree species were not included in the floristic ordination. Preliminary ordinations based on the quantitative forist ic composit ion of stands, indicated an outlier which was subsequently removed from the analysis. Spe- cies with only one occurrence in the sample stands were not included in any ordination. Interpretation of the floristic ordinations was based on a method described by Persson (1981) but using the known

96

autecology of plants in Ireland. Ordinations were also carried out using stand environmental attrib- utes.

Standardisation of the data did not prove helpful to interpretation, except with the environmental ordinations, when each variable was expressed as a percentage of its maximum recorded value. A sub- jective floristic classification into stand groups based on the partition of ordination space (Gauch & Whittaker, 1981) was carried out after the failure of single linkage (Davies, 1971) and minimum group variance (Ward, 1963) cluster analysis, to select groups compatible with stand distribution on either the floristic or environmental ordinations. The subjectively defined groups were used simply as an aid to describing the ordination diagrams. Correlation analysis was carried out using the com- puter program SPSS described by Nie et al. (1970).

Results

Floristic ordinations

Species with a low loading on the first axis of the floristic ordination (Fig. 2) include the grasses Bra- chypodium sylvaticum, Deschampsia caespitosa and Agrostis capillaris, and the ruderals, Cirsium arvense and Seneciojacobaea. The majority of the remaining species, in particular, Oxalis acetosella, Geranium robertianum and Hyacinthoides non- scriptus, have a higher loading. They are typically woodland plants and show little ability to tolerate disturbance. The first axis may, therefore, represent a gradient of site perturbation.

Species with a high loading on the second axis, for example, Heracleum sphondylium, Filipendula ulmaria and Deschampsia caespitosa can be found in wet meadows as well as damp woods. The species with a low loading on the second axis, such as Polypodium vulgare, Lonicera periclymenum and Umbilicus rupestris, usually grow in rocky habitats and may be more tolerant of soil moisture stress. The third axis separated shade-tolerant species, in- cluding Circaea sp., Anemone nemorosa, Athy- rium filix-femina and Polystichum setiferum from plants of more open habitats.

Since species and stand ordinations are per- formed simultaneously in reciprocal averaging and are functions of one another, the variation along

100-

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VS

DC Cf Ra

S~ Ud

Pa Gu

Ca Rr Am

Ht FrN SI Fv Ta RI Dd Ga Ac AS SePv pp

TS Vr Em Hh H~ Hr L0 AI Vc Df

LCPS Pse PC Ha Ao C An Rau

LDe Sh Pov Oa

Gf

Ut

50 100 Axis 1

Fig. 2. Projection of the first two axes of a reciprocal averaging species ordination: Agrostis capillaris ( Ac); A rrhenatherum ela- tius (Ae); Athyrium filix-femina (Af); Arum maculatum (Am); Anemone nemorosa (An); Anthoxanthum odoratum (Ao); An- gelica sylvestris (As); Brachypodium sylvaticum (Bs); Circaea spp. (C); Cirsium arvense (Ca); Cardamine flexuosa (CO; De- schampsia caespitosa (Dc); Dryopteris dilatata (Dd); Dryopteris filix-mas (DO; Dactylis glomerata (Dg); Epilobium montanum (Em); Filipendula ulmaria (Fu); Fragaria vesca (Fv); Galium aparine (Ga); Geranium robertianum (Gr); Geum urbanum (Gu); Hypericum androsaemum (Ha); Hedera helix (Hh); Hol- cus lanatus (HI); Holcus mollis (Hm); Hyacinthoides non-scrip- tus (Hn); Hypericum pulchrum (Hp); Hypochoeris radicata (Hr); Heracleum sphondylium (Hs); Lapsana communis (Lc); Lamium purpureum (Lp); Lonicera periclymenum (Lpe); Oxalis acetosella (Oa); Pteridium aquilinum (Pa); Poa pratensis (Pp); Polypodium vulgare (Pov); Potentilla sterilis (Ps); Polystichum setiferum (Pse); Poa trivialis (Pt); Prunella vulgaris (Pnv); Primula vulgaris (Pv); Ranunculus acris (Ra); Ranunculus auri- comis (Rau); Rosa canina (Rc); Rubus fruticosus (RI); Ranun- culus repens (Rr); Rumex sanguineus (Rs); Sanicula europaea (Se); Stellaria holostea (Sh); Seneciojacobaea (S j); Succisa pra- tensis (Sp); Taraxacum sp. (Ta); Teucrium scorodonia (Ts); Urtica dioica (Ud); Umbilicus rupestris (Ur); Veronica cha- maedrys (Vc); Viola riviniana (Vr); Vicia sepium (Vs).

any two corresponding axes can be interpreted in the same way. Disturbance, soil water status and light intensity may, therefore, be considered the three main environmental factors controlling the species composition of the five groups of stands delimited in Figure 3. The lack of spatial separation of stands from grazed and ungrazed parts of the study area indicate their floristic similarity. The composition of the sample quadrats is shown in Table I.

Interpretation of the stand ordination is streng-

100-

I¢1 '~ 50. ,< 0 0

• °

- - _ O

%o 0 0

O 0

D

D

~b Axis 1

. !

100

Fig. 3. Projection of the first two axes of a reciprocal averaging stand ordination based on floristic attributes. Stands have been partitioned into groups 1 (u), 2 (Q), 3 (O), 4 (o) and 5 (z~). Underlined stands are grazed by farm stock.

97

thened by a positive correlation (0.52) between stand loading on the first axis and the abundance of large boulders present in each quadrat, and a nega- tive correlation with slope (-0.47). There was a positive correlation (0.54) of stand loading on axis one, with soil water content. The negative correla- tion (-0.35) with soil pH provides no evidence for enrichment with nutrient bases at these sites, at least in the surface horizons.

There was a positive correlation (0.30) between stand loading on the second axis and soil water content, and a negative correlation (-0.27) between stand loading on the third axis and the intensity of PAR. A further positive correlation (0.38) with the amount of bare soil, also along this axis further indicates a poor ground flora, perhaps related to low light intensity.

Vascular plant species diversity in the samples was greatest on the steeply sloping, higher elevation soils, as shown by a positive correlation of species number with slope (0.33) and negative correlations with the abundance of boulders (-0.43) and soil moisture content (-0.33). Diversity was particularly

T a b l e 1. Mean percentage cover of herbaceous plants in the five communities delimited on a reciprocal averaging ordination of sample quadrats (Fig. 3). Species present in less than five quadrats have been omitted. Matrix values are deciles (+ and 1-9 with - for absence).

Species Community number Species Community number 1 2 3 4 5 1 2 3 4 5

R u m e x s a n q u i n e u s + + + - P o l y s t i c h u m s e t i f e r u m + + + + +

H y a c i n t h o i d e s n o n - s c r i p t u s 2 - + + - A r u m m a c u l a t u m + + + + +

G a l i u m apar ine + + + + - P r i m u l a vu lgar is + + + + +

R o s a c a n i n a + + + + - R a n u n c u l u s a u r i c o m i s + + +

P o a p r a t e n s i s + + + - S a n i c u l a e u r o p a e a + + + + +

Vicia s e p i u m + + - + - S e n e c i o j a c o b a e a + + + + +

F i l i p e n d u l a u l m a r i a + + + - + V e r o n i c a c h a m a e d r y s + + + + +

Oxa l i s ace to se l l a 2 + + + + Viola r iv in iana + + + + +

Urt ica d io ica + + + + - A g r o s t i s c a n i n a + + + + +

A r r h e n a t h e r u m ela t ius + + + + - B r a c h y p o d i u m s y l v a t i c u m 2 + + + 1

C a r d a m i n e f l e x u o s a + + + - D a c t y l i s g l o m e r a t a + + + + +

Ci rcaea l u t e t i a n a + + + 1 + H o l c u s l a n a t u s + + + + +

E p i l o b i u m m o n t a n u m + + + + + A n g e l i c a sy lves t r i s + + -

Fragar ia ve sca + + + + + P o t e n t i l l a sterilis + + + +

G e r a n i u m r o b e r t i a n u m 4 1 + + + R a n u n c u l u s r e p e n s + + -

G e u m u r b a n u m + + + + + T a r a x a c u m of f ic inal i s - + + + +

H e r a c l e u m s p h o n d y l i u m + 9 1 + + P o l y p o d i u m vu lgare - + + +

H e d e r a he l i x 2 3 5 1 1 T e u c r i u m s c o r o d o n i a + - + +

L o n i c e r a p e r i c l y m e n u m 4 + 1 + 1 A n t h o x a n t h u m o d o r a t u m + + +

R u b u s f r u t i c o s u s + + 1 I + A n e m o n e n e m o r o s a + - - + +

A t h y r i u m f i l i x - f e m i n a + + + + + H y p e r i c u m p u l c h r u m - - + +

D r y o p t e r i s d i la ta ta + + + + + A g r o s t i s capil laris - - - + +

D r y o p t e r i s f i l i x - m a s + + + + + H y p o c h a e r i s rad ica ta - - + -

98

low at the wetter, low elevation soils and on wooded rock piles.

Environmental ordinations

The first axis of the ordination of measured en- vironmental attributes shows that those attributes which summarise habitat instability, have a high axis loading (Fig. 4). The low loading of the abun- dances of boulders and rocks on the first axis is linked with a low loading of stand distance from the cliff base. This confirms the field observation that steeper soils of scree slopes at higher elevations and relatively stable rock piles, provide the two main habitat types within the wood. Soil water content is separated f rom stand slope and the intensity of PAR on the second axis. The third axis indicates an association of bare soil with the shallower, less stable scree slopes.

These factors also provide the most useful sum- mary of sample variability (Fig. 5). Conclusions drawn from the second and third axes however, must be tempered with the observation that these two axis account for only 15% and 9% of sample variation respectively, as compared with 42% for the first axis.

100,

N

• ~ 50

Mo D~

$1 As

Ra

pH Ss

Sd

6 5o lOO Ax is 1

Fig. 4. Projection of the first two axes of a reciprocal averaging ordination of stand environmental attributes. Aspect (As); bare soil (Bs); distance from cliff base (Di); photosynthetically active radiation (Pa); boulders, >50 cm (Bo); rocks, 5 50 cm (Rc); stones, <5 cm on the soil surface (St); rock accumulat ion behind Corylus avellana stools (Ra); soil depth (Sd); slope (SI); soil water content (Mo); soil pH (pH); stones in soil (Ss).

100

~ 5O

0

O

O u o_ •

[ ] _O - - 0

O O • ~O •

o © © ~ • 0 0

0

0 •0 • ~

5'0 160 Ax is 1

Fig. 5. Projection of the first two axes of a reciprocal averaging stand ordination based on environmental attributes. Stand groups 1 (u), 2 (O), 3 (O), 4 (e) and 5 (A), which were delimited according to their species composition after an ordination based on their floristic attributes, have been plotted. Underlined stands are grazed by farm stock.

The five main groups of stands delimited on the floristic ordination, have been plotted on the stand environmental ordination (Fig. 5), This allows summary of the environmental characteristics of the groups and illustrates the similarities between the floristic and environmental ordinations. The stands in group -1 occupy well drained, wooded rock piles at lower elevations. There is mineral soil between the boulders and the stands are relatively stable, with shallow, rather moist, organic soils of low pH, occupying slight to moderate slopes. The second group of stands are found on the wettest sites, again at low elevations, with a slight to mod- erate slope. The soils are deeper and the stands are not dominated by boulders and large rocks. They are less acid and more stoney than the previous group, but are still strongly organic. The ground flora is well shaded.

Stands of group -3 occupy moderate to steeply sloping sites of higher elevation. They have well- drained, rocky and stoney soils and an abundance of large boulders delimits them from group -4. Whilst the soils are shallow, their percentage cover of woodland plants is high. The essentially unstable nature of the soil, however, is highlighted by the deep accumulations of rocky debris behind trees.

The fourth group of stands is found on the most

steeply sloping, higher elevation sites whose lower boulder content is probably due to gravitational loss. The soils are deep but mobile and have a higher pH and angular stone fraction. They have a relatively open, but continuous canopy and are dry. The high loading of the stands in group -5 shows that they occupy higher elevation, but less steeply sloping sites.

The distribution of stands from ungrazed habi- tats on the environmental ordination, indicates that they are directly comparable to the main body of samples from the ungrazed woodland. The high loading of the grazed stands on the first axis, how- ever, shows that they possess a limited set of geo- morphological and physicochemical characterist- ics. They are steeper and have deeper, more mobile soils than samples found at lower elevations in the ungrazed woodland. This accounts for the absence of stands in the grazed section of the wood, which have a floristic composition allied to group -4.

Tree size structure and composition

Stools of Corylus avellana form an almost con- tinuous cover except on the wetter, low elevation soils (Fig. 6a). There is, however, a large variation in stool density which can be linked to differences in ground flora composition and stand environmental conditions. The density of stools is greater in the relatively unstable, steeply sloping stands of group -3 and group -4 on the floristic ordination (Fig. 3). Negative correlations between the density of Cory- lus avellana and maximum stem and stool diame- ters in each stand, and a strong correlation between maximum stem and stool diameter (Table 2), indi- cate a predominance of young individuals where density is highest. The correlation matrix also shows that the largest maximum stem diameters are associated with the highest frequencies of boulders in the lower elevation stands.

Steeply sloping stands have a greater proportion of stems in the lower age classes, when compared with the less steeply sloping stands (Fig. 7). Figure 8 shows a greater number of dead stems in the 3-4 cm diameter class (median age of 17.8 yr) on the steeply sloping stands. Regeneration occurs sparingly, mainly on the partially stabilised scree soils of group -3 and group -4 (Fig. 3).

Corylus avellana is not found on the wetter min- eral soils, usually dominated by Prunus spinosa.

99

100"

"~ 50" • 8

1®~ ,o

2

2

26 7 r®@

6 1 4 ®

O, 6 o 5'0 16o

Axis 1

100.~

b)

0 0

0 00~ 0 • , • ",~• (9

. ...

®"

® ~60 ~b

Axis 1

Fig. 6a + b. The density of a) Corylus avellana and b) Fraxinus excelsior, in sample quadrats distributed along the first two axes of a reciprocal averaging ordination of the ground flora. Sam- ples containing individuals with a maximum stem diameter, at 1.3 m, of less than 5 cm, have been circled. Solid circles indicate absence of a species from a quadrat.

This is reflected in negative correlations between their population parameters (Table 2). The few stands at higher elevation, containing Prunus spi- nosa, are gully sites with a high proport ion of stony drift. On the less steeply sloping sites, a greater number of individuals of Prunus spinosa were in the larger size classes of the frequency distribution of Figure 9.

100

Table2. Matrix of correlation coefficients between environmental factors and tree population parameters. Only coefficients which were significant at the 5% probability level are included.

r),lus avellana

r of Corylus avellana

r of Fraxinus excelsior

Stem diameter of Fraxinus excelsior

nsity of Crataegus monogyna

0.77 nmodiyameter of Crataegus

nsity of Prunus spinosa

Stem diameter of

- I I . . . . I NNPrunusspin°sa

Fraxinus excelsior is sparingly distributed in the sample stands (Fig. 6b), and there is no relationship between the measured population parameters and any of the environmental factors, except for a cor- relation between the intensity of PAR and density (Table 2). The size structure of the population, however, does show extremely poor seedling regen- eration on steep slopes (Fig. 9).

Crataegus monogyna is confined, as scattered individuals, to the steeper, higher elevation mineral soils (Fig. 9). There is a correlation between its

density and stand slope, and negative correlations between both density, and stem diameter and boulder frequency (Table 2). Active seedling regen- eration is not taking place. The distribution of Cra- taegus monogyna contrasts with that of Sambucus nigra, which is confined, on the ungrazed sites, to the less steeply sloping soils. Its distribution paral- lels that of Prunus spinosa. Acer pseudoplatanus also occurred in the sample quadrats, but in such small numbers that few conclusions could be drawn about its distribution or reproductive dynamics.

(a) 40-

30-

_>

=_ 20-

"5

E Z 10.

11 - 2 0

12~ 3 131 141 151 1 - - 0 - 4 0 - 5 0 - _ O > 6 1

Age- class (yr)

(b) 3o-

-> 20-

t~ 10" E Z

11 i 21 -20 - 30

I

'3, '.1 '5,6o' - 4 0 - 5 0 >61

A g e - c l a s s ( y r )

Fig. 7. The age classes of the largest diameter stem taller than 1.3 m, present in each stool of Corylus avellana occurring on (a) steeply sloping (>300 ) and (b) less steeply sloping (<30 ° ) sample stands.

Discussion

Reciprocal averaging proved to be an gffective technique for simplifying the field data to a form which conveyed the essential attributes of the woodland vegetation. The similarity of the floristic and environmental ordinations, indicates the over- riding influence of topographic and geomorpholog- ical characteristics on the woodland ground flora is through their influence mainly on habitat stability and soil water status.

Physicochemical factors also influence the dis- tribution and age structure of Corylus avellana,

101

(a)

o~ m

-g_

¢-

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E Z

15-

10-

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] I

I i i 5 7 9 11 - 6 - 8 -10 -12

Dead stem diameter-class (cm)

(b)

x2 "o .=_.

..Q E Z

1 0 -

5-

0 0 - 2

Dead

U I i i i

3 5 - 4 - 6

7 9 11 - 8 -10 -12

stem diameter-class (cm)

Fig. 8. The diameter size classes of dead stems of Corylus avella- ha, taller than 1.3 m, present in (a) steeply sloping (>30 ° ) and (b) less steeply sloping (<30 ° ) sample stands.

Prunus spinosa, Fraxinus excelsior a n d Crataegus monogyna. The predominance of young stems and small stools in dense stands of Corylus avellana (Table 2) and the correlation between max imum stem and stool diameter may be due to either the successional invasion of individuals, or an earlier stem or stool mortality in some habitats. Three lines of evidence make the hypothesis, that scree and soil creep, together with rock falls, play a major part in preventing the establishment of a closed canopy of mature individuals, most likely. Firstly, density is greatest on the steeply sloping, less stable soils. Secondly, the largest diameter, older stems and stools are more frequent on the lower elevation sites. Thirdly, there is a greater stem mortality in the smaller stem size classes, on the steeper stands.

Site instability is also associated with the most diverse ground flora communities, at least in terms

102

Prunus spinosa (a)

20 -

(n (D

"5

d z

10-

[ ' ! i ! ! 1 I

0 2 4 6 8 10 -1 - 3 - 5 - 7 - 9 - I ! >11

Stem d iame te r s i z e - c l a s s ( cm)

(b)

(/1 (D

"5

z

20-

10-

0

F-t ]k

/

0 1 2 ~ 4 v 6 ; 8 i10 i - ! - 3 - 5 - 7 - 9 -11 >11

S tem d iamete r s i z e - c lass ( cm)

(a) Fraxinus excelsior

20-

to

I0- 0

d z

I ' - - " 1 ][ I l I I 0 5 9 13 17 21 >25 - 4 - 8 -12 -16 -20 -24

S tem d iameter s i z e - c lass ( c m )

(b) 20-

cn ,D

10- O

Z

o L I - - ' 3

0 v 5 I 9 v13 f 1 7 v 21 | 4 725 -4 -8 -12 -16 -20 -24

S tem d iameter s i z e - c l a s s ( c m )

Crataegus monogyna (a) (b)

Z v ~ Z v v i !

2 5 7 9 11 13 715 5 ! 7 ~ 9 ~ 11 •13 r ~15 - 4 - G -8 -10 -12 -14 - 4 - 6 - 8 -10 -12 -14

c , . .m d iameter s i z e - c l a s s ( c m ) S tem d iamete r s i z e - c l a s s ( c m )

Fig. 9. The stem diameter size classes of Prunus spinosa, Fraxinus excelsior and Crataegus monogyna, taller than 1.3 m, present in (a) steeply sloping (>30 °) and (b) less steeply sloping (<30 ° ) sample stands.

of vascular plants. The more stable woodland sites at higher elevations have an impoverished ground flora, with a larger proportion of grassland and heath plants. Stability at lower elevations, associat- ed with less steeply sloping rock piles, or wetter sites, also gives rise to a less diverse, but different ground flora to that of the stable sites at higher elevations. Scrub dominated by Corylus avellana might, therefore, in the less steeply sloping, stable sites, be considered to be a naturally occurring vegetation with an intrinsically high species diversi- ty.

It is often difficult to distinguish the effects of past biotic disturbance, from those of the physico- chemical environment. For example, it is impossi- ble to say, to what extent a more intensive herbivore activity in the past, might account for the more grassland-like nature of the woodland ground flora at higher elevations. Similarly, the dominance of Prunus spinosa at some low elevation sites may be due to its tolerance of wetter soils or to an interac- tion between site accessibility and timber removal by man. The species is known to be an early colon- iser of disturbed sites (Webb, 1977).

Presuming a more intensive grazing pressure and management in the wood in the past, it is likely that the rock piles acted as a refuge for the more typical- ly woodland species. This might account for the abundance of Geranium robertianum, Oxalis ace- tosella and Hyacinthoides non-scriptus in these habitats. Alternatively, their distribution may simply reflect habitat tolerances and preferences for good drainage and stability.

Fraxinus excelsior seems to be invasive in some habitats at Knockagh, probably in response to past changes in the land use. This might explain the general lack of significant correlations between its population parameters and the measured environ- mental factors. The correlation between density and the intensity of PAR may be interpreted as reflecting the invasion and establishment of Fraxi- nus excelsior, as a tree which might achieve greater abundance in the future. The relatively low density of Fraxinus excelsior in the samples, however, may have masked other species-environment interac- tions, since its population size structure does show poor seedling regeneration on steep slopes, when compared with the less steeply sloping, rockier soils (Fig. 9). This provides further evidence for the resil- ience of scrub dominated by Corylus avellana in the

103

less stable, more steeply sloping habitats. Differences in the composition and structure of

sample stands can not be ascribed entirely to varia- tion in the physicochemical environment. Ash & Barkham (1976) for example, demonstrated the importance of cyclic disturbance by man, in main- taining the diversity of the ground flora in coppiced woodland in Norfolk. Merton (1970) showed the effect of socioeconomic factors on the composition and age structure of woodland on limestone in Derbyshire and Staffordshire.

There is indirect evidence that management has been intensive at Knockagh in the past. McSkimin (1909) described the wood as containing large quantities of 'white thorn' in 1728, most probably the pioneer species, Crataegus monogyna and Pru- nus spinosa. Grazing must have been much more intense before the landscape was enclosed by a sys- tem of ditch, wall and hedge boundaries during the last century (Symons, 1963). A reduction in the local population of Lepus cuniculus around 1954 has probably led to some successional changes, at least on the less steeply sloping, more stable sites, at higher elevations. This may account for the large proport ion of grassland species in group -3 of the floristic ordination. That grazing pressure has re- laxed in the past, can be confirmed by the field observation that the invasion of seedlings of Cory- lus avellana, into cliff gullies, above the study area, is occurring. Changes in traditional farming meth- ods also mean that the wood is probably less dis- turbed at present, than in the recent past. The minor floristic differences between the grazed and un- grazed woodland samples point, for example, to a much heavier grazing pressure in the past, in the currently ungrazed sections of the wood.

The complexity of physicochemical, biotic and temporal variables makes prediction of future changes in the woodland flora, difficult. Whilst scree instability undoubtedly accounts for the open nature of the canopy and high species diversity in the numerically most abundant stands, the possibil- ity of further canopy closure due to invasion by Fraxinus excelsior and Acer pseudoplatanus, re- mains. Its prevention should be considered as a priority in any management programme. The rem- oval of trees capable of forming a dense, closed canopy on the escarpment site would effectively reduce the size of its minimum dynamic area. This is defined by Pickett & Thompson (1978), as the smal-

104

lest area with a natural disturbance regime which maintains internal recolonisation sources and hence minimises extinctions. It is an important considera- tion in view of the usually small size of basaltic escarpment woods.

References

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McSkimin, S., 1909. The History and Antiquities of The County of The Town of Carrickfergus. Mullah, Belfast. 495 pp.

McVean, D. N., 1964. The forest zone. In: J. H. Burnett (ed.), The Vegetation of Scotland. Oliver & Boyd, Edinburgh.

McVean, D. N. & Ratcliffe, D. A., 1962. Plant Communities of The Scottish Highlands. H.M.S.O., London.

Merton, L. F. H., 1970. The history and status of the woodlands of the Derbyshire Limestone. J. Ecol. 58:723 744.

Mueller-Dombois, D. & Ellenberg, H., 1974. Aims and Methods of Vegetation Ecology. Wiley, New York. 547 pp.

Nie, N. H., Dale, H. B. & Hull, C. H., 1970. Statistical Package for The Social Sciences. McGraw-Hill, London. 356 pp.

Norris, J. M. & Barkham, J. P., 1970. A Comparison of some Cotswolds beechwoods using multiple-discriminant analy- sis. J. Ecol. 58: 603-619.

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Rackham, O., 1980. Ancient Woodland. Arnold, London. 402 pp.

Steele, R. C., Penistan, M. J. & Phillips, J. C. C., 1968. Report of Visiting Groups (Woodlands) Northern Ireland, 19-26 July. Nature Reserves Committee, Belfast.

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Accepted 15.6.1983.