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INFLUENCE OF ALTERNATIVE VEGETATION MANAGEMENT TREATMENTS ON PLANT COMMUNITY ATTRIBUTES: ABUNDANCE, SPECIES DIVERSITY,
AND STRUCTURAL DIVERSITY.
by
Pontus Mauritz Fredrik Lindgren
B.Sc. (Forestry), The University of British Columbia, 1995
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
in
THE FACULTY OF GRADUATE STUDIES Department of Forest Sciences
Faculty of Forestry
We accept this thesis as conforming to the
required standard
THE UNIVERSITY OF BRITISH COLUMBIA December 1999
© Pontus Mauritz Fredrik Lindgren, 1999
In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head of my department or by h i s or her r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission.
Department of
The U n i v e r s i t y of B r i t i s h Columbia Vancouver, Canada
Abstract
This study was designed to test the hypothesis that alternative vegetation management
treatments (manual cutting and cut-stump applications of glyphosate herbicide), applied
to young plantations, would decrease plant community abundance (crown volume index),
species diversity, and structural diversity. The experimental design consisted of nine
operational-sized plantations stratified on the basis of location and elevation into 3 blocks
(i.e., 1 control, 1 manual, and 1 cut-stump plantation per block), and with 5 permanent
strip-transects to sample vegetation within each plantation. Vegetation management
treatments did not significantly (P > 0.10) affect the crown volume index of the herb,
shrub, or coniferous tree layers. However, both manual and cut-stump treatments
significantly reduced the crown volume index of deciduous trees in the first post-
treatment year (P = 0.05 and P < 0.01, respectively). Due to prolific growth of stump
sprouts, the manual treatment effect did not extend past the first post-treatment year. In
contrast, the cut-stump treatment impeded sprouting and, relative to control and manual
treatments, significantly suppressed deciduous growth for at least four years (P < 0.05).
Species richness, diversity, and turnover of the herb, shrub, and tree layers were not
significantly (P > 0.10) different between treatments and control. Similarly, the structural
diversity of herb, shrub, and tree layers were also not significantly (P > 0.10) different
between treatments and control. By opening the canopy and decreasing the dominance of
the deciduous tree layer, both manual and cut-stump treatments showed greater total
structural diversity (herb, shrub, and tree layers combined) relative to the control.
However, differences in total structural diversity between treatments and control were,
for the most part, not significant (P > 0.10). Therefore, the vegetation management
treatments used in this study decreased only the volume of the targeted deciduous tree
layer and did not adversely affect the species richness, diversity, turnover, or structural
diversity of the plant community.
Table of Contents
Abstract ii
List of Tables vi
List of Figures vii
Acknowledgements viii
Introduction 1
Study Areas 6
Materials and Methods 7
Experimental Design 7
Vegetation Sampling 9 Crown Volume Index 11 Species Richness 11 Species Turnover 11 Species Diversity 12 Structural Diversity 12
Statistical Analysis 13
Results 15
Site Similarity 15
Crown Volume Index 15 Herb Layer 15 Shrub Layer 21 Deciduous Tree Layer 22 Coniferous Tree Layer 23
Species Richness 24
Species Turnover 26 Herb Layer 26 Shrub Layer 26 Tree Layer 28
Species Diversity 29 Herb Layer 29 Shrub Layer 29 Tree Layer 29
Structural Diversity 31 Herb Layer 31 Shrub Layer 31 Tree Layer 34 Total (herb, shrub, and tree layers combined) 34
iv
Discussion 35
Experimental Design 35 Crown Volume Index 36
Herb and Shrub Layers 36 Deciduous Tree Layer 38 Coniferous Tree Layer 39
Species Richness 42 Species Turnover 43
Herb Layer 43 Shrub Layer 44 Tree Layer 45
Species Diversity 45 Herb and Shrub Layers 46 Tree Layer 48
Structural Diversity 49 Herb Layer 49 Shrub Layer 50 Tree Layer 51 Total (herb, shrub, and tree layers combined) 51
Conclusions 52
Literature Cited 53
Appendix 1 63
Appendix 2 66
Appendix 3 68
List of Tables
Table 1. Repeated measures analysis of variance (RM-ANOVA) model used for
investigating plant volume, species diversity, and structural diversity 14
Table 2. Statistical results (RM-ANOVA) 16
Table 3. Crown volume indices (m /0.01 ha) of prominent herbs, shrub, and trees 17
VI
List of Figures
Figure 1. Design of strip-transects used to sample vegetation 10
Figure 2. Mean total crown volume index (m3/0.01ha) for herb, shrub, deciduous tree, and coniferous tree layers among control, manually, and cut-stump treated plantations 20
Figure 3. Mean species richness for herb, shrub, and tree layers among control, manually, and cut-stump treated plantations 25
Figure 4. Mean species turnover for herb, shrub, and tree layers layers within control, manually, and cut-stump treated plantations . 27
Figure 5. Mean species diversity indices (Simpson's and Shannon-Wiener) for herb, shrub and tree layers among control, manually, and cut-stump treated plantations .. 30
Figure 6. Mean Simpson's structural diversity indices for herb, shrub, tree, and combined total layers among control, manually, and cut-stump treated plantations 32
Figure 7. Mean Shannon-Wiener structural diversity indices for herb, shrub, tree, and combined total layers among control, manually, and cut-stump treated plantations. 33
vii
Acknowledgements
I thank the British Columbia Ministry of Forests, Research Branch, Victoria, B.C., and
the Salmon Arm Forest District for financial and logistical support. Additional funding
was provided by the Natural Sciences and Engineering Research Council of Canada. For
their assistance with field work, I also thank S. Allen, M. Burwash, B. Hammond, C.
Houwers, S. Milne, C. Nowotny, G. Ozawa, M. Porter, D. Ransome, E. Roberts, G.
Ryznar, and C. von Trebra. And finally, I thank L. Zabek, P. Marshall, and M . Pitt for
assistance with statistics.
viii
Introduction
Vegetation management is increasingly important in temperate forests of North America
as a means to increase the survival and growth rates of crop trees, and to more quickly
achieve free-to-grow status for regenerating stands (Newton and Comeau, 1990). If
allowable annual cut levels are to be maintained, accelerated development of new
plantations, as well as rehabilitation of backlog sites is required. Intensive silviculture
programs must deal with the problem of reducing competing herbs, shrubs, hardwoods,
and other non-commercial species to increase production of merchantable trees
(McDonald and Radosevitch, 1992). Vegetation management is, therefore, a tool which
can help provide rates of tree growth necessary to sustain the forest industry.
To accelerate the development of a second-growth stand of crop trees, vegetation
management is particularly important during the first few years following planting. This
is the time when a site is naturally dominated by pioneer species which, depending on the
objectives established for the site, may be considered as non-crop species. Species often
considered as non-crop competitors with coniferous crop trees are species of the genera
Betula (birch), Alnus (alder), Populus (poplars and aspens), Acer (maple), Prunus
(cherry), and Rubus (raspberry and thimbleberry). There are several methods for
reducing the competition created by these non-crop species such as 1) manual (hand-held
chain saws, brush saws, and girdling tools), 2) mechanical (machinery that mow, rake,
1
crush, and chip), 3) burning, 4) biological (use of grazing livestock and pathogens), and
5) herbicides.
Because the goal of vegetation management is to modify the plant community to favor
the rapid development of crop trees, it undoubtedly has profound effects on the
abundance of some plant species inhabiting treated areas (Santillo et al., 1989). Although
the benefits of vegetation management treatments on the growth and survival of
coniferous crop trees is well documented (Reukema, 1964; Crossley, 1976; Brix, 1981;
Berry, 1982; Harrington and Reukema, 1983; MacLean and Morgan, 1983; Maguire,
1983; Lavigne, 1988; Yang, 1991, Wang etal, 1995; Simard and Heineman, 1996a,
19966, and 1996c, and others), its effects on non-timber plant community attributes are
still not well understood (Wagner, 1993).
Because of growing public concern about the environment, forest managers are no longer
charged with only regenerating forests (Lautenschlager, 1986; McGee and Levy, 1988;
Freedman, 1991; Brand, 1992; Lautenschlager, 1993; Halpern and Spies, 1995). Today,
in addition to regenerating forests for future harvest, the conservation of biodiversity is
becoming an integral part of forest management. For example, the recent development of
the Forest Practices Code of British Columbia and the publication of a series of
guidebooks such as the Biodiversity Guidebook (B.C. Ministry of Forests, 1995), are
indications of this change within B.C.'s forest industry. However, as outlined by Wagner
(1993), there is a serious lack of information about the effects that vegetation
management has on the environment. Therefore, if forest management is to encompass
2
both timber production and the conservation of biodiversity, vegetation management
procedures must be critically examined within the context of both of these objectives.
The most common form of vegetation management within Canadian forests is aerial
spraying of the herbicide glyphosate (Vision®, commercial formulation containing
glyphosate, 356 g/L present as an isopropylamine salt) (Campbell, 1990). At prescribed
rates, glyphosate poses minimal toxicological risks, in terms of mortality and reduced
reproduction in wildlife, and does not accumulate in the environment (Morrison and
Meslow, 1983; Newton et al., 1984; Freedman 1991), however, public opinion about the
use of any forest herbicides is often very negative (Mitchell, 1990; Turpin, 1990;
Freedman, 1991; Glover, 1994). Research has shown that glyphosate does not have a
direct effect on the survival or reproduction of small mammals (Sullivan and Sullivan,
1981; Sullivan, 1990a; Sullivan et al., 1997). In addition, several studies have shown
that small mammal population responses to aerial application of this herbicide have
ranged from an overall increase in population density (Anthony and Morrison, 1985), to
no change in density (Sullivan and Sullivan, 1982; D'Anieri et al, 1987; Sullivan,
19906), to others which have reported a decrease in abundance (Clough, 1987; Santillo et
ah, 1989). Other studies have also reported that glyphosate does not have any significant
long-term effects on the survival or health of ungulates (Sullivan and Sullivan, 1979;
Cambell et al, 1981; Jones and Forbes, 1984). However, more information is still
needed to assess the effects that vegetation management treatments, particularly
alternative treatments (Campbell, 1990), have on both the timber resource and non-timber
values such as habitat quality.
3
One such alternative vegetation management treatment is the ground-based method of
applying glyphosate to the cut-stump surface of manually-cut deciduous trees. The non-
broadcast and species-specific approach of this treatment suggests that it may be an
effective and environmentally sensitive method of vegetation management. One study
reported that this cut-stump method, although significantly reducing crown volume index
of the targeted deciduous tree layer, had no apparent effect on the abundance of herbs,
shrubs, and coniferous trees, and did not affect the survival or reproduction of small
mammals (Runciman and Sullivan, 1996).
Studies designed to investigate the effects of vegetation management on biodiversity
often monitor changes in plant abundance and diversity (Tomkins and Grant, 1977;
Pollack ef al, 1990; Freedman et al, 1993; Sullivan, 1994; Sullivan et al, 1996; and
others). However, a habitat attribute that is frequently ignored by such studies is
structural diversity. Because of the well documented direct relationship between the
structural diversity of a habitat and the diversity of species that live there (MacArthur and
MacArthur, 1961; MacArthur, 1964; MacArthur, 1965; Balda, 1969; Sutton and Hudson,
1980; Adler, 1987; Harney and Dueser, 1987, Hunter, 1990; and others), structural
diversity should be carefully examined to determine the effects of vegetation
management treatments on habitat quality.
This study was designed to test the hypothesis that vegetation management treatments
(manual spacing and cut-stump applications of glyphosate) applied in young mixed-
4
conifer plantations would adversely affect the plant community (herbs, shrubs, and trees)
by decreasing 1) plant abundance, 2) species richness, 3) species turnover, 4) species
diversity, and 5) structural diversity.
5
Study Areas
This study was conducted within two similar areas in the Shuswap Highlands of south-
central British Columbia, Canada. The Eagle Bay (50° 55' N, 119° 11' W) and Sicamous
(50° 52' N , 118° 59' W) sites are both located within the Thompson Moist Warm variant
of the Interior Cedar-Hemlock biogeoclimatic zone (Lloyd et al., 1990). These areas are
characterized by similar topography, elevation, climate, and climax vegetation. The
topography of this area is hilly to steeply sloping and elevation ranges from 550 to 1237
m. Summers are generally warm and dry and winters cool and wet. Mean annual
temperatures range from 2 to 8.7° C and precipitation ranges from 500 to 1200 mm, with
as much as 50% falling as snow (Ketcheson et al., 1991). Climax forests on mesic sites
are characterized by western redcedar (Thuja plicata Donn ex D. Don) and western
hemlock (Tsuga heterophylla (Raf.) Sarg.), while interior Douglas-fir (Pseudotsuga
menziesii var. glauca (Beissin.) Franco), lodgepole pine (Pinus contorta Dougl. Ex Loud,
var. latifolia Engelm.), and paper birch (Betulapapyrifera Marsh.) are common serai
species. Nomenclature for all plant species follows Hitchcock and Cronquist (1973).
Nine plantations, ranging from 16 to 47 ha in area, and 2 to 9 years in age when selected,
were chosen in conjunction with the Salmon Arm Forest District silvicultural staff on the
basis of operational scale, proximity, and initial similarity in requiring vegetation
management treatments. The six plantations found at the Eagle Bay site were located on
a north to north-east facing slope. These areas were logged between 1978 and 1986 and
6
planted predominantly to lodgepole pine between 1985 and 1990. Site inspections
indicated that Douglas-fir, western larch (Larix occidentalis Nutt), and hybrid spruce
{Picea engelmannii Parry X P. glauca (Moench) Voss) were also present. The remaining
three plantations at the Sicamous site were located on a southeast slope. These areas
were logged between 1977 and 1988 and planted between 1982 and 1989, largely with
Douglas-fir. Inspections of these sites indicated that lodgepole pine was also present with
lesser amounts of hybrid spruce and western white pine (Pinus monticola Dougl.).
Materials and Methods
Experimental Design
This study follows a randomized-block design. Study plots were established in eight of
the nine plantations during September 1991, and in the ninth during April 1992.
Assignment of control and vegetation management treatments was done subjectively
among these nine plantations due to logistical and economic constraints. The first of
these constraints was related to the close proximity of the Eagle Bay plantations to a
residential community. To facilitate the approval of the herbicide application permits, the
cut-stump herbicide treatments had to be applied to the two plantations furthest from
these residents. The second constraint was related to a minimum acceptable outcome
anticipated for the cost of the vegetation management treatments. This required that the
more expensive treatment, the cut-stump herbicide treatment, be applied to the Sicamous
plantation that was most dominated with deciduous trees.
The nine plantations were stratified into three blocks on the basis of 1) location and 2)
elevation. Consequently, plantations A, B, and C (Sicamous site) formed Block 1, and
the remaining six plantations from the Eagle Bay site (D to I) were stratified into Blocks
2 and 3, on the basis of elevation.
In addition to untreated controls, the vegetation management treatments used in this study
were applied in the following manner. Manually treated plantations had all trembling
aspen (Populus tremuloides Michx.), paper birch, and black cottonwood (Populus
trichocarpa T. & G.) cut with power saws, except for a few individual stems that were
left in the openings of the plantation. A l l stems of willow (Salix spp.), Douglas maple
(Acer glabrum Ton-.), bitter cherry (Prunus emarginata (Dougl.) Walp.), and speckled
alder (Alnus incana (L.) Moench) that were within 1 m of a crop tree were also cut. A l l
other vegetation was left uncut.
Cut-stump treated plantations were treated in a similar manner as those receiving the
manual treatment, with the additional application of the herbicide glyphosate, diluted 2:1
in water, to the cut stump surfaces of the felled deciduous trees. A small amount of
Basacid Blue® dye was added to the herbicide mixture to mark the treated stumps. Most
maple, cherry, and alder stems were left uncut. These prescriptions were applied between
September 25 and October 22, 1992.
8
Vegetation Sampling
Vegetation was assessed annually within each plantation to determine the presence and
abundance of individual species for the purpose of monitoring the plant community's
composition and structure. Five permanent 5 x 25-m strip-transects, each consisting of
five contiguous 5 x 5-m subplots with nested 3 x 3-m and 1 x 1-m subplots (Figure 1),
were established within each of the nine plantations (as outlined in Stickney, 1980;
1985). Transects were randomly placed within each plantation as long as they were at
least 50 m from the nearest stand edge and they did not cross any roads, skid-trails, or
landings.
Within a strip-transect, each of the 3 different sized subplots were used to sample
2 2
different plant forms: the 5 x 5-m (25-m ) subplots for sampling trees, the 3 x 3-m (9-m )
subplots for sampling shrubs, and the 1 x 1-m (1-m ) subplots for sampling herbs (as
outlined in Ritchie and Sullivan, 1989). In addition, trees, shrubs, and herbs were each
sampled within 6 height classes of 0-0.25, 0.25-0.50, 0.50-1.0, 1.0-2.0, 2.0-3.0, and 3.0-
4.0 m (Walmsley et al, 1980). Species abundance was estimated within each of the 6
height classes that the plants occurred in by a visual estimate of percent cover. Individual
plants were only measured once within the height class containing the topmost growth of
that plant.
All plants can be classified into one of three life forms: herb, shrub, or tree. However,
these classes can be further subdivided or, conversely, grouped together, to create
additional classes of plants. For example, the tree layer may be subdivided into
deciduous and coniferous tree classes. Also, herbs, shrubs, and trees are sometimes
9
grouped together to form a class including all plant forms. Hereafter, the tree class w i l l
include both deciduous and coniferous trees and the total class w i l l include all plant
forms.
5 x 25 m Transect
Trees
Shrubs
Herbs
Figure 1. Design of strip-transects used to sample vegetation.
10
Crown Volume Index
A crown volume index was calculated for each species within each of the five nested
subplots of a transect by multiplying the percent cover values by the top of the
corresponding height class (Stickney, 1985). The product of these values gives the
volume of a cylindroid representing the space occupied by the plant in m3/0.01 ha.
Species Richness
Species richness (S) was calculated as the total number of species of a given plant form
(herb, shrub, or tree) sampled within a transect (Krebs, 1989).
Species Turnover
Species turnover (TO) was also calculated for herb, shrub, and tree layers. TO is defined
as the number of species lost and gained during a set time period, divided by the total
number of species sampled during the same period (Schoonmaker and McKee, 1988),
and is calculated as follows:
TO = / (A + B)
where: L is the number of species lost and G is the number of species gained during a
defined time period from ti to t2 and A and B are the total number of species sampled
during times ti and t2, respectively. Because a species turnover calculation requires a
11
minimum of two sample periods, this measure is undefined for the first year of sampling
(pre-treatment year).
Species Diversity
Two diversity indices are reported within this paper: Simpson's index, which is sensitive
to changes in abundant species (Simpson, 1949) and the Shannon-Wiener index, which is
sensitive to changes in rare species (Pielou, 1966a; Peet, 1974). Simpson's index of
species diversity is the probability of picking two organisms at random that are different
species, and ranges from 0 to almost 1. The Shannon-Wiener index of diversity is based
on information theory and the degree of difficulty in predicting correctly the next
individual sampled. As such, this index increases with number of species sampled, and
ranges from 0 to approximately 5 for biological communities.
Structural Diversity
The above definition of Simpson's and Shannon-Wiener diversity indices, although
referring to species diversity, applies equally to structural diversity. While species is the
object of a species diversity index, height class is the object of a structural diversity
index. The indices are calculated as follows:
Simpson's diversity index (D)
D = l-[Z(Plf]
Shannon-Wiener diversity index (H')
H' = -I(Pi)(log2Pi)
12
where: p, = proportion of mean total crown volume index (Pielou, 1966a) belonging to
the i'h species (for species diversity indices) or height class (for structural diversity
indices).
Pre-treatment vegetation sampling was initiated in late July 1992. The first post-
treatment sampling of vegetation was conducted in July 1993 and was conducted
annually to July 1996, when the study was terminated.
All plant community attributes (e.g., crown volume index, diversity indices) were
calculated on a subplot basis, and then averaged across the five subplots of a transect.
Consequently, each transect represents one datum.
Statistical Analysis
A repeated measures analysis of variance (RM-ANOVA, SPSS Institute Inc., 1997) was
used to test for significant differences among treatment means, as shown in Table 1.
Both pre- and four post-treatment years were analyzed together, resulting in five levels
(years) for the within-subjects factor (time). Both treatment and block were assigned as
between-subjects factors. Before performing any analyses, data not conforming to
properties of normality and equal variance were subjected to various transformations to
best approximate these assumptions required by any ANOVA (Zar, 1984). Mauchly's W
test statistic was used to test for sphericity (independence of data among repeated
measures) (Littel, 1989; Kuehl, 1994). For data found to be correlated among years, the
Huynh-Feldt correction was used to adjust the degrees of freedom of the within-subjects
13
F-ratio. The Bonferroni post-hoc test (adjusted for multiple contrasts) was used to locate
differences among treatment means within each sample year (Rosenthal and Rosnow,
1985). Significance levels for all analyses (i.e., RM-ANOVA and Bonferroni
significance tests) was set at a = 0.10. Although conclusions made from statistical
results with the more standard significance level of 0.05 may be stated with more
confidence than those resulting from tests with a probabilty of 0.10, significance tests
with P-0.10 are likely biologically significant and deserve comment. Tests regarding
data properties, such as Leven's homogeneity test and Mauchly's W sphericity test, were
made with a significance level of a = 0.05.
Table 1. Repeated measures analysis of variance (RM-ANOVA) model used for investigating crown volume, species richness, species turnover, species diversity, and structural diversity.
Source of variation
Factor type
Level Degrees of freedom F-test (d.f.)
Block Random n = 3 n-l=2 Treatment Fixed k = 3 k-l=2 MS-Treat. / MS-Err. I (2,4) Error I (n-l)(k-l)=4 Time Time X Treatment Error II
Fixed t = 5a t-l=4 (t-l)(k-l)=8 k(n-l)(t-l)=24
MS-T.xTreat. / MS-Err. II (8,24)
" There are five levels of time (t = 5) for all plant attributes except species turnover. Because species turnover is not defined for the pre-treatment year, there are only four levels (t = 4) for this attribute.
14
Results
Site Similarity
There were no statistical differences among the treatment and control plantations for any
of the plant attributes (crown volume index, species richness, species diversity indices,
and structural diversity indices) during the pre-treatment year (Table 2; Figures 2, 3 and 5
to 7).
Crown Volume Index
Herb Layer
Prominent herb species found within the study area included fireweed (Epilobium
angustifolium), white hawkweed (Hieracium albiflorum), common dandelion
{Taraxacum officinale), wild strawberry (Fragaria virginiana), and pearly everlasting
(Anaphalis margaritacea). The crown volume index of these and other prominent herbs
are provided, by treatment and sample year, in Table 3.
Although the mean total crown volume index (m /0.01 ha) of the herb layer changed over
time (Figure 2), differences among control and treatment plantations were not significant
(F2,4 = 1.26; P = 0.38). Both the control and treatment groups showed similar trends
during the four post-treatment years. The mean total crown volume index of the herb
layer for the control, manually, and cut-stump treated plantations peaked during the
15
Table 2. Statistical results obtained from repeated measures analysis of variance (RM-ANOVA) conducted on several plant community attributes. Five years of data (one pre-treatment year and four post-treatment years) were included in these analyses.
Attribute Treatment Effects Time X Treatment Interaction Attribute r(2.4> P P
Volume Index Herbs 1.26 0.38 F(6,19) = 0.73 0.63
Shrubs 0.86 0.49 F (8,24) = 1.15 0.37 Deciduous trees 40.63 0.002 F(6,19) = 9.94 < 0.001 Coniferous trees 0.99 0.45 F(8,24) = 0.33 0.94
Species Richness Herbs 3.29 0.14 F(8,24) = 1.77 0.13
Shrubs 2.75 0.18 F(8,24) = 0.31 0.95 Trees (decid. and conif.) 1.17 0.40 F(8.24) = 0.85 0.57
Species Turnover Herbs 0.72 0.54 F(6,18) = 0.37 0.89
Shrubs 1.66 0.30 F(6,18) = 1.24 0.33 Trees (decid. and conif.) 2.14 0.23 Fr6.i8i= 1.06 0.42
Species Diversity - Simpson's Herbs 3.34 0.14 F a ,22)= 0.34 0.93
Shrubs 1.26 0.38 F(7,22) = 0.29 0.95 Trees (decid. and conif.) 1.49 0.33 F(7.22) = 2.66 0.04
Species Diversitv - Shannon's Herbs 2.78 0.18 F(7,22) = 0.24 0.97
Shrubs 1.17 0.40 F(7,21) = 0.36 0.92 Trees (decid. and conif.) 1.22 0.39 F(7.23) = 2.70 0.03
Structural Diversitv - Simpson's Herbs 0.09 0.92 F(8,24) = 0.24 0.97
Shrubs 0.05 0.95 F(8,24) = 0.62 0.75 Trees (decid. and conif.) 3.68 0.12 F(7,23) = 1.73 0.15
Total (herbs, shrubs, and trees) 6.34 0.06 F(7,23) = 2.39 0.05
Structural Diversitv - Shannon's Herbs 0.16 0.85 F(8,24) = 0.30 0.96
Shrubs 0.09 0.92 F(8,24) = 0.70 0.69 Trees (decid. and conif.) 3.70 0.12 F(7,23) = 1.73 0.15
Total (herbs, shrubs, and trees) 7.09 0.05 F(7.22) = 1.60 0.19
a Degrees of freedom for tests of Time x Treatment interactions are 8 and 24 (or 6 and 18 for species turnover) when the data are not correlated among years. Correlation among repeated measures data is a violation of ANOVA assumptions and requires an adjustment (Huynh-Feldt correction) which decreases the df from those mentioned above. Note: Significant P-values (a = 0.10) are indicated in bold text.
16
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19
Control Manual Cut-stump
Mean Total Crown Volume Index
Herbs Shrubs
40 <?30 1 20
'92 | '93 '94 Yea
'95 '96 '92 '93 '94 Yea
'95 '96
Figure 2. Mean total crown volume index (m /O.Olha) for herb, shrub, deciduous tree, and coniferous tree layers among control, manually, and cut-stump treated plantations. No statistical differences (a = 0.10) were observed for herb, shrub, or coniferous tree volume, however, the deciduous tree layer was significantly affected by the treatments. Arrow on horizontal axis indicates timing of treatments. * P < 0.10, **P< 0.05, *** P < 0.01; significance by Bonferroni post-hoc test.
20
second post-treatment year (1994) and then gradually decreased during the final two
years of the study. Both the manually and cut-stump treated plantations showed a
decrease in volume of herbs in the first post-treatment year, in contrast to the increase in
herb volume observed within the control during this same period, although differences
were not significant (Figure 2).
Shrub Layer
Prominent shrub species found within the study area include falsebox (Pachistima
myrsinites), thimbleberry (Rubus parviflorus), red raspberry (Rubus idaeus), bitter cherry
(Primus emarginatd), and willow (Salix spp.). The crown volume index of these and
other prominent shrubs are provided, by treatment and sample year, in Table 3.
Although the mean total crown volume index of the shrub layer changed over time
(Figure 2), differences among control and treatment plantations were not significant (F2;4
= 0.86; P = 0.49). The control plantations showed a steady increase in mean total crown
volume index of shrubs throughout the five years of the study. Both the manually and
cut-stump treated plantations also increased in shrub volume during the post-treatment
years. Like the herb layer, the shrub layer experienced a slight decrease in volume during
the first post-treatment year within the treated plantations; however, differences were not
significant (Figure 2).
21
Deciduous Tree Layer
The three deciduous tree species that were found within the study area were paper birch,
black cottonwood, and trembling aspen. The crown volume index of these deciduous tree
species are provided, by treatment and sample year, in Table 3.
The mean total crown volume indices of deciduous trees were not statistically different
(Bonferroni; P = > 0.17) among the control and treatment plantations during the pre-
treatment year. The treatments resulted in siginificant (F2,4 = 40.63; P < 0.002) decreases
in deciduous tree volume during the first post-treatment year (Table 2; Figure 2). In the
first post-treatment year, the mean total crown volume index of deciduous trees decreased
by 87% (106.5 to 13.5 m3/0.01ha) and 95% (46.8 to 2.2 m3/0.01ha) within the manually
and cut-stump treated plantations, respectively. This decline was in direct contrast to the
control plantations, which experienced an increase in mean total crown volume index of
38% (53.2 to 73.3 rrrVO.Olha) during this same period (Figure 2). Consequently, both the
manually and cut-stump treated plantations had significantly less (Bonferroni; P = 0.05
and P < 0.01, respectively) deciduous tree volume than that of the control during the first
post-treatment year. In addition, during the first post-treatment year, the cut-stump
treated plantations had significantly less (Bonferroni; P = 0.02) deciduous tree crown
volume than those treated manually (Figure 2).
Both the control and cut-stump treated plantations showed a gradual increase in mean
total crown volume index of deciduous trees throughout the four post-treatment years.
The dramatic suppression of deciduous tree volume observed one year after treatment is
22
maintained during the post-treatment years for the cut-stump treatment. As a result,
deciduous tree volume within the cut-stump treatment is less (Bonferroni; P < 0.02) than
that of the control during all four post-treatment years. The suppression of deciduous tree
volume within the manually treated plantations was, however, only short-lived. There
was a rapid increase in growth by the second post-treatment year (1994), resulting in
deciduous crown volumes becoming similar (Bonferroni; P = 1.00) to that of the control
during the 1994 sample year, and thereafter (Figure 2).
Coniferous Tree Layer
Prominent coniferous tree species sampled within the study area included interior
Douglas-fir, western redcedar, lodgepole pine, western hemlock, hybrid spruce, and
western white pine. Western larch, subalpine fir (Abies lasiocarpa), and western yew
(Taxus brevifolia) were also found within the study area, but were much less common
than the coniferous tree species listed above. The crown volume index of these
coniferous tree species are listed, by treatment and sample year, in Table 3.
Although the mean total crown volume index of the coniferous tree layer changed over
time (Table 3, Figure 2), differences among control and treatment groups were not
significant (F2,4 = 0.99; P = 0.45). The control plantations showed only a slight increase
in mean total crown volume index of coniferous trees throughout the five years of this
study. However, both the manually and cut-stump treated plantations exhibited
accelerated growth rates of coniferous tree volume relative to that of the control during
23
the post-treatment years. During the four post-treatment years, the mean annual
percentage increment of coniferous crown volume (see formula below) was 5%, 44%,
and 29% for control, manually, and cut-stump treated plantations, respectively.
where: A is the crown volume at time 1, B is the crown volume at time 2, and C is the
number of years between time 1 and 2.
Although the differences in coniferous tree crown volume among treatment and control
plantations were not statistically significant, increased coniferous growth rates of 5 to 8
times that of control plantations suggested a biologically and likely economically
significant treatment effect.
Species Richness
During this study, a total of 75 herb, 37 shrub, and 12 tree species were sampled. A list
of these herb, shrub, and tree species is provided in Appendices 1, 2, and 3, respectively.
There was no statistical difference in mean species richness among the control and
treatment plantations for herbs (F2,4 = 3.29; P = 0.14), shrubs (F2 > 4 = 2.75; P = 0.18), or
trees (F2,4 = 1.17; P = 0.40), at any time during this study (Table 2; Figure 3). Some time
trends, common to both control and treatment groups did occur. Although not
Mean annual percentage increment of crown volume
24
Control Manual C U Cut-stump
Species Richness
Herbs Shrubs
I '94 Year
'95 '96
Trees
Figure 3. Mean species richness for herb, shrub, and tree layers among control, manually, and cut-stump treated plantations. No statistical differences (a = 0.10) in species richness were observed among the control and treatment plantations for any of the plant forms (herbs, shrubs, or trees) during any of the five sample years. Arrow on the horizontal axis indicates timing of treatments.
25
statistically significant, the observed peak in mean herb species richness two years post-
treatment (1994), was followed by a gradual decline until the completion of the study
(Figure 3). The mean species richness of the shrub layer, although more static than that
of the herbs, also peaked in the second and third post-treatment years (1994 and 1995)
(Figure 3). The mean species richness of the tree layer was the most static of all, with
only very minor differences observed from the start to the end of this 5-year study
(Figure 3).
Species Turnover
Herb Layer
There were no significant (F2,4 = 0.72; P = 0.54) differences in herb species turnover
observed among control and treatment plantations during any consecutive year intervals
(Table 2; Figure 4). The lowest species turnover was observed between the pre-treatment
and first post-treatment year (1992 to 1993), and the highest was observed the next year
(1993 to 1994). Control, manual, and cut-stump treatments all showed similar time
trends throughout the study.
Shrub Layer
There were no significant (F2,4 = 1.66; P = 0.30) differences in shrub species turnover
observed among control and treatment plantations during any consecutive year intervals
(Table 2; Figure 4). Although not statistically significant, differences in shrub species
turnover among treatment and control plantations do suggest some trends. Both manual
26
Control I I Manual CZ] Cut-stump
Species Turnover
Herbs
Shrubs
0.1 0.08
o0.06 H 0.04
0.02 0
Trees
'92-'93
t
'93-'94 '94-'95 Time interval (years)
'95-'96
Figure 4. Mean species turnover for herb, shrub, and tree layers within control, manually, and cut-stump treated plantations. No statistical differences (a = 0.10) in species turnover were observed among the control and treatment plantations for any of the plant forms (herbs, shrubs, or trees) during any of the five sample years. Arrow indicates timing of treatment.
27
and cut-stump treated plantations experienced the highest species turnover during the pre-
treatment and first post-treatment time interval (1992 to 1993) and then declined
thereafter. This is in contrast to the shrub species turnover observed to be its lowest
within the control plantations during this same time period, followed by an increase
during consecutive years, peaking during the 1994 to 1995 time interval.
Tree Layer
There were no significant (F2,4 = 2.14; P = 0.23) differences in tree species turnover
observed among control and treatment plantations during any consecutive year intervals
(Table 2; Figure 4). The small changes in tree species turnover observed during the
course of this study were caused by a single species being gained or lost within a given
time interval. An exception to this was observed during the pre-treatment to first post-
treatment year interval (1992 to 1993) when, within one of the three manually treated
plantations, two tree species were gained.
By the final post-treatment year (1996), no tree species present during the pre-treatment
year had been lost within any of the control or treatment plantations. However, during
this same time interval, some species were gained. Western yew, western white pine, and
western larch were gained within the control, manually, and cut-stump treated
plantations, respectively (Table 3).
28
Species Diversity
Herb Layer
At no time during this study did either the Simpson's (F^ = 3.34; P = 0.14) or Shannon-
Wiener (F2,4 — 2.78; P = 0.18) mean species diversity of the herb layer differ statistically
among the control and treatment plantations (Table 2; Figure 5). Both Simpson's and the
Shannon-Wiener index of diversity showed similar time trends among the control and
treatment plantations. Specifically, the species diversity of the herb layer appeared to
peak one year post-treatment (1993) and then gradually decline during the remaining
three years of the study (Figure 5).
Shrub Layer
At no time during this study did either the Simpson's (F2>4 = 1.26; P = 0.38) or Shannon-
Wiener (F2 j4 = 1.17; P = 0.40) mean species diversity of the shrub layer differ statistically
among the control and treatment plantations (Table 2; Figure 5). Both control and
treatment plantations demonstrated the highest level of shrub species diversity during the
pre-treatment year and showed a gradual decline in diversity thereafter.
Tree Layer
At no time during this study did either the Simpson's (F^ = 1.49; P = 0.33) or Shannon-
Wiener (F2 ;4 = 1.22; P = 0.39) mean species diversity of the tree layer differ statistically
among the control and treatment plantations (Table 2; Figure 5). The greatest change in
both Simpson's and Shannon-Wiener tree species diversity indices occurred between the
pre-treatment and first post-treatment year and accounts for the significant (¥7,22 = 2.66; P
29
Control Manual E 3 Cut-stump
Simpson's Index
Herbs
Trees
Species Diversity
2 1.5 1
0.5 0
Shannon-Wiener Index
Herbs
'92 '93 '94 Year
'95 '96
Trees
'92 | '93 '94 Year
'95 '96
Figure 5. Mean species diversity indices (Simpson's and Shannon-Wiener) for herb, shrub and tree layers among control, manually, and cut-stump treated plantations. No statistical differences (a = 0.10) in species diversity were observed among the control and treatment plantations for any of the plant forms (herbs, shrubs, or trees) during any of the five sample years. Arrow on horizontal axis indicates timing of treatments.
30
= 0.04 and F7 23 = 2.70; P = 0.03, respectively) Time x Treatment interactions reported in
Table 2.
Structural Diversity
Herb Layer
The mean structural diversity of the herb layer for both the Simpson's (F 2 4 = 0.09; P =
0.92) and Shannon-Wiener (F2,4 = 0.16; P = 0.85) indices were statistically similar
among the control and treatment plantations throughout the study (Table 2; Figures 6 and
7). The control plantations were observed to peak in herb structural diversity in the first
post-treatment year (1993) and gradually decreased thereafter. Similarly, the plantations
that received treatments were also observed to peak in structural diversity, however, this
peak lagged behind that of the control by one year (i.e., peaked during the second post-
treatment year).
Shrub Layer
The mean structural diversity of the shrub layer for both the Simpson's (F2,4 = 0.05; P =
0.95) and Shannon-Wiener (F2,4 = 0.09; P = 0.92) indices were similar among the control
and treatment plantations throughout the study (Table 2; Figures 6 and 7). The structural
diversity indices of the shrub layer changed very little during the five year course of this
study. The only notable exception was a slight decrease in structural
31
Control [ZD Manual Cut-stump
Simpson's Structural Diversity
Herbs Shrubs
Trees Total
Figure 6. Mean Simpson's structural diversity indices for herb, shrub, tree, and combined total layers among control, manually, and cut-stump treated plantations. Bonferroni post-hoc tests indicated no statistical differences (a = 0.10) in structural diversity among any of the control and treatment plantations within the herb and shrub layers, pre- and post-treatment. However, some statistical differences among the treatment and control groups were observed within the combined total layer. Arrow on horizontal axis indicates timing of treatments. * P < 0.10, ** P < 0.05, *** P < 0.01; significance by Bonferroni post-hoc test.
32
Control Manual I I Cut-stump • •
Shannon-Wiener Structural Diversitv
Figure 7. Mean Shannon-Wiener structural diversity indices for herb, shrub, tree, and combined total layers among control, manually, and cut-stump treated plantations. A Bonferroni post-hoc test suggested no statistical differences (a = 0.10) in structural diversity among any of the control and treatment units within the herb, shrub, and tree layers, pre- and post-treatment. However, some statistical differences among the treatment and control groups were observed within the combined total layer. Arrow on horizontal axis indicates timing of treatments. * P < 0.10, ** P < 0.05, *** P < 0.01; significance by Bonferroni post-hoc test.
33
diversity observed immediately following treatment within both manually and cut-stump
treated plantations.
Tree Layer
The mean structural diversity of the tree layer for both the Simpson's (F^ = 3.68; P =
0.12) and Shannon-Wiener (F2,4 = 3.70; P = 0.12) indices were similar among the control
and treatment plantations throughout the study (Table 2; Figures 6 and 7). The structural
diversity of the tree layer decreased for both the control and cut-stump treated plantations
from the pre-treatment year through to the final year of sampling. The manually treated
plantations increased slightly in tree layer structural diversity in the first post-treatment
year, followed by a gradual decline during the final three years of sampling. The rate of
decrease in tree layer structural diversity (both Simpson's and Shannon-Wiener) was
greatest for the control and least for the cut-stump treated plantations.
Total (herb, shrub, and tree layers combined)
The mean total structural diversity indices (both Simpson's and Shannon-Wiener) for the
control and treatment plantations gradually declined following a single peak. However,
the year in which this peak occurs differs among control, manual, and cut-stump treated
plantations (1992, 1993, and 1994, respectively) (Figures 6 and 7). Differences in total
structural diversity among control and treatment plantations were indicated by a
significant (F2,4 = 7.09; P = 0.05) treatment effect for total Shannon-Wiener structural
diversity (Table 2). During the second post-treatment year, total Shannon-Wiener
structural diversity within both manual (Bonferroni; P = 0.08) and cut-stump treated
34
plantations (Bonferroni; P = 0.01) was greater than that of the control plantations (Figure
7).
A significant Time X Treatment interaction for mean total Simpson's structural diversity
(F723 = 2.39; P = 0.05) made it difficult to comment on the significant main treatment
effect. However, a significant treatment effect (F^ = 7.37; P = 0.04) without interaction
(F6,i8 = 1.63; P = 0.20) was observed when a RM-ANOVA was performed on the post-
treatment years only. This suggested that the interaction was caused by a change in total
Simpson's structural diversity among control and treatment plantations from the pre- to
post-treatment year and that a treatment effect did exist during the post-treatment years.
Post-hoc tests indicated that, as with the Shannon-Wiener index, total Simpson's
structural diversity was significantly greater than that of the control within both the
manual (Bonferroni; P = 0.02) and cut-stump treated plantations (Bonferroni; P < 0.01),
during the second post-treatment year (Figure 6).
Discussion
Experimental Design
The subjective allocation of the cut-stump herbicide treatment was a potential concern
because this was not consistent with a randomized-block design. However, although
natural variation was apparent among the plantations, the statistical similarity of control
and treatment units during the pre-treatment year (Figures 2 to 7) suggested that the
35
subjective allocation of cut-stump treatments did not significantly bias the experimental
design of this study. In addition, treatment allocation was well interspersed among the
nine plantations (and among the three blocks), a condition that is considered by some to
be more important than randomization. Hurlbert (1984) states that ".. .interspersion is the
more critical concept or feature; randomization is simply a way of achieving interspersion
in a way that eliminates the possibility of bias and allows accurate specification of the
probability of a type I error". This is especially true when replication is low, as is often
the case with large-scale ecological studies.
Crown Volume Index
Herb and Shrub Layers
Because the manual and cut-stump treatments targeted only specific deciduous tree
species, it is not surprising that there was not a significant effect on the crown volume
index of herbs or shrubs (Table 2; Figure 2). However, these treatments appeared to
temporarily depress the growth of these plant forms, relative to the control. Within the
control plantations, during the first post-treatment year (1993), the mean crown volume
index of the herb layer increased by 59% from the pre-treatment year (12.3 to 19.6
m /O.Olha). Whereas, during the same period, decreases of 24% (10.8 to 8.2 m /O.Olha)
and 17% (13.2 to 11.0 m70.01ha) were observed for the manual and cut-stump
treatments, respectively. Similarly, from 1992 to 1993, the mean volume index of the
shrub layer increased by 11% (26.6 to 29.6 m /O.Olha) within the control plantations.
The shrub layer decreased in volume by 50% (36.7 to 18.5 m3/0.01ha) and 19% (28.5 to
36
23.1 m3/0.01ha) within the manual and cut-stump treatments, respectively, during this
period.
The small and temporary decrease in abundance of understory vegetation (both herbs and
shrubs) noted in response to treatments during this study was in contrast to the significant
decreases in understory biomass reported in several studies treated with broadcast
applications of glyphosate (Pollack et al., 1990; Sullivan, 19906; MacKinnon and
Freedman, 1993; Sullivan, 1994; Simard and Heineman, 1996a; 19966; 1996c; Sullivan
et al, 1998; Whitehead and Harper, 1998; and others). This suggested that, relative to
the more commonly prescribed broadcast applications of herbicides, the species-specific
approach of cut-stump herbicide applications appeared to have minimal effects on the
understory vegetation biomass.
From the second post-treatment year (1994) to the final year of sampling (1996), both
herb and shrub crown volume indices fluctuated in a similar manner among the control
and treatment plantations presumably in response to natural successional processes.
The deciduous tree slash resulting from the vegetation management treatments likely
resulted in the short-term decrease in herb and shrub volume observed one year after
treatment. The physical obstruction caused by the deciduous tree slash may have 1)
impeded the growth of both herbs and shrubs, and 2) damaged some perennial shrubs.
The expected recovery of both herb and shrub volume observed by the second post-
37
treatment year (1994) was likely due to plants taking advantage of the increased
resources (light and moisture) created by the treatments.
Deciduous Tree Layer
Because treatments targeted the deciduous tree layer, the dramatic decrease in mean
deciduous crown volume was expected in the first post-treatment year (Figure 2). Both
the manually and cut-stump treated plantations had significantly less (Bonferroni; P =
0.05 and P < 0.01, respectively) deciduous tree volume than that of the control during this
first post-treatment year. However, rapid regrowth of paper birch and trembling aspen
trees via prolific stump sprouts occurred within manually treated plantations.
Consequently, by the second post-treatment year (1994), deciduous tree volume within
the manually treated plantations had returned to levels similar to that of the control.
Other trials have also recorded the vigorous sprouting ability of paper birch and other
hardwood species following manual cutting (Hart and Comeau, 1992; Simard and
Heineman, 1996a). As reported in other studies (Johansson, 1985; Marrs, 1985), the
treatment of cut-stumps with the systemic herbicide glyphosate limited the sprouting
behavior of hardwoods, resulting in continued suppression of the deciduous tree layer for
the four post-treatment years of this study. Relative to the volume of deciduous trees
within the control plantations, the manually treated plantations had a treatment effect
which lasted only one year.
38
My findings and those of other studies (Christensen, 1984; Lund-Heie, 1984; Johansson,
1985; Marrs, 1985; Wall, 1990), suggest that treatments with the objective of suppressing
deciduous tree species capable of growing prolific stump sprouts, such as paper birch and
trembling aspen, should employ measures to impede this sprouting behavior. My results
indicate that the application of glyphosate to cut-stumps of paper birch and trembling
aspen is an effective approach to reduce competition of these species with coniferous
crop trees. In addition, the species-specific nature of the cut-stump method appeared to
benefit the growth of coniferous crop trees and had no significant effects on other plant
community attributes, such as species richness, species turnover, species diversity, and
structural diversity.
Coniferous Tree Layer
Although the sampling methods employed during this study did not measure specific
attributes of coniferous tree growth (such as diameter at breast height and exact heights),
the mean crown volume index did measure the growth response of the entire coniferous
tree (branches and bole). Therefore, the crown volume index of the coniferous tree layer
is likely directly related to coniferous bole production, which is of primary importance to
the silviculturalist gauging the success of vegetation management treatments.
Although no statistical differences in coniferous crown volumes were observed among
control and treatment plantations, pre or post-treatment, trends in growth response during
the post-treatment years do suggest a treatment effect (Figure 2). The mean annual
39
percentage increment of coniferous tree volume (accumulation of volume) within the
control units was very gradual during the four post-treatment years (5% annual increase).
This was in contrast to the 44% and 29% mean annual percentage increment of
coniferous tree volume observed during this same period among the manually and cut-
stump treated plantations, respectively. This suggests that the coniferous tree layer did
benefit from the vegetation management treatments in terms of an increased rate of
growth.
Differences in coniferous tree growth within treated plantations may increase relative to
that of the control with time. Simard (1996a) reported that responses of Douglas-fir
height growth to vegetation management treatments were not fully expressed even nine
years after treatment. Also, a more sensitive measure to determine the treatment effects
on crop tree growth may have been possible if the sampling method had included
measurements of stem diameter. Simard (1990, 1996a) reported that stem diameter was
the most sensitive measure of early response of lodgepole pine to release from competing
vegetation.
The slight decrease in coniferous tree volume observed in the first post-treatment year
among the manually and cut-stump treated plantations can likely be attributed to a
phenomenon known as "thinning shock" (Reukema, 1964; Brix, 1981; Harrington and
Reukema, 1983; Maguire, 1983). Thinning shock, of course, does not actually reduce the
stem volume of an affected tree, but rather it temporarily reduces the rate of tree growth
due to increased exposure (e.g., sunscald) and increased physical damage (e.g., wind,
40
snow, and ice) (Harrington and Reukema, 1983). The decrease in coniferous tree cover
observed within the treated plantations during the first post-treatment year was consistent
with the effects of thinning shock.
In addition to the short-term negative effects associated with thinning shock, vegetation
management treatments may have a more serious long-term effect on a stand's
productivity. In deciduous/coniferous mixedwood stands, the potential for deciduous
trees to function as nurse trees for a coniferous crop suggests that a stand may experience
a decrease in production following treatments that suppress the deciduous tree layer (Man
and Lieffers, 1999). Several studies have indicated that deciduous trees can have positive
effects on coniferous tree growth by improving nutrient cycling, interplant transfer of
nutrients and carbon through mycorrhizae, decreasing competition, reducing pest attack,
and by offering physical protection from snow and wind (Navratil et al., 1991; Lieffers
and Beck, 1994, Man and Lieffers, 1999). These studies have all been conducted within
boreal forests and it is not clear how well their results apply to the milder conditions of
the Interior Cedar-Hemlock biogeoclimatic zone. It is clear, however, that no vegetation
management prescription should be chosen without considering the natural ecology of the
area and the non-timber resource values. In addition, the bias towards coniferous
production is not always appropriate with the growing importance of hardwoods in
today's wood products (Man and Lieffers, 1999).
41
Species Richness
Because differences in herb, shrub, and tree species richness observed among control and
treatment plantations were not significant, and time trends were similar among all
plantations, the observed fluctuations can be assumed to be associated with naturally
occurring environmental and successional events. Other studies have also reported no
significant differences in species richness among control plantations and these treated
with manual methods or glyphosate applications (Boyd et al, 1995; Simard and
Heineman, 1996a; 19966; Sullivan etal, 1996; and others).
The sampling method used during this study recorded the presence of a plant, no matter
how small a plant was. Even a plant covering less than 1% was recorded as having trace
coverage within a given sample plot. With this fine-filter approach for vegetation
sampling, a common scenario might be for a small and/or rare species to be sampled
during year 1 and 3, but not during year 2, due to mortality and later germination of that
particular species. This does not imply that this species was necessarily lost from the
site, nor should the reappearance of this species the following year suggest that this
species was gained. Therefore, it was difficult to comment on which species were lost or
gained as changes in species composition, especially for the herb and shrub layers, were
often only temporary.
42
Species Turnover
Herb Layer
The similarity in herb species turnover among control and treatment plantations, as well
as similar trends over time, suggested that herb turnover was not affected by treatments
and that changes may have been the result of naturally occurring successional events.
The number of herb species gained was always greater than species lost during the first
two or three years of the study within control and treatment plantations. The contrary
was true for the final years of the study, when more herb species were lost than gained.
This suggested that the five-year sampling period of this study may have captured a
transition period for the herb layer, perhaps changing from a community of shade-
intolerant to more shade-tolerant herbs in association with increased canopy closure of
the tree layer. The greater number of species gained early in the study may have
represented the last wave of shade-intolerant herbs invading the sites while light
conditions still permitted germination, and the greater number of species lost during the
final years of this study may have represented the loss of shade-intolerant species as light
conditions declined.
The peak in herb species richness observed midway through this study further supported
the hypothesis that the peak in herb species turnover may have been associated with a
period of overlapping herb assemblages (one shade-intolerant and the other more shade-
tolerant), as one would expect greater species richness during such transitions, before
species are lost. Schoonmaker and McKee (1988) reported similar findings during such
transitional periods in a study of secondary succession within coniferous forests of the
43
western Cascade Mountains. They described the transition period from one plant
assemblage to another as a time of unresolved competition.
Unfortunately, I can only speculate as to the cause of the changes in herb species turnover
during this study. Moreover, the lack of statistical difference, within and between years,
suggested that the observed changes in herb species turnover may have been the result of
random variation and not that of the suggested successional progression of plant species.
An analysis that groups herb species into classes of shade-tolerance, as well as a
sampling method that specifically measures canopy closure and light conditions beneath
the forest canopy, would be important for testing this hypothesis (Halpern and Spies,
1995).
Shrub Layer
The similarity in shrub species turnover among control and treatment plantations
suggested that shrub turnover was not affected by treatments and that changes may have
been the result of naturally occurring successional events. However, a weak trend of
increasing shrub turnover within control plantations and a decrease in shrub turnover
within the treated plantations over the 5-year period of this study suggested a possible
treatment effect. Number of species gained and lost within manually and cut-stump
treated plantations remained relatively low and constant throughout the study (average of
0 to 3 species lost and gained within consecutive year intervals). Whereas control
plantations experienced a slightly greater and more variable gain and loss of species
(average of 1 to 4 species lost and gained within consecutive year intervals). Perhaps the
44
treatments have maintained a more stable shrub community by delaying the canopy
closure of the tree layer. The contrary being the case for the control plantations which
may have experienced a more rapid change in shrub community due to the decreasing
light conditions associated with the closing canopy of the tree layer. The decrease in
shrub species turnover observed within both the manual and cut-stump treatments during
the final two years of this study suggested that the delay in canopy closure was only
temporary and that the shrub community within the treated plantations may have begun
to change at this point. As with the herb layer, an analysis that groups shrub species into
classes of shade-tolerance, as well as a sampling method that specifically measures
canopy closure and light conditions beneath the forest canopy, would have been useful
for testing this hypothesis.
Tree Layer
The lack of difference in tree species turnover among control and treatment plantations,
as well as among years, suggested that the treatments did not affect tree species turnover
and that observed changes were likely the result of naturally occurring successional
events.
Species Diversity
Two diversity indices were calculated (Simpson's and Shannon-Wiener) because of the
different interpretations possible from each type of index. However, because time trends
were similar for both indices within the herb, shrub, and tree layers (Figure 5),
45
interpretations of the two diversity indices would also be similar. Therefore, to avoid
redundancy, both Simpson's and Shannon-Wiener indices are discussed together.
When interpreting species diversity results, it is important to consider the two
components that define diversity: 1) the number of species sampled, or species richness,
and 2) the frequency distribution, or relative abundance, of these species. For this study,
crown volume index was used to calculate the proportions of species sampled within an
area. Therefore, crown volume index is useful for inferring information about the
proportion component of a diversity index.
Herb and Shrub Layers
The lack of statistical difference and similar time trends observed among the treatment
and control groups, pre- and post-treatment, suggested that the fluctuations in species
diversity within the herb and shrub layers may be the result of naturally occurring
successional events, and not to the treatments.
Herb species diversity began to decline in 1994, even though this was the year that
experienced the greatest species richness of any of the sample years. However, when
combined with the fact that the greatest herb volume of any of the sample years was also
observed during 1994 (Figure 2), it follows that a few herb species thrived (causing the
increased herb volume) and dominated the herb layer during this time, causing the
observed decline in herb species diversity. Domination was observed to increase from
46
the first to second post-treatment years for several herb species within control and
treatment plantations. In particular, during 1993, the combined crown volumes of
fireweed and grass made up 44, 62, and 63% of the total crown volume of the common
herbs (Table 3) within control, manual, and cut-stump treated plantations, respectively.
In the following year (1994), these same two herbs had increased in dominance and made
up 57, 80, and 67% of the total crown volume of the common herbs within control,
manual, and cut-stump treated plantations, respectively.
The shrub layer species diversity gradually declined from the pre-treatment year (1992)
to the end of the study (1996) among the control, manually, and cut-stump treated
plantations (Figure 5). Although shrub species richness was relatively constant
throughout all five sample years, shrub volume generally increased during the four post-
treatment years. This inverse relationship between shrub species diversity and shrub
volume suggested that a few shrubs became increasingly dominant, causing the observed
decline in diversity, as was concluded for the herb layer. Domination was observed to
increase from the first to second post-treatment years for several shrub species within
control and treatment plantations. In particular, during the first post-treatment year
(1993), the combined crown volumes of thimbleberry, bitter cherry, and willow made up
77, 83, and 63% of the total crown volume of the common shrubs (Table 3) within
control, manual, and cut-stump treated plantations, respectively. By the final year of
sampling (1996), these same three shrub species had increased in dominance so that their
combined volumes made up 86, 91, and 70% of the total crown volume of the common
shrubs within control, manual, and cut-stump treated plantations, respectively.
47
Tree Layer
The manual treatment increased tree species diversity while the cut-stump treatment
resulted in a decrease in diversity from the pre-treatment to post-treatment year period.
Tree species diversity within the control plantations remained relatively unchanged
during this same period. The different treatment effects on tree species diversity between
these two years (1992 and 1993) accounts for the significant Time X Treatment
interaction observed in Table 2, and was confirmed by the nonsignificant interaction
(F 6 j, 8 = 0.68; P = 0.66 and F 6 > i 8 = 0.87; P = 0.53 for Simpson's and Shannon-Wiener
indices, respectively) that resulted from a RM-ANOVA without the pre-treatment year.
Once again, the two components of diversity (number of species (species richness) and
relative proportions of these species (inferred via crown volume index)) must be
investigated to interpret and explain the observed response in tree species diversity.
Because tree species richness was relatively constant and coniferous tree volume changed
little in the first post-treatment year, the dramatic changes in deciduous tree volume were
suspected to be the cause of changes in tree species diversity within the manual and cut-
stump treatments. The dramatic reduction of deciduous tree volume observed within the
cut-stump treated plantations (95% decrease relative to pre-treatment year), while not
eliminating any deciduous species, increased the relative dominance of the coniferous
trees. Of the total crown volume index for the tree layer, coniferous trees made up 50%
during the pre-treatment year. During the first post-treatment year, the proportion of
48
coniferous trees had increased to 94% within plantations receiving the cut-stump
treatment. Moreover, during this first post-treatment year, 88% of the total tree crown
volume was made up of only 4 of the 12 tree species found within the cut-stump treated
plantations, all coniferous (Douglas-fir, western redcedar, lodgepole pine, and western
hemlock). Conifer dominance is, therefore, the most likely cause for the decline in tree
species diversity during the first post-treatment year.
Manually treated plantations also had the deciduous tree layer volume significantly
reduced (87% decrease relative to pre-treatment year). During the pre-treatment year,
76% of the total tree volume was made up of just 3 of the 11 tree species found within the
manually treated plantations, all deciduous (paper birch, black cottonwood, and trembling
aspen). During the year following treatment, these three deciduous tree species
comprised only 37% of the total tree volume. The removal of a substantial proportion of
this deciduous tree layer resulted in a more evenly distributed, and therefore, more
diverse tree layer during the first post-treatment year within manually treated plantations.
Pielou (19666) described this type of increase in species diversity (i.e., one caused by the
thinning of a dominant tree layer) as resulting from an increase in pattern-diversity.
Structural Diversity
Herb Layer
The similar structural diversity (both Simpson's and Shannon-Wiener) of the herb layer
observed among control and treatment plantations, pre- and post-treatment, suggested
49
that changes in structural diversity may have been caused by naturally occurring
successional events, and not by the treatments. The gradual decline in structural diversity
of the herb layer within control and treatment plantations was likely correlated with
increasing canopy closure of the tree layer. By removing a substantial portion of the tree
canopy, the time to canopy closure was delayed within plantations that received
treatments. The observed delay for the herb layer to reach peak structural diversity
within the manual and cut-stump treated plantations further suggested that structural
diversity of the herb layer was related to the overstory canopy closure.
I predicted that the treatments should not significantly alter the structural diversity of the
herb layer. This was because of the annual nature of herbs which allow this plant layer to
respond quickly to changing environmental conditions, growing into many of the same
height classes as it had the year before. In contrast, perennial plants such as shrubs,
because of their interdependence on the previous year's growth, were predicted to take a
little longer, as a group, to respond to new environmental conditions.
Shrub Layer
The lack of statistical difference among the control and treatment plantations for
structural diversity of the shrub layer suggested that the minor fluctuations in diversity
may have been caused by naturally occurring successional events. However, the slight
decrease and increase in structural diversity observed within treated and control
plantations, repectively, during the first post-treatment year suggested a possible
treatment effect. By felling a substantial portion of the deciduous tree layer during
50
treatment, it is speculated that the obstruction and physical damage caused to the shrub
layer (particularly the taller height classes) may have temporarily simplified the structural
diversity of this layer.
Tree Layer
As a stand of trees ages, shading created by increasing canopy closure begins to thin out
and simplify the understory height classes (Pielou, 19666). By removing a substantial
portion of the canopy, the manual and cut-stump treatments had effectively delayed the
onset of canopy closure within the treated plantations. The result was that the plantations
receiving treatments had greater structural diversity than the control plantations, although
this difference was not significant. The control plantations moved quickest towards
canopy closure, and as such, decreased in structural diversity faster than either of the two
treatment groups.
Total (herb, shrub, and tree layers combined)
Because overstory shading, to a large extent, governs what can grow in the understory,
canopy closure is a driving force that will largely determine the total (herb, shrub, and
tree layers combined) structural diversity of a stand. Therefore, as the control plantations
grew unimpeded towards canopy closure, there was a steady decline in structural
diversity throughout all five years of this study. Treatments, on the other hand,
temporarily opened the overstory canopy, and consequently the total structural diversity,
although decreasing through the post-treatment years, was statistically more structurally
diverse than that of the control in 1994.
51
Conclusions
This study suggested that conifer release from deciduous trees (capable of stump sprouts)
was best achieved with a cut-stump application of herbicide, such as glyphosate.
Methods that do not employ some means of impeding stump sprouts, such as the manual
cutting method, are not likely to maintain suppression of deciduous trees for extended
periods following treatment.
Although the cut-stump herbicide application of glyphosate significantly reduced
deciduous tree volume for four post-treatment years, this treatment had little effect on
species richness, species diversity, and structural diversity of the plant community (herb,
shrub, and tree layers). In fact, reduced dominance of the deciduous tree layer and
opening of the tree canopy created by both treatments appeared to increase the total
structural diversity (all layers combined) during the post-treatment years, relative to that
of the control.
This study appears to be the first to analyze a plant community's response to alternative
vegetation management treatments applied to young mixed-conifer plantations. Results
suggested that the species-specific approach of the cut-stump herbicide treatment should
achieve its objective of conifer release without adversely affecting the species richness,
species diversity, or structural diversity of the plant community, while suppressing the
targeted deciduous tree layer volume.
52
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Appendix 1
Alphabetical list of all of the herb species study (75 species in total). Nomenclature
that were sampled during the five years of this follows Hitchcock and Cronquist, (1973).
Scientific (Latin) name Common name Achillea millefolium L.
Actea rubra (Ait.) Willd.
Adenocaulon bicolor Hook.
Anaphalis margaritacea L.
Antennaria microphylla Rydb.
Apocynum androsaemifolium L.
Aquilegia formosa Fisch.
Aralia nudicaulis L.
Arnica cordifolia Hook.
Asarum caudatum Lindl.
Aster ciliolatus Lindl.
Aster foliaceus Lindl.
Athyrium filix-femina (L.) Roth.
Calypso bulbosa (L.) Oakes.
Car ex spp. L.
Castilleja miniata Dougl.
Cerastium arvense L.
Chrysanthemum leaucanthemum L.
Cirsium arvense (L.) Scop.
Cirsium vulgare (Savi) Tenore
Clintonia uniflora (Schult.) Kunth.
Cornus canadensis L.
Disporum hookeri (Torr.) Nicholson
Epilobium angustifolium L.
Epilobium minutum Lindl.
Yarrow
Baneberry
Pathfinder
Pearly everlasting
Rosy pussytoes
Spreading dogbane
Red columbine
Wild sarsaparilla
Heart-leaved arnica
Wild ginger
Lindley's aster
Leafy aster
Lady fern
Fairyslipper
Sedge
Common red paintbrush
Field chickweed
Oxeye daisy
Canada thistle
Bull thistle
Quene's cup
Bunchberry
Hooker's fairybells
Fireweed
Small-flowered willowherb
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Appendix 1 Continued Scientific (Latin) name Common name Epilobium watsonii Barbey Purple-leaved willowherb
Equisetum arvense L. Common horsetail
Fragaria virginiana Duchesne Wild strawberry
Galium triflorum Michx. Sweet-scented bedstraw
Geranium sp. L. Geranium
Geum macrophullum Willd. Large-leaved avens
Goody era oblongifolia Raf. Rattlesnake plantain
Gramineae spp. Grass
Gymnocarpium dryopteris (L.) Newm. Oak fern
Habenaria unalescensis (Spreng.) Wats. Alaska rein-orchid
Hieracium albiflorum Hook. White hawkweed
Hieracium aurantiacum L. Orange hawkweed
Hieracium cynoglossoides Arv.-Touv. Hounds-tongue hawkweed
Hieracium gracile Hook. Slender hawkweed
Hieracium umbellatum L. Narrow-leaved hawkweed
Lactuca canadensis L. Canadian wild lettuce
Lactuca muralis (L.) Fresen. Wall lettuce
Lactucapulchella (Pursh) DC. Blue lettuce
Lathyrus ochroleucus Hook. Creamy peavine
Lilium columbianum Hanson Tiger lily
Lupinus latifolius var. subalpinus (Piper &
Robins) Smith .TA.1. C 1 U U 1 1 I C
Medicago sativa L. Alfalfa
Melilotus alba Desr. White sweet-clover
Melilotus officinalis (L.) Lam. Yellow sweet-clover
Mentha arvensis L. Field mint
64
Appendix 1 Continued Scientific (Latin) name Common name Mitella nuda L.
Osmorhiza chilensis H. & A.
Osmorhiza purpurea (Coult. & Rose)
Suksd.
Prunella vulgaris L.
Pteridium aquilinum (L.) Kuhn.
Pyrola asarifolia Michx.
Pyrola picta Smith
Pyrola secunda L.
Ranunculus sp. L.
Saxifraga occidentalis Wats.
Smilacina racemosa (L.) Desf.
Smilacina stellata (L.) Desf.
Sonchus arvensis L.
Streptopus amplexifolius (L.) DC.
Streptopus roseus Michx.
Streptopus streptopoides (Ledeb.) Frye &
Rigg Taraxacum officinale Weber
Tiarella trifoliata var. trifoliata L.
Tiarella trifoliata var. unifoliata (Hook.)
Kurtz.
Tragopogon dubius Scop.
Trifolium pratense L.
Trifolium repens L.
Verbascum thapsus L.
Vicia americana Muhl.
Viola spp. (L.)
Common mitrewort
Mountain sweet-cicely
Purple sweet-cicely
Self-heal
Bracken
Pink wintergreen
White-viened wintergreen
One-sided wintergreen
Buttercup
Western saxifrage
False Solomon' s-seal
Star-flowered false Solomon's-seal
Perennial sow-thistle
Clasping twisted stalk
Rosy twisted stalk
Small twisted stalk
Common dandelion Three-leaved foamflower
One-leaved foamflower
Yellow salsify
Red clover
White clover
Great Mullein
American vetch
Violet
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Appendix 2
Alphabetical list of all of the shrub species that were sampled during the five years of this study (37 species in total). Nomenclature follows Hitchcock and Cronquist, (1973). Scientific (Latin) name Common name Acer glabrum var. douglasii (Hook.)
Dippel Alnus incana (L.) Moench.
Amelanchier alnifolia Nutt.
Arctostaphylos uva-ursi (L.) Spreng.
Berberis aquifolium Pursh
Ceanothus velutinus Dougl.
Chimaphila umbellata (L.) Bart.
Cornus stolonifera Michx.
Gaultheria ovatifolia Gray
Holodiscus discolor (Pursh) Maxim
Linnaea borealis (L.)
Lonicera ciliosa (Pursh) DC.
Lonicera involucrata (Rich.) Banks
Lonicera utahensis Wats.
Menziesia ferruginea Smith
Pachistima myrsinites (Pursh) Raf.
Prunus emarginata (Dougl.) Walp.
Rhamnus purshiana DC.
Ribes lacustre (Pers.) Poir.
Ribes viscosissimum Pursh.
Rosa acicularis Lindl.
Rosa gymnocarpa Nutt.
Rubus idaeus L.
Rubus leucodermis Dougl.
Rubus parviflorus Nutt.
Douglas maple
Speckled alder
Saskatoon
Kinnikinnick
Tall Oregon-grape
Snowbrush
Prince's-pine
Red-osier dogwood
Oregon wintergreen
Ocean spray
Twinflower
Orange honeysuckle
Black twinberry
Utah honeysuckle
False azalea
Falsebox
Bitter cherry
Cascara
Black gooseberry
Sticky current
Prickly rose
Baldhip rose
Red raspberry
Black raspberry
Thimbleberry
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Appendix 2 Continued Scientific (Latin) name Common name Rubus pedatus J. E . Smith Five-leaved bramble
Rubus pubescens Raf. Trailing raspberry
Salix spp. L. Willow
Sambucus racemosa L. Red elderberry
Sheperdia canadensis (L.) Nutt. Soopolallie
Sorbus sitchensis Roemer Sitka mountain-ash
Spiraea betulifolia Pall. Birch-leaved spirea
Symphoricarpos albus (L.) Blake Common snowberry
Vaccinium caespitosum Michx. Dwarf blueberry
Vaccinium membranaceum Dougl. Black huckleberry
Vaccinium ovalifolium Smith Oval-leaved blueberry
Vaccinium parvifolium Smith Red huckleberry
67
Appendix 3
Alphabetical list of all of the tree species that were sampled during the five years of this study (12 species in total). Nomenclature follows Hitchcock and Cronquist (1973). Scientific (Latin) name Common name Abies lasiocarpa (Hook.) Nutt.
Betula papyrifera Marsh.
Larix occidentalis Nutt. Picea engelmannii Parry X Picea glauca (Moench) Voss Pinus contorta Dougl. ex Loud. var. latifolia Engelm. Pinus monticola Dougl.
Populus tichocarpa T. & G.
Populus tremuloides Michx. Pseudotsuga menziesii var. glauca (Beissin.) Franco Taxus brevifolia Nutt.
Thuja plicata Donn ex D. Don
Tsuga heterophylla (Raf.) Sarg.
Subalpine fir
Paper birch
Western larch
Hybrid spruce
Lodgepole pine
Western white pine
Black cottonwood
Trembling aspen
Interior Douglas-fir
Western yew
Western redcedar
Western hemlock
68