Willamette
National Forest |
Stand
Structure and Vegetation Changes Five Years after Applying Three Thinning
Treatments and Control
Preliminary Results
Liane
Beggs
and Klaus Puettmann,
Oregon
State University, Corvallis, OR 97333
Introduction
As
a comprehensive and integrated long-term ecological study, the Young
Stand Thinning and Diversity Study (YSTDS) is designed to test the efficacy
of thinning, underplanting, and snag creation in accelerating development of
late-successional habitat. It has
been proposed that this combination of techniques will lead to earlier increases
in stand heterogeneity, thereby enhancing flora and fauna diversity while
maintaining timber production goals. Impacts
of these treatments on understory vegetation, chanterelles,
small mammals, amphibians, birds, and invertebrates are being studied.
This
paper reports preliminary results of understory vegetation response to
thinning four to seven years following treatments.
It is hypothesized that thinning will alter microsite conditions,
overstory stand structure, and composition, structure and diversity of the
understory vegetation. Previous
work lends support to this hypothesis (Alaback and Herman 1988, Carey and
Johnson 1995, Bailey et al. 1998, Harrington and Edwards 1999, Thomas et
al.1999, Thysell and Carey 2000). However other studies found no change in the
understory following thinning (He and Barclay 2000).
None, however, were able to track the dynamic response over longer
periods of time. Examining this
relatively early period of vegetation response will provide a foundation for
interpretation of later results.
Methods
Study
Design
The
YSTDS consists of a randomized block design comprised of four treatments within
four blocks. Study
blocks are designated as: Cougar
Reservoir (CR), Christy Flats (CF), Sidewalk Creek (SC), and Mill Creek (MC).
All blocks are located in the Willamette National Forest on the western
slope of the Cascade Range of Oregon and are composed of 40-50 year old planted
Douglas-fir stands occurring within the Western Hemlock (Tsuga heterophylla)
zone (Franklin and Dyrness 1973). Mean
annual precipitation for the area is 230 cm.
Elevation of sites ranges from 439 to 800 meters above sea level.
Each block contains four treatments: Control (C), Light Thin (L), Heavy Thin (H), and Light Thin with Gaps (LG) (see Table 1 for site characteristics of each treatment area). C treatments retained original stand densities of approximately 250 trees per acre (tpa). L and H treatments were evenly thinned to approximately 100-120 tpa and 50 tpa, respectively. LG treatments were lightly thinned to 100-120 tpa with additional cutting of randomly dispersed 0.5 acre circular gaps. Therefore, the LG treatment can be divided into 3 sub-treatments: Gap (LGG), Edge (LGE), and Matrix (LGM). All treatments were applied between 1994–1996. Table 2 displays one-year post treatment average overstory characteristics of the four treatments.
Table
1: Site
characteristics of each treatment area
Block |
Treatment |
Total
# of Subplots |
Area
(Ha) |
Stand
Age |
Elev.
(m) |
Aspect |
Slope
% |
CR |
Control |
23 |
30 |
40 |
805 |
E |
18.8 |
CR |
Heavy |
13 |
19.4 |
40 |
792 |
E |
24 |
CR |
Light |
26 |
37.2 |
38 |
610 |
E |
17.1 |
CR |
Light
with Gaps |
29* |
14.6 |
40 |
792 |
ENE |
16 |
MC |
Control |
25 |
52.6 |
42 |
902 |
SSEE |
21.1 |
MC |
Heavy |
23 |
34.8 |
42 |
658 |
SE |
22.9 |
MC |
Light |
30 |
37.2 |
43 |
524 |
S |
20 |
MC |
Light
with Gaps |
29* |
19.8 |
42 |
439 |
SSW |
8.9 |
CF |
Control |
23 |
30.8 |
39 |
878 |
SE |
6.2 |
CF |
Heavy |
15 |
20.2 |
36 |
905 |
SE |
0 |
CF |
Light |
24 |
32 |
39 |
902 |
SE |
5.3 |
CF |
Light
with Gaps |
30 |
38.9 |
40 |
905 |
SSEE |
5.3 |
SC |
Control |
17 |
51 |
37 |
634 |
N |
11.4 |
SC |
Heavy |
13 |
19 |
35 |
652 |
NW |
16 |
SC |
Light |
15 |
22.3 |
33 |
646 |
NNE |
21.8 |
SC |
Light
with Gaps |
30 |
30.4 |
39 |
671 |
N |
14.5 |
Table
2: Overstory stand characteristics one
year after thinning for all trees ≥
8 cm dbh (exception: Overstory % Cover was for all trees ≥ 5
cm dbh). (Averages across four
treatment areas.)
|
Treatment
|
Overstory
Cover (%) |
DBH (cm)
|
Conifer
DBH (cm) |
BA (m2
/ ha) |
TPH
|
Control
|
79
|
24.8
|
25.3
|
44.6
|
767
|
|
Heavy |
29
|
29.9
|
31.4
|
15.1
|
182
|
|
Light
|
50
|
29.5
|
32.5
|
24.3
|
302
|
|
Light
with Gaps |
42
|
29.0
|
32.0
|
19.8
|
247
|
Sampling
Design
“First-year”
vegetation sampling occurred during the summer of 1995, 1996, or 1997, depending
on time of harvest completion. For
ease of communication, this data will be referred to as “1997” data,
inferring it is data describing the earliest post-harvest response. Resampling was completed during the summer of 2001; the data
depict vegetation response 4-6 years post harvest.
All
vegetation sampling was conducted using 0.1
ha circular permanent plots. The
number of plots was selected to ensure sampling of 7.5% of each treatment area;
therefore the total number of plots in each treatment areas varied (see Table
1). The LG treatment areas were
an exception. Due to the variability and the desire to sufficiently sample
each of the three “sub-treatments,” each LG treatment area contained 30
plots, with 10 plots in each sub-treatment.
In each plot, overstory percent cover was
measured centrally and at four cardinal directions
(see Plot
Diagram). Overstory trees were also tallied for DBH and species.
Understory tall shrub and small tree
(diameter at breast height (dbh) < 5cm but taller than 10cm) cover was also
recorded within each plot along two 14.5m transects using the Transect
Line-Intercept method. Nested within each plot were sixteen, 0.1 m2 subplots
placed along the two 14.5 m transects (see Plot
Diagram). Percent cover of all herbs, low shrubs, graminoids and
bryophytes, as well as percentage of exposed mineral soil, coarse litter and
fine litter were recorded within each subplot.
Taxonomic nomenclature follows Hitchcock and Cronquist (1973).
Data
Analysis
All
data from the 16 subplots were first averaged to provide mean cover values for
each plot. Because
cumulative cover estimates were not taken for separate lifeform groups (Tall
Shrubs/Small Trees, Low Shrubs, Herbs, Bryophytes, and Exotics), cover estimates
for these groups represent the average combined cover for the species in each
group.
Data from all plots within a treatment were then averaged to provide mean values
for each treatment area. For the LG
treatment areas, data from plots were averaged for each sub-treatment. Sub-treatment averages were multiplied by the proportion of
each sub-treatment area in the plot to provide a weighted treatment average.
Note that due to the sampling
design, gap sub-treatment sampling is more representative of the area within
a 10.5 m radius of gap center than the entire gap and edge sub-treatment
sampling is more representative of the area 10.25 m from the gap perimeter than
the entire “edge” area. Weighted
sub-treatment averages were then added together to obtain overall averages for
each LG treatment.
Multivariate
analysis of the understory plant communities was also conducted. For
this analysis, the LG sub-treatments (LGG, LGE, and LGM) were kept separate.
This provided a total of 6 treatments per block and an overall
total of
24 treatment areas for each of the two measurement years. Cover
of understory species, mineral soil, fine litter, and coarse litter
from
each subplot as well as overstory and tall shrub/small tree species
covers were first averaged for each plot. Mean cover of each species and environmental variable in each treatment area
was then calculated by averaging the mean values of the plots.
From
the treatment area averages, two data matrices were constructed.
The species matrix consisted of 48 treatment areas (sample units) and 177
species. The environmental matrix was composed of 48 treatment areas and
categorical (treatment, block, year) and quantitative (percent cover of
overstory, understory, mineral soil, coarse litter, and fine litter) data.
Overstory, understory, coarse litter, and fine litter were used as
indicators of stand and habitat conditions for 1997 and 2001.
Because exposed mineral soil was an indicator of post-harvest soil
disturbance only the values from the 1997 data set were used.
Multivariate
community analysis was conducted using PC-ORD
v. 4.0 (McCune and Medford 1999). All
data matrices were first examined for outliers.
For the species matrix, several moderately strong outliers both among the
sample units and among the species were discovered.
Initial ordinations also displayed a high degree of noise, making
patterns difficult to detect. To
correct for this, rare species (species occurring in less than 2 sample units)
were dropped from the analysis. Dropping
these species reduced the original 48 x 177 species matrix to 48 x 124, but
failed to eliminate the outliers. In
order to reduce the influence of outliers and decrease the high amount of
variation in the data (average percent cover values ranged over 4 orders of
magnitude), a log transformation was preformed.
To retain all zero values, 0.01 (derived from McCune and Grace 2002) was
added to all values prior to transformation.
Following transformation, one sample unit, the LGG treatment from the CR
block in 1997, remained a moderately strong outlier.
This sample unit had a relatively high percent cover of Sencecio
sylvaticus, an invasive, exotic plant typical of disturbed soil. Ordinations were run with and without it.
Exclusion of the site lead to a slightly decreased emphasis on the
importance of S. sylvaticus in final interpretations and a slight shift
of sample units along the ordination axis, but overall interpretations did not
change. Therefore, this site was
included in the analysis.
Groups
defined by treatment and by year were first tested for differences using
multi-response blocked permutation procedure (MRBP; Mielke 1984).
Because it does not require distributional assumptions, this
non-parametric technique is well suited for testing for differences among groups
in species space. Euclidean
distance measure was used due to the incompatibility of the Sørensen distance
with the median alignment of blocks used in the procedure (McCune and Grace
2002).
The
test of groups defined by year was segmented into two separate analyses: a test
of difference across time of thinned treatments (excluding controls) and a test
of difference across time of only control treatments.
Excluding controls permitted shifts of the thinned treatments over time
to be revealed that were otherwise concealed by a relatively static control.
Indicator
species analysis (Dufrêne and Legendre 1997) was used to identify differences
in species composition among treatment groups and also between years.
This method calculates an indicator value (IV) from relative abundance
and relative frequency of each species in each group.
The statistical significance of this value is evaluated by comparison
with 1000 randomizations of the data using a Monte Carlo test.
To
illustrate patterns of species distribution, ordination was conducted with
non-metric multi-dimensional scaling (NMS; Kruskal1 1964, Mather 1976).
Because assumptions of linear relationships among variables and normality
are not necessary with this method, NMS is well suited for community data.
Ordination was run using the Sørensen distance measure on the PC-ORD
“slow and thorough” autopilot setting (maximum iterations=400; runs with
real data=40; stability criterion=0.00001).
Using
the species matrix, a final 3-dimensional ordination was generated.
While a 2-D solution was preferred for ease of interpretation, the high
stress of the 2-D solution (final stress=21.412) may have resulted in
misinterpretation and thus rendered it less reliable than the 3-D solution.
A Monte Carlo test of statistical significance was used to test the
validity of the final 3-D solution by comparison with 50 randomized runs
(p=0.0196, final stress= 14.043, final instability=0.00001, 80 iterations).
The
ordination was rotated to maximize correlations of environmental variables
(cover of overstory, understory, and exposed mineral soil) along a single axis,
thereby facilitating interpretation. To
illustrate the relation of treatment to the community patterns, treatment types
were overlaid on each ordination. A
vector overlay connecting sample units within the same year and block was also
used to highlight differences among blocks and time and also to display
segregation of treatments within each block.
Results
Overstory
Thinning resulted in significant changes in the overstory conditions in each treatment that persisted four to six years following thinning. In 2001, overstory cover in the H and LG treatments remained significantly lower than in the control (Figure 1; p = 0.0002). While the L treatment did not differ from the LG treatment or the control, it did have significantly more cover than the H treatment ( p = 0.0002). Though statistical tests have not yet been performed, canopy cover in all thinning treatments has increased since initial post-treatment conditions (Figure 1).
Figure 1: Average cover of overstory trees ≥ 5 cm dbh for 1997 and 2001 (averages across four treatment areas).
Understory
Tall
shrubs/small trees and low shrubs exhibited similar responses following thinning
(Figures 2 and 3). For these
groups, all thinning treatments had significantly less cover than the control in
1997, but did not differ from each other (Tall Shrubs:
p = 0.0042; Low Shrubs: p < 0.0001).
However, by 2001, none of the thinning treatments differed significantly
from each other or from the control treatment (Tall Shrubs:
p = 0.0639; Low Shrubs: p = 0.5421).
Figure
2: Average species cover of tall
shrubs and small trees for 1997 and 2001 (averages across four treatment areas).
Figure
3: Average species cover of low
shrubs for 1997 and 2001 (averages across four treatment areas).
For
bryophytes, only the H treatment had significantly less cover than the control
in 1997 (p = 0.0171; Figure 4). Thinning
treatments did not differ from each other in bryophyte cover in 1997.
By 2001, none of the treatments differed significantly (p = 0.7495).
Figure
4: Average cover of bryophytes for
1997 and 2001 (averages across four treatment areas).
Herbs
responded somewhat differently than the other lifeform groups (Figure 5).
In 1997, thinning treatments did not differed statistically in herb cover
from each other or from the control (p = 0.0763).
However, by 2001, the H treatment had significantly more herb cover than
the control (p = 0.0412). Neither
the L nor the LGG treatments differed from the control in 2001.
Figure
5: Average species cover of herbs
for 1997 and 2001 (averages across four treatment areas).
Though
cover of exotic species was low in all treatments, the LGG treatment did have
significantly higher cover of exotics than the other thinning treatments and the
control in 1997 (p = 0.0321; Figure 6). By
2001, none of the treatments differed from each other or the control.
Figure
6: Average species cover of exotics for 1997 and 2001 (averages across four
treatment areas).
In
the multivariate analysis, understory
plant communities showed significant differentiation among treatment types
(MRBP; 1997 and 2001 combined: A=0.073, p<0.001; Only 1997:
A=0.105, p=0.021; Only 2001: A=0.062,
p=0.001), as well as a weak divergence over time in the
thinned treatments (A= 0.123, p=0.055). No
difference between controls of 1997 and 2001 was present (A=0.081, p=0.1590).
Ordination
of the sample units in species space clearly illustrated the separation of
treatments and years. Of the total variation, 84 .4% was accounted for by the three
axis in the ordination. A 114°
rotation was used to maximize alignment of environmental variables along Axis 2. Accounting for 29.3% of the variation, Axis 2 (Figure
7) was strongly correlated with treatment and stand conditions, showing a
strong positive relationship to overstory
and understory tall shrub/small tree cover
of 1997 and 2001(r
= +0.626 and r = +0.627, respectively) and a strong negative relationship to
harvesting disturbance (i.e., exposed mineral soil in 1997, r = -0.584).
Overlays with the main matrix revealed that plants, such as bryophytes, Gaultheria
shallon, and Chimaphila umbellata, (r > +0.6 for all) fell on the
positive end of the second axis while plants, such as Epilobium watsonii,
Cirsium spp., and Senecio sylvaticus, fell toward the negative end (r
< -0.5 for all). Axis 3,
which was successful in capturing 36.2% of the variation, represented
differences in species composition among treatment blocks.
The CR block was heavily dominated by Oxalis oregana, Montia sibirica,
and Anaphalis margaritacea (r > +0.5 for all), while the CF
block contained high abundances of Linnaea borealis, Rubus ursinus, Smilacina
stellata, and Rubus nivalis (r < -0.5 for all), causing the two
blocks to strongly differentiate along opposite ends of the axis.
The MC and SC blocks were similar in abundance of Anemone deltoidea,
Trifolium latifolium, and Whipplea modesta, resulting in similar
scores and an intermediate positioning for the third axis.
Figure
7: Ordination of treatment areas in
species space. Vectors connect treatments within same block, same year.
'CR' = Cougar Reservoir Block; 'CF' = Christy Flats Block'; 'MC' = Mill Creek
Block; 'SC' = Sidewalk Creek Block.
Axis
1, which accounted for 21% of the total variation, illustrated compositional
shifts of the communities over time (Figure 8).
Graminoid and bryophyte species as well as species such as W. modesta,
Hieracium albiflorum, Berberis nervosa, R.
ursinus, Trientalis latifolia, and Satureja douglasii strongly
correlated to the negative range of the first axis (r < -0.4 for all), while
species such as S. sylvaticus,
Cirsium spp., Stachys mexicana, Rosa gymnocarpa, and Tiarella
trifoliata var. unifoliata fell within the positive range of axis 1
(r > +0.3 for all).
Figure
8: Ordination of treatment areas in species space. Vectors connect
1997 sites to their corresponding 2001 sites. 'CR' = Cougar Reservoir
Block; 'CF' = Christy Flats Block; 'MC' = Mill Creek Block; 'SC' = Sidewalk
Creek Block.
Indicator
species analysis revealed a distinction of species among treatments similar to
that noted along axis 2 (Table 3).
Between years, a separation of species comparable to axis 1 was also
produced by analysis of indicator species (Table 4).
Table 3: Species
representative of understory plant communities within each treatment
(* p < 0.05; ** p < 0.01)
TREATMENT |
INDICATOR SPECIES |
CONTROL |
Aster
canescens * |
|
Berberis
nervosa * |
|
Blechnum
spicant * |
|
Boykinia
elata * |
|
Chimaphila
umbellata * |
|
Goodyera
oblongifolia * |
|
Rubus
nivalis * |
|
Trillium
ovatum * |
|
Bryophytes
** |
|
Cornus
canadensis ** |
|
Asarum
caudatum ** |
LIGHT |
Acer
circinatum * |
|
Adiantum
pedatum * |
LIGHT
GAPS -- EDGE |
None
Identified |
LIGHT
GAPS -- GAP |
Cirsium
vulgare * |
|
Conyza
canadensis * |
|
Epilobium
angustifolium * |
|
Epilobium
paniculatum * |
|
Epilobium
watsonii * |
LIGHT
GAPS -- MATRIX |
Smilacina
stellata ** |
|
Linnaea
borealis * |
Table
4: Species representative of understory
plant communities within each year
(* p <
0.05; ** p < 0.01)
YEAR |
INDICATOR SPECIES |
1997 |
Aster
canescens ** |
|
Cirsium
spp. * |
|
Rosa
gymnocarpa * |
|
Rubus
parviflorus * |
|
Senecio
sylvaticus * |
|
Stachys
mexicana * |
|
Tiarella
trifoliata var.
unifoliata * |
|
Vaccinium
parvifolium |
|
Viola
spp. * |
2001 |
Achlys
triphylla ** |
|
Asarum
caudatum ** |
|
Asplenium
viride * |
|
Berberis
nervosa * |
|
Bryophytes
* |
|
Campanula
scouleri * |
|
Cirsium
vulgare * |
|
Epilobium
angustifolium * |
|
Fragaria
virginiana * |
|
Gnaphalium
microcephalum * |
|
Graminoid
spp. * |
|
Hieracium
albiflorum * |
|
Hypericum
perforatum * |
|
Lactuca
muralis * |
|
Linnaea
borealis * |
|
Polystichum
munitum ** |
|
Rhododendron
macrophyllum ** |
|
Rubus
ursinus * |
|
Satureja
douglasii ** |
|
Stachys
spp. * |
|
Symphoricarpos
mollis * |
|
Trientalis
latifolia * |
|
Violoa
oregana * |
|
Viola
sempervirens * |
|
Whipplea
modesta * |
Discussion
Thinning
of overstory trees in young Douglas-fir stands produced distinctive changes in
the understory plant communities among treatments.
Immediately following treatment, all thinned treatments had significantly
less cover of tall shrubs/small trees and low shrubs than the control,
presumably due to damage in the harvesting operation.
However, the rapid recovery of these lifeform groups indicates that this
initial decline was likely a result of harvest disturbance rather than a
response to changes in resource conditions.
While
it is likely that bryophytes also suffered from harvest disturbance, much of
their initial decline may have been due to alterations in microclimate. Thinning probably resulted in decreased soil moisture due to
increased solar radiation reaching the forest floor. Because several bryophyte species generally depend on moist
environments, this drop in moisture may have resulted in desiccation and initial
decline of cover. However, as the
canopy closes, bryophytes seem to be recovering to levels similar to those found
in the control.
As
a group, herbs seemed to respond favorably to thinning.
Unlike other lifeform groups, they did not appear to suffer significantly
from harvest disturbance nor did they exhibit initial declines due to altered
microclimate. The increase in cover
4-6 years following thinning indicates a gradual positive response likely due to
increased resource availability.
Pre-treatment
conditions, simulated by the control, harbored species more sensitive to
disturbance and indicative of shaded environments while
species typical of high light environments and disturbed soil dominated the LGG
treatments. Opening of the canopy
and thereby increasing resource availability likely permitted these more
opportunistic species to quickly colonize and displace the less competitive
species (Grime 1979). Epilobium spp.
and other similar early-successional species are capable of rapid seed
dispersal, allowing them to quickly occupy a disturbed site with abundant
exposed mineral soil and exclude other species possessing slower reproductive
mechanisms (Meier et al. 1995). However,
the strong tendency for species composition to remain relatively stable among
blocks (Figure 7, axis 3) illustrates the important
influence pre-harvest conditions hold on post-treatment development (Hughes and
Fahey 1991).
Though
inclusion of early-successional species may increase species diversity, the high
degree of soil disturbance and complete opening of canopy within the gaps did
permit limited introduction of problematic invasive and exotic species.
Within the LGG treatments, invasive and exotic species flourished as
evident by the location of the LGG sites in the lowest negative range of axis 1.
It is possible that the relatively young understory plant communities in
the pre-thinned forest were not robust enough to defend against these types of
invasions following significant disturbance (Sakai et al. 2001).
In this sense, artificial gap formation may be counterproductive to the
overall goal of accelerating late-successional species composition.
However, in this study, exotics were not dominant in any of the thinning
treatments and none differed from the control after 4-6 years.
This may indicate a lack of seed source or that exotic invasion may be an
immediate effect of thinning, but may not have long-term impacts on plant
communities.
Other
thinning treatments (H, L, LGE, and LGM) did not show a strong differentiation.
Indicator species analysis revealed only a few species demonstrating
strong associations with these treatments.
Among these, L. borealis, a plant capable of capitalizing on
heterogeneous resource availability (Antos and Zobel 1984, Spies 1991) did
strongly associate with the LGM treatment, signifying a variety of habitat
conditions within this treatment. However, within the species ordination (Figure
7) the treatments lie intermediately scattered along the second axis.
It may be that more subtle distinctions are occurring within these
treatments that are obscured by the drastic distinctions between the C and the
LGG treatments. To investigate this
possibility, future analysis may need to either exclude the C and LGG treatments
in order to magnify any potential differences or examine within treatment
variation more closely at the macroplot level.
Within
all thinning treatments, conspicuous changes in plant communities occurred
between the initial post-harvest conditions and conditions 4-6 years later.
Even within this brief post-harvest time, the decrease in treatment
effects of the four years indicates treatments may be homogenizing rather than
diversifying. This may be, in part,
to an overall shift in all treatments from pioneer species to a shrub-dominated
understory. As the overstory begins
to close in and effects of soil disturbance begin to diminish,
early-successional, shade-intolerant species are slowly being displaced by a
shrub community of Berberis nervosa, R. ursinus, and W. modesta,
along with more shade-tolerant herbs. Combinations
of these species along with “weedier” species in the 2001 sites suggest the
understory plant communities are undergoing a transition from an extreme
early-successional community structure to a more shrub dominated young–seral
composition.
Conclusions
Thinning
successfully created a range of overstory densities resulting in a variation of
understory light conditions. Preliminary
analysis of the understory vegetation response to the thinning treatments
indicates that there is subtle differentiation of plant communities among all
the thinning treatments, with the gap treatment and control being the most
dissimilar. Increased similarity of
these communities over time indicates that a transition from an
early-successional community to a more shrub-dominated community may be
occurring. Future analysis of the understory response will explore
questions such as: what are the strongest patterns, what is driving the changes,
and more in-depth examination of how the communities are changing over time.
Acknowledgments
This
study is made possible through cooperation from
Oregon State University, University
of Oregon, Cascade Center
for Ecosystem Management, Willamette
National Forest, and the
USDA Forest Service Pacific Northwest
Research Station.
We would also like to thank John Cissel, Maureen Duane, Steve Garman,
James Mayo, Brenda McComb, John Tappeiner, Gabriel Tucker and numerous others
for providing data and background information for the study.
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