Reorganization of understory diversity over three decades
in an old-growth cool-temperate forest
SUMMARY:
4. Fine-scale understory diversity loss may be driven by
competitive sorting. Woods (2007) suggests that declining
canopy α-diversity in the same forest is due to
competitive dynamics following rare disturbances. Longlasting influence of disturbance on both diversity in both
strata has implications for conservation and management.
Canopy composition has been monitored since 1935
on 248 fifth-acre circular plots at the 100-ha, neverlogged Dukes RNA (Hiawatha National Forest)
(Woods 2009). Canopy composition (below, for 19781980) varies with soil properties: Thuja and Fraxinus
dominate on swampy peats; Acer saccharum on welldrained calcareous soils; Tsuga on deep sands; and
mixed hardwoods (with Fagus) and Tsuga where
hardpans impede drainage (right). Total plot basal area
ranges from 18.9 to 93.0 m2/ha; the overall average is
39.6 m2/ha.
North
12
2
2
Pie charts (left) show proportional basal area by species in 1978-1980 for
permanent plots at the Dukes RNA, MI. Area of circles is proportional to total plot
basal area (max = 93.0 m2/ha). Dominance is spatially patchy and corresponds to
soil types; relief is slight. Plots with mixtures of Fagus, Acer, Tsuga are assigned
to the ‘mixed’ type in figures at right; Thuja and Fraxinus-dominated sites to
swamp type. Four 1-m2 understory quadrats are placed in each permanent
canopy plot
6
4
1979
2004
1992
1979
2004
0
120
Ephemeral richness, 1 m
8
2
4
3
2
1979
1
2009
0
50
100
40
80
Ephemeral cover (%)
Dukes RNA habitats: Tsuga-dominated (upper left), Acer
saccharum-dominated (lower left), Thuja-dominated swamp forest
(upper right), canopy of Acer-dominated stand (lower right).
Canopy height averages about 30 m.
We calculated changes in average cover and frequency
for individual species and examined patterns with
respect to original frequency, a niche-breadth index
(Fridley et al. 2007), and known sensitivity to deer
browsing (Balgooyen and Waller 1995, Kraft et al. 2004,
Heckel et al. 2010, and others).
East
Richness in 1-m2 quadrats decreased significantly (about 20% on average)
between sample periods, for all habitats. Richness losses in maple-dominated
sites were limited to the spring ephemeral guild. Total cover increased
significantly in swamp forests and remained unchanged in other stands.
10
Cover estimates were recoded to class mid-points. Some
difficult Carex species were pooled by Section, but most
ambiguous taxa were confidently identified as distinct
within a quadrat, and so retained for diversity calculation.
For each quadrat and each sample period, we calculated
species richness (S), Shannon diversity index (H), and
total covver. Tree seedlings were not included.
200 m
Hypotheses for loss of understory diversity include:
1) competitive, trophic, or indirect interactions with invasive species (e.g.,
Yurkonis et al. 2005 for invasive plants; Holdsworth et al. 2007 for
earthworms). However, invasive plants and earthworms are rare at this site.
2) intensified deer herbivory (e.g., Rooney et al. 2004). However, species
documented as tolerant of or susceptible to deer herbivory do not show distinct
patterns of change in frequency or cover (below left).
3) cimate change. We can’t exclude this possibility, but nearly all species
increase in cover in quadrats where present (below right) and species of
northern affinity do not show disproportionate frequency decreases.
4) changes in canopy influence due to increased dominance by evergreen or
dense-canopied species (Tsuga, Thuja and Fagus) (Woods 2007). This, may
be inconsistent with increasing cover for nearly all species in quadrats where
present, and loss of diversity in little-change A. saccharum stands.
5) long-persisting response to historical disturbance. Woods (2007) attributes
loss of canopy alpha-diversity to long-lasting competitive sorting following rare
disturbances. Competition among understory species may lead similarly to
intensified patchiness and reduced point diversity at finer scales long after
disturbance. If this is the case, conservation of local diversity in all strata may
be dependent on rare disturbances that reset competitive dynamics.
Understory assemblages in Tsuga-dominated
(top right) and Acer-dominated (bottom right)
stands have distinct composition but similar
diversity when ephemerals are included.
In 1978-80, JS established four 1-m2 quadrats, 15 and 35
ft E and W of plot center in each canopy plot and
estimated cover for all species. In 1992-94, DH
remeasured quadrats in 112 plots in upland forests. KW
remeasured understory quadrats in upland plots and 76
plots in peat swamps in 2002-2007. Spring ephemerals
were sampled in most upland plots in 1978-80 (JS) and
again in 2009 (KW). All subsequent analysis concerns
only quadrats sampled in at least two of these periods.
Acer saccharum
Fagus grandifolia
Betula alleghaniensis
Tsuga canadensis
Acer rubrum
Thuja occidentalis
Fraxinus nigra
other
Understory composition varied with habitat and canopy composition (right).
Both species richness (below) and Shannon diversity (not shown) for squaremeter understory quadrats were highest in swamp stands. When ephemeral
samples (below right) were pooled with other understory species, richness did
not vary significantly across upland habitats within sample years. Total
understory cover was also similar across habitats.
Diversity losses were not an artifact of taxonomic inconsistencies; no species
from earlier samples were lacking in later samples, and several species were
unique to the most recent sample dates. In upland stands, diversity was
generally intermediate at the middle sample period (1992-1994)
I. Study Site and Data-Set
Since 1935, canopy diversity decreased within plots.
Shade-tolerant species (Tusga, Fagus, Acer
saccharum, Thuja) increased in dominance) while
other species – particularly Betula alleghaniensis and
Fraxinus nigra -- declined markedly (Woods 2000,
2007). Trends suggest slow, local competitive sorting
in response to intermediate disturbances with return
times on the order of centuries (Woods 2004)
III. Accounting for Diversity Loss
Species richness (S), 1 m
2. Over 25 years, understory richness in 1-m2 plots
declined by ~20% across a range of habitats and
communities, while total cover remained unchanged or
increased. At coarser scales diversity changes were
smaller or undetectable.
3. Most species declined in frequency, but not in cover.
Frequency change was unrelated to initial abundance,
ecological amplitude, or vulnerability to deer. Nonindigenous plants and invasive earthworms are largely
absent.
II. Results: Alpha-diversity Loss, Increasing Cover
Total cover (%)
1. Diversity is concentrated in the understory in temperate
forests, but understory diversity patterns in time and
space are not well-documented, particularly for oldgrowth forests. We use multi-decade permanent plots to
assess the stability and spatial structure of understory
diversity in a well-studied old-growth tract in MI, USA.
Kerry D. Woods, Bennington College, Bennington VT
David J. Hicks, Manchester College
Jan Schultz, USDA Forest Service
60
40
20
0
Of 42 species with more than 50 total quadrat occurrences, nearly all showed decreasing
frequency and increasing cover in 1-m2 quadrats where present. Species thought to be particularly tolerant of or susceptible to deer herbivory did not show distinctive patterns of change.
30
20
10
Acknowledgements:
0
swamp
n=41
transition
n=27
hemlock
n=21
mixed
n=29
maple
n=67
mixed
n=15
maple
n=55
Species richness in 1-m2 quadrats decreased over time (t-test, p<0.05) in all canopy types; total understory cover increased in some habitats.
Box plots for each canopy/habitat type and sampling period show mean values (solid line), median (dotted line), middle two quartiles (box),
and 5th and 95th percentiles (whiskers) ‘n’ indicates number of canopy plots; each plot includes four 1-m2 understory quadrats. Left-hand plot
includes all non-tree species except spring ephemerals. Spring ephemerals were sampled separately (right). Changes in maple-dominated
plots are significant only with inclusion of ephemerals.
Diversity change depended on spatial scale. Richness
decreases were smaller but significant (except for mapledominated statnds) when four 1-m2 quadrats were pooled
within canopy plots, and changes in Shannon diversity
were not significant (not shown).
Species accumulation curves (left), assembled by pooling
1-m2 quadrats within habitat types (PC-Ord V5.33),
showed no significant differences between sampling
periods for areas > 4-6 m2 (based on 95% confidence
intervals) and directions of change were inconsistent.
Species accumulation curves derived by repeated pooling of 1-m2
quadrats (PC-Ord 5.3) show no significant difference between
sample dates for areas greater than 4-6 quadrats pooled.
Research was funded by grants from the National Science Foundation, The Andrew J. Mellon Foundation, United
States Forest Service, the Huron Mountain Wildlife Foundation, and Bennington College. Analyses presented here
were conducted while KW was a Center Fellow at the National Center for Ecological Analysis and Synthesis, a
Center funded by NSF (Grant #EF-0553768), the University of California, Santa Barbara, and the State of California.
Original (1978-80) data-collection was by JS and associates from the U.S. Forest Service. Resampling from 1992
through 2009 was by KW and DH, with assistance from many Bennington College undergraduates.
References:
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A pdf of this poster may be downloaded at fac.bennington.edu/~kwoods