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Effects of Stress on Community Assemblages in Boreal Forests
Andrew Barta, Ellen Hermanson, Matthew Jacobson, Tessa Wirz, Evan Weiher,
and Faculty Mentor Todd Wellnitz
Results
Introduction
• Two types of plant community assemblage have been studied: stochastic,
or random, assemblage and niche-based assemblage.
• In plant communities with a high stress filter, niche assembly becomes
stronger over stochastic assembly because fewer species are able to
tolerate the conditions. Those that can tolerate these conditions are
similar in composition and have established their niche as a community in
the ecosystem.
These graphs condense multiple dimensions into one 2D graph in which units are not necessary. It is best to imagine it similar to a
shadow on the wall and as you rotate your hand, the 2D image on the wall changes. These graphs represent the view of the multiple
dimensions that best show the factors that “pull” on the data. Sites are represented by circles and individual species are represented
by crosses. Sites that share the most species will be physically closer on the graph than those that share few or no species.
• Non-metric Multidimensional Scaling (NMDS) shows how
similar sites are based on all data collected.
• The seven sites with the lowest slope are circled in green.
These sites have a wide range meaning they are less similar
and have more variation. This pattern indicates that
stochastic assembly is more important in low slope sites than
in high slope sites.
• In low stress communities, stochastic methods of assemblage are
stronger than in high stress communities because more species are able to
tolerate these mild conditions and are therefore more varied in
composition (Chase, 2007).
•The seven sites with the highest slope are circled in red.
These sites are much closer together on the graph. This means
that they are more similar and have less variation. In high
slope sites more emphasis is placed on niche selection than in
low slope sites.
Chase, J.M. “Drought mediates the importance of stochastic community assembly.” PNAS Oct 30 2007: Vol 104 no. 44 pp. 17430-17434.
Low
Strength of stochastic assemblage
over a stress gradient
Methods
• Randomized transects were used to generate random plots from which
we gathered vegetation data on low and high slope sites.
• All herbaceous plants within a one
meter radius were identified to species,
while shrubs, vines and trees were
identified to species within a five meter
radius from the center of the
herbaceous plot.
• Species abundance was determined
by estimating percent coverage within
the plot by each species
• Slope, aspect and soil profiles (at greatest practical depth), were
taken and recorded for each plot.
•The base of the triangle at the head of the aspect vector arrow shows
there is more variation in species on a southern aspect. The point of the
triangle further down the aspect arrow shows less variation on a northern
aspect.
•High slope and/or north aspect sites with little variation have more
emphasis on niche selection while low slope and/or south sites show more
stochastic assembly.
•Since the soil depth vector is opposite the slope vector this implies an
inverse interaction; as slope increases soil depth tends to decrease.
• Box plot showing the
differences in variance in the
first non-metric
multidimensional axis between
high and low slope sites.
2
Boundary Waters Canoe Area Wilderness,
located in northern MN, where our data was
collected.
•The point of the triangle at the head of the slope vector arrow shows the
similarity of composition between sites at high slope, with greater
variation at low slope.
0
Stress
-2
http://commons.wikimedia.org/wiki/File:Bwca_map.png
-4
Low
-6
High
• Constrained Correspondents Analysis (CCA) allows us to choose specific
vectors (represented by blue arrows) to get a better idea of their effect.
The direction the arrow is pointing represents a high value (high slope),
while the opposite direction represents decreasing values (low slope).
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Niche
Stochastic
High
high
low
•Low slope communities
showed greater variance than
high slope communities (F =
0.0865, df = 6, p-value =
0.008933).
Discussion
• Our results show that an environmental
stressor such as slope changes the strength
of niche or stochastic assembly patterns in
plant communities.
•Furthermore, it shows that this relationship
is still valid in a natural setting that has
assembled over a long time period.