Comparison of Reconstructed and Remnant Prairies
Perhaps the most important question in restoration ecology is, how well can we
restore a native ecosystem? To address this question, we have been comparing
restored
prairies to remnants. We
are comparing plant species diversity, net primary productivity, and community
stability at several different paired sites in Iowa and Texas. Results so far
suggest that restorations are able to establish the common native species and
reduce exotic biomass to remnant levels (Polley et al. 2005, Martin et al.
2005). However, species diversity (especially species richness) at all scales
remains well
below remnant levels (Polley et al. 2005, Martin et al. 2005). The proportion
of beta diversity can be lower (Polley et al. 2005) or higher (Martin et al.
2005) in restorations compared to remnants, depending on the amount of
topographic relief, seeding regimes and patch size in recruiting plants.
Current
research (with Leanne Martin) is testing whether low
diversity in restorations is caused by seed limitation or by a lack of
disturbance by native ungulates (bison and elk) at Neal Smith National Wildlife
Refuge. Results suggest that seedling emergence of rare species can be
increased with a combination of seed additions and grazing.
Prairie remnant (foreground and right background) and red cedar invasion of old
fields (left background) during winter in Texas Blackland prairie region. See
Polley et al. (2005) and Polley et al. (2007) for further details.
Native cover crops and community assembly
We are conducting an experiment that
compares how prairie communities assemble and how species diversity varies
among plots as a function of early-emerging plant species composition.
Historical effects, dispersal limitations, or timing of disturbance can cause
very different species to be initially present among patches or fields. We are
testing how these differences in
species identity will affect the outcome of community development. Community
assembly theory predicts that different
species compositions will be found depending on which species is present
initially, and predicts that alternate stable states will occur.
Alternatively,
succession theory predicts that
species composition will not vary across initial species treatments, and that
species composition will converge on a small subset of species best adapted to
the climate and soil conditions of the site. Neutral theory
predicts that species composition will closely match the seed-mix composition.
If differences in species composition are found among treatments, it would be
important because this type of patchiness is what underlies beta diversity,
which is expecially high in tallgrass prairies (Wilsey et al. 2005). An
experiment was established in 2004 that varied the identity of the early
emerging species. Species treatments were black-
eyed susan Rudbeckia hirta, Canada wildrye Elymus canadensis,
partridge pea Chamaecrista fasciculata, Illiniois bundleflower
Desmanthus illinensis, or side-oats
grama Bouteloua curtipendula, as well as a control (no initial species
present). These species emerge early and have the potential to impact
species diversity as prairie communities develop. Species were allowed
to establish in replicated plots for one growing season before a prairie seed
mix of 27 species was added. Data from our experiment will help differentiate
among these important alternative hypotheses for how prairies establish.
Furthermore, our results will provide one of the first experimental tests of
whether native cover crops can help to establish prairies.
Prairie plots containing Rudbeckia hirta left or control right.
