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Climate change impacts on population dynamics in tallgrass prairie: implications for species codominance




Gray, Jesse E., author
Smith, Melinda D., advisor
Knapp, Alan K., committee member
Ocheltree, Troy W., committee member

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Two grass species, Andropogon gerardii and Sorghastrum nutans, together account for the great majority of individuals, biomass, and possibly genetic diversity in plant communities of the tallgrass prairies of the Great Plains, US. As competitors with similar functional traits and what appears to be overlapping niches, it is not clear what mechanisms facilitate their co-dominance, but it may rely on the high variability of environmental conditions that characterize grassland ecosystems. Because these abundant grasses strongly influence plant community structure and ecosystem function, it is critical that we understand the factors influencing the population dynamics of these species, and how climate change might alter those relationships. We found an asynchrony in population dynamics in which A. gerardii begins each growing season at higher tiller densities, with attrition of tillers starting mid-season. Concurrent gains of S. nutans tillers results in A. gerardii becoming the less abundant by the end of most growing seasons. We hypothesized that this differentiation in tillering strategies causes each species to be vulnerable to unfavorable environmental conditions during different parts of the growing season, thus enabling their coexistence by preventing an inter-annually consistent competitive advantage of either species. We found that greater tiller density asynchrony was associated with higher population densities of S. nutans and of aggregate tiller densities of both species. Experimental increases in temperature and rainfall variability reduced population-level asynchrony while exacerbating population declines and overall community productivity, suggesting this mechanism of co-dominance may rely on current levels of environmental variability, and may be vulnerable to projected increases in that variability with climate change.


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