Long-term responses of the grassland co-dominants Andropogon gerardii and Sorghastrum nutans to changes in climate and management

摘要

Understanding responses of both dominant and sub-dominant plant species to management regimes and climatic variables is a critical component of understanding an ecosystem's potential response to local or regional changes in such parameters. The dominant vegetation of much of the North American tallgrass prairie is composed of C 4 grasses. As the most abundant species, Andropogon gerardii has been well characterized ecologically. The second most abundant, co-dominant species, Sorghastrum nutans, has received less attention, mainly because it has been considered the "ecological equivalent" of A. gerardii and is assumed to have responses similar to those of A. gerardii. The objective of this study was to assess the validity of this assumption by examining long-term abundance patterns and responses to climate, fire, and grazing for both species. Based on 15 years (1983-1997) of plant species composition and cover data collected at Konza Prairie Biological Station in northeastern Kansas, USA, we were able to detect important differences between these two grasses. Regression analysis showed that long-term reductions in the cover of A. gerardii were correlated with decreasing summer maximum temperatures, whereas inter-annual variations in the cover of S. nutans were related to annual precipitation. Additionally, the cover of S. nutans increased with increasing fire frequency, while the cover of A. gerardii was not significantly affected by fire frequency. Bison grazing did not significantly affect the cover of either species studied. The mean cover of A. gerardii decreased significantly over the 15 years, but only a slight change was noted in the cover of S. nutans. The decrease in A. gerardii corresponded to increased plant species diversity and evenness and has the potential to impact the tallgrass prairie ecosystem from population through ecosystem levels. The observed differences in these co-dominant species may prove important under climate change scenarios predicted for the tallgrass prairie of North America.