An ecological assessment of core and edge populations of two dominant Great Plains grasses: Implications for climate change

Background/Question/Methods   Species with broad distributions can be characterized as having core and edge populations for those that reside in the center vs. the periphery, respectively, of the overall species range. These populations are often separated by large distances and can even be located in different climatic zones if the edge species inhabit small scale habitats that provide them with an appropriate micro-environment for success. Previous research has focused on dominant species in core populations. However, because the rate of climate change is likely to be greater than the rate of species migration, edge not core populations may be the sources of both individuals and propagules that comprise future generations. The purpose of this study was to compare core and edge populations of two dominant C4 grasses, Bouteloua gracilis (blue grama) and Andropogon gerardii (big bluestem), in a reciprocal common garden transplant experiment. Common gardens were set up in eastern Kansas at the Konza Prairie LTER and in Colorado at the Shortgrass Steppe LTER site. Physiological and phenotypic responses in core vs. edge populations, collected from each state, to differing degrees of water stress were assessed to determine if important ecological differences exist between these populations.

Results/Conclusions   Edge populations of A. gerardii were more phenologically advanced than core populations at both gardens. At edge's home garden, both populations produced similar amounts of total biomass, though edge population's reproductive percentage of biomass was double. However, core populations produced reproductive tillers that were significantly larger in both gardens. This suggests that these populations have different reproductive strategies. Edge populations appear to take advantage of resources when present and reproduce rapidly, whereas core populations develop reproductive structures more slowly and invest more resources in each tiller. In contrast, core populations of B. gracilis were more phenologically advanced in the core's home garden, and no difference was noted between populations in edge's home garden. There was no difference between populations in total biomass at either garden. However in edge's home garden, there was a significantly higher percentage of reproductive biomass. Overall, reproduction for core population of B. gracilis was similar in both gardens, but edge population performed significantly better in their home garden with regard to both total biomass and maximum height of reproductive tillers. These results suggest that for both of these dominant grass species, care needs to be taken when traits from core populations are used to forecast responses of these species to future climates.