Dominant Prairie Grass in Reciprocal Gardens Across the Rainfall Gradient of the US Central Grasslands: Decadal Patterns

Background/Question/Methods

Big bluestem (Andropogon gerardii) is a dominant, native, foundational grass species for forage, conservation, and restoration. This grass has a wide distribution across the Great Plains precipitation gradient (500-1200 mm rain/yr), leading to formation of wet to dry ecotypes across this gradient. Local adaptation is a fundamental phenomenon in evolutionary biology with implications for conservation, restoration, and climate change. To study local adaptation, we established reciprocal gardens in 2009 in four garden sites across this gradient (dry to wet: Colby, Hays and Manhattan, KS to Carbondale, IL) and measured the biomass and cover produced by each ecotype in plots sown with seed mixes containing each ecotype under ambient and reduced precipitation. We predicted that each ecotype would outperform in their respective home-sites compared to the foreign ecotype and a pattern of adaptation would strengthen over years. We expected strong local adaptation of an ecotype, resulting in “extension of the phenotype” and competitive dominance over the surrounding community. To examine drought effects, precipitation was reduced by 50% using rainout shelters in three sites. We also quantified cover and biomass of broad functional groups in the surrounding communities.

Results/Conclusions

Wet and dry ecotypes performed best in their home site of origin, demonstrating local adaptation to precipitation. Across all years the mesic ecotype had intermediate cover and biomass across sites, suggesting this ecotype is a generalist and can survive well across this precipitation gradient. Experimental precipitation reduction supported the pattern observed by the natural precipitation gradient. Rainout shelters resulted in decreased plant performance under rainouts of the wet ecotype in the dry site and increased cover of the dry ecotype in the wet site. These results indicate the prominent role of ecotypes across the natural and experimental precipitation gradient and strengthening of patterns of adaptation over time. Notably, local adaptation in some sites was not observed in earlier years, demonstrating the need for long-term study to detect these phenomena. Finally, strong local adaptation often resulted in competitive dominance over the neighboring plant communities, where maladaptation resulted in competitive release. These results indicate the important role of ecotypes as “extended phenotypes” in the Great Plains. Due to predicted droughts because of ongoing climate change, restoration ecologists should consider utilizing locally adapted ecotypes as tools for mitigating drought and preparing for future climate change.