COS 284-1 - Dominant Prairie Grass in Reciprocal Gardens across the Rainfall Gradient of the US Midwest: Decadal Patterns of Adaptation and Trait Variation in Response to Drought

Abstract: Drought is one of the most severe abiotic stresses resulting in the greatest losses in plant productivity. Many studies have addressed community and ecosystem responses to drought. However, little is known about intraspecific variation in plants responding to drought. We used wet, mesic, and dry grass ecotypes to test the role of intraspecific variation on plant function across the rainfall gradient. Our focal species, big bluestem (Andropogon gerardii), is the dominant grass that is important for cattle forage, conservation, and restoration. It has a wide distribution across the Midwest rainfall gradient (500-1200 mm rain/yr) giving rise to ecotypes adapted to regional rainfall. Reciprocal gardens were established in 2009 in an ecological community and measured for over a decade. Cover and biomass were monitored over time in four sites (driest to wettest: Colby, Hays and Manhattan, KS to Carbondale IL). To examine drought, rainfall was reduced by up to 50% using rainout shelters in three sites. For single plants, height, blade width, SPAD, photosynthetic rate, biomass, and reproductive phenology were measured. At the plot-level, we measured cover and biomass. The objectives were to 1) characterize functional traits of A. gerardii ecotypes across the natural gradient, 2) measure functional trait response to experimental drought using rainouts and 3) determine the impact of local adaptation on the surrounding community. First, we expected: local adaptation, or home-site advantage, of A. gerardii ecotypes in response to rainfall; traits aiding in drought tolerance would be observed in the dry ecotype and in dry sites; experimental drought would delay reproductive phenology. Finally, strong local adaptation of the A. gerardii ecotype would result in competitive dominance over the surrounding community. At the plant-level, the dry ecotype had higher photosynthetic rates and SPAD, narrower blade width, and shorter height compared to the wet ecotype. Experimental drought delayed and reduced flowering of all ecotypes and decreased biomass. At the plot-level, wet and dry ecotypes had highest cover and biomass in their home site, demonstrating local adaptation. Rainouts increased cover and biomass of the dry ecotype in the wet site, demonstrating its adaptation to low rainfall. Lastly, the local ecotype became dominant over the surrounding plant community whereas low cover of the non-local ecotype resulted in competitive release of the other forbs and grasses. These results indicate the deterministic role of ecotypes across a rainfall gradient on community assembly and informs the use of climate-adapted ecotypes in anticipation of future droughts.