Interspecific variation in plant communities is known to affect ecosystem productivity, stability, and resilience. Intraspecific variation is often due to local selective pressures, leading to local adaptation. From reciprocal gardens, Andropogon gerardii, the dominant grass species in tallgrass prairie, exhibited local adaptation to precipitation and occurred as wet and dry ecotypes in their home sites of southern Illinois (SIL ecotype) and central Kansas (CKS ecotype), respectively. Here, we characterize mechanisms of intraspecific competition between ecotypes of A. gerardii in watered and drought treatments in a greenhouse experiment. We hypothesize that: 1) because ecotypes are under strong heterogenous selection pressure to rainfall, we expect functional traits differences in wet and dry ecotypes in response to precipitation differences, 2) wet and dry ecotypes should respond differentially to drought, with dry ecotypes better able to tolerate drought, and 3) if differences between ecotypes are strong, then plants of the same ecotype should compete more with each other than with plants of a different ecotype. To test these, CKS and SIL ecotypes were grown in pots (4 plants/pot, 4 replicates, 80 pots total, 320 plants total) in mixed ecotype and single ecotype combinations for 24 weeks, under drought (watered 1/3 compared to controls).
Results/Conclusions
We report on metrics of plant performance including height, number of tillers, blade width, flower timing, SPAD, and photosynthesis, and end-of-season biomass (vegetative and seeds). Flower timing and height showed significant ecotype and drought effects. CKS flowered significantly earlier compared to SIL. Drought slowed flowering regardless of ecotype. Drought reduced height, disproportionately in SIL, suggesting the wet ecotype is more sensitive to drought. Several trait responses (blade width, flowering stalk diameter, and number of tillers) showed a significant ecotype effect, but no drought effect. SIL had wider leaves, greater stalk diameter and fewer tillers compared to CKS. CKS showed enhanced photosynthesis and SPAD under drought compared to SIL. For the ecotype neighbor effect, for most response variables, no effect of identity of neighboring ecotype (same or different ecotype) was evident, at least after 6 months, although longer timescales may be needed to elucidate responses. Combined, these results suggest that ecotypes of Andropogon gerardii have functionally different growth strategy tradeoffs. This work provides additional evidence that local selection pressures have resulted in local adaptation and ecotypic differences in big bluestem.