Mechanisms of selection: Phenotypic differences among genotypes explain patterns of selection in a dominant species

Predicted changes in precipitation means and variability are expected to alter genotype composition of plant populations; however, it remains unclear whether selection will be for trait differences among genotypes or phenotypic plasticity. This is especially true for more variable precipitation patterns that simultaneously alter soil moisture means and variability. In a previous study we found that a decade of more variable growing-season precipitation patterns changed the genotypic composition of a dominant C-4 grass population (Andropogon gerardii) in native tallgrass prairie located in northeastern Kansas. Here, we assessed potential mechanisms underlying the changes observed in population structure of this species by studying how changes in both the size and variability of watering events affected ecophysiological, growth, biomass-allocation, and fitness traits of five common genotypes of A. gerardii in a greenhouse experiment. Three of these genotypes had greater abundances or were only present in field plots receiving 10 years of greater intra-annual variability in growing-season precipitation patterns. In a fully factorial experiment, we subjected the five genotypes to three water amounts (average for Kansan study site, a 40% decrease, and a 60% increase) and two watering frequency treatments (every 5 or 10 days) to produce differences in soil moisture amount and variability, respectively. We found genotype3water amount interactions for traits related to leaf-level physiology and biomass allocation; in many cases genotypes that performed better under low soil moisture conditions were outperformed by other genotypes under high soil moisture conditions. For the three genotypes that had greater abundance in field plots that received more variable precipitation patterns, we found evidence that genotypes differed in their allocation to above-vs. belowground biomass, demonstrating phenotypic trait divergence. Our results suggest that a genetically diverse population can have enough trait variation among genotypes for adaptation to occur, and thus, for dominant species, microevolution may be an important aspect of adaptation to changing environmental conditions.