Print PageNiche differences are thought to allow coexistence and maintain diversity within communities (species coexistence) and populations (genotype coexistence) through niche complementarity. Looking at competitive interactions between genotypes across a combination of resource limitations gives us insight into how above- and/or below-ground niche differentiation maintains intraspecific diversity. In the tallgrass prairie, we have observed variation in abundance among naturally co-occurring genotypes of the dominant C4 grass species, Andropogon gerardii, in an annually burned site, with a few very common and a number of uncommon genotypes. We conducted a greenhouse study in 2010 with the 4 most common genotypes found in the field and competed them in pair-wise combinations under all combinations of high and low resource conditions (water, nitrogen, and light). We measured height, leaf area, and growth rate for each individual monthly throughout the growing season (May-August) and root and shoot biomass, root:shoot ratio, number of vegetative stalks, and bolting at the end of the growing season. Our objectives were to 1) determine if there were differences in phenotype between genotypes and 2) determine if these phenotypic differences explain mechanisms by which naturally co-occurring genotypes coexist, thus advancing our understanding of how diversity is maintained in this dominant species.
Results/Conclusions
Early in the growing season, we see significant differences between genotypes in aboveground traits, such as growth rate, height, and biomass. However, at the end of the growing season, we predominantly see convergence in aboveground traits and instead see significant differences between genotypes in belowground traits (root biomass). Furthermore, differences between genotypes in root:shoot ratio indicates that certain genotypes have different strategies when it comes to investing in above versus belowground biomass. Not surprisingly, we find that light availability has a strong overriding effect on all genotypes, where all genotypes have low growth rate in low light conditions and underlying differences between genotypes in all above and belowground traits are held constant. When given ample light, water availability drives differences between genotypes in root biomass and vegetative output. Overall, competitive hierarchies of which genotypes have the greatest aboveground biomass, height, and vegetative output differ between low and high light conditions, while water availability drives differences in belowground traits such as root biomass and root:shoot ratio. Together, these results give us important insight into how different resource limitations mediate coexistence between common genotypes of a dominant species, which has implications on the functioning of the surrounding community and ecosystem.
