Abstract:

Within-species genetic and phenotypic variation have well-known effects on evolutionary processes, but less is known about how within-species variation may influence community-level processes. Ecologically meaningful intraspecific variation might be particularly important in the context of anthropogenic impacts on natural systems, such as agriculture and species invasion, because human actions can cause strong selection pressures.

In a greenhouse study, we explored intraspecific (30 accessions) and ecotypic variation (representing agricultural and nonagricultural habitats) in biomass and rhizome production in response to inter- and intraspecific competition and soil fertility of Johnsongrass (Sorghum halepense), a widespread invasive species and agricultural weed.

Contrary to our expectations and previous results, we did not find variation in biomass production among Johnsongrass ecotypes at this early life stage. However, we did find that Johnsongrass biomass varied substantially depending on competitor identity, soil fertility treatments and among accessions. Rhizomes were 11% larger in the agricultural ecotype and up to 3-fold larger in fertilized treatment; while rhizome biomass increased by ~50% when fertilized, but did not differ among ecotypes. Interestingly, in competition, Johnsongrass produced 32% less biomass and 20% less rhizome mass with a conspecific than when competing interspecifically with corn. Our results indicate species-specific competitive responses and changes in rhizome allocation in response to neighbor identity; suggesting the possibility of adaptation by Johnsongrass to shift allocation under competition.

Key words: adaptation, agriculture, competition, exotic invasion, Johnsongrass, selection, Sorghum halepense