The Effects of Defoliation and Competition on Regrowth of Tillers of Two North American Mixed-Grass Prairie Graminoids

Abstract

Field defoliation experiments were employed on a mixed-grass prairie site to ascertain the tradeoff between competition and intraplant resource sharing. Tillers (ramets) of Andropogon gerardi (big bluestem) and Carex filifolia (threadleaf sedge) were subjected to various defoliation regimes under full and reduced competition. Subsequent trends in leaf biomass production, leaf exsertion kinetics, tiller mortality, and biomass of storage organs were measured over 70 days. Tillers defoliated under conditions of full competition produced significantly less leaf biomass than tillers defoliated under reduced competition. Apparent mortality of tillers defoliated biweekly was 95% under full competition and 20% under reduced competition. Andropogon tillers subjected to multiple defoliation under reduced competition produced approximately three more leaves per tiller than nondefoliated tillers or tillers similarly defoliated under full competition. All defoliated tillers had substantial reductions in biomass of storage organs relative to nondefoliated tillers. Tillers defoliated under full competition had stembase biomass reductions comparable to those of tillers defoliated under reduced competition but produced significantly less leaf biomass. The results suggest that the positive aspects of resource sharing among interconnected tillers were outweighed by the negative aspects of intraand interspecific competition. Because of the potential importance of competitive interactions on individual plant response to defoliation, care must be taken when designing field and laboratory defoliation experiments and interpreting and extrapolating their results. At the community level, herbivores may mediate competitive interactions among primary producers through the differential defoliation of plants.