PARTITIONING THE EFFECTS OF PLANT BIOMASS AND LITTER ON ANDROPOGON GERARDI IN OLD-FIELD VEGETATION

We examined the effects of living plant neighbors and litter on the pe rformance of a native C-4 grass, Andropogon gerardi, at five old-field sites that differ in community biomass and soil fertility. We used pl ant removal experiments in which both living neighbors and litter were manipulated in a factorial arrangement of treatments over one growing season. Andropogon was added to treatment plots as seeds and as estab lished transplants to examine the effects of the surrounding plant com munity on the recruitment and established phases of its life history. Neighbors negatively affected Andropogon performance at all sites, ind icating that resource exploitation by living plants was an important c onstraint to seedling recruitment and growth across the range of commu nity biomass examined. Plant litter negatively affected recruitment at sites with the greatest community biomass, but had no effect on the g rowth of established transplants at any of the sites. The total effect of the surrounding plant community on recruitment was positively corr elated with community biomass due to an increasing impact of plant lit ter. However, the total effect of the surrounding community on the gro wth of established transplants was unrelated to community biomass. The results suggest that it may be during the recruitment phase of the li fe history, when seeds and seedlings are especially susceptible to the effects of both litter and living neighbors, that Andropogon is most sensitive to variation in community biomass. Variation in the effects of litter on recruitment may be important in regulating plant species diversity and the distribution of native C-4 grasses along old-field b iomass gradients in southwest Michigan.