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 perfor- mance of a native C4 grass, Andropogon gerardi, at five old-field sites that differ in com- munity biomass and soil fertility. We used plant 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 established transplants to examine the effects of the surrounding plant community on the recruitment and established phases of its life history. Neighbors negatively affected Andropogon performance at all sites, indicating that resource exploitation by living plants was an important constraint to seedling recruitment and growth across the range of community biomass examined. Plant litter negatively affected recruitment at sites with the greatest community biomass, but had no effect on the growth of established transplants at any of the sites. The total effect of the surrounding plant community on recruitment was positively correlated with community biomass due to an increasing impact of plant litter. However, the total effect of the surrounding community on the growth of established transplants was unrelated to community biomass. The results suggest that it may be during the recruitment phase of the life history, when seeds and seedlings are especially susceptible to the effects of both litter and living neigh- bors, 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 C4 grasses along old-field biomass gradients in southwest Michigan.

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