DENSITY-DEPENDENT DEMOGRAPHY IN 2 GRASSES - A 5-YEAR STUDY

To determine the strength, frequency, and nature of density-dependent processes in natural plant populations, the densities of two perennial grass species, Bouteloua rigidiseta and Aristida longiseta, were pert urbed in an otherwise undisturbed community. Both seed additions and a dult removals, of each species separately, were used. Individual plant survival, growth, and reproduction were monitored, as well as recruit ment to the population and the numbers in each size class of each spec ies, for 5 yr. The responses to the experimental perturbations were sm all relative to the magnitude of the perturbations, indicating that de nsity dependence in demographic parameters in both populations was wea k throughout the period. The results therefore do not support the sugg estion of Fowler (1986) that the weak density dependence in the Boutel oua rigidiseta population in the ist yr of the experiment was a tempor ary result of drought. Significant responses to the density manipulati ons were scattered among annual intervals, characters, and replicates. This may be simply a result of testing responses at the limits of det ectability, or it may reflect intermittent and scattered density depen dence. Density effects were generally much smaller than other differen ces among quadrats and differences among plants within quadrats, indic ating that other factors had much more effect on a plant's fate.