Diversity loss, recruitment limitation, and ecosystem functioning: lessonslearned from a removal experiment

A five-year removal experiment in which plant functional group diversity wa s manipulated found strong limitation of ecosystem functioning caused by th e differing abilities of remaining functional groups to recruit into space left unoccupied by the plants removed. We manipulated functional group dive rsity and composition by removing all possible combinations of zero. one, o r two plant functional groups (forbs, C-3 graminoids, and C-4 graminoids), as well as randomly chosen biomass at levels corresponding to the functiona l group removals, from a prairie grassland community. Although random bioma ss removal treatments showed no significant effect of removing biomass in g eneral on ecosystem functions measured (P > 0.05), the loss of particular f unctional groups led to significant differences in above(P<0.001) and below ground (P<0.001) biomass, rooting-zone (P=0.001) and leached (P=0.01) nitro gen, nitrogen miniralization (P<0.001), and community drought resistance (P = 0.002). Many of these differences stemmed from the marked difference in the ways remaining functional groups responded to the experimental removals . Strong recruitment limitation of C-4 graminoids resulted in large areas o f open ground, high nutrient leaching. and high community drought resistanc e in plots containing just this functional group. In contrast, rhizomatous C-3 graminoids quickly colonized space and used soil resources made availab le by the removal of other groups, leading to lower soil nitrate in plots c ontaining C-3 graminoids. These effects of recruitment limitation on ecosys tem functioning illustrate possible effects of diversity loss not captured by synthetic experiments in which diversity gradients are created by adding high densities of seeds to bare soil.