Fertilization effects on species density and primary productivity in herbaceous plant communities

Fertilization experiments in plant communities are often interpreted in the context of a hump-shaped relationship between species richness and product ivity. We analyze results of fertilization experiments from seven terrestri al plant communities representing a productivity gradient (arctic and alpin e tundra, two old-field habitats, desert, short- and tall-grass prairie) to determine if the response of species richness to experimentally increased productivity is consistent with the hump-shaped curve. In this analysis, we compared ratios of the mean response in nitrogen-fertilized plots to the m ean in control plots for aboveground net primary productivity (ANPP) and sp ecies density (D; number of species per plot of fixed unit area). In genera l, ANPP increased and plant species density decreased following nitrogen ad dition, although considerable variation characterized the magnitude of resp onse. We also analyzed a subset of the data limited to the longest running studies at each site (greater than or equal to 4 yr), and found that adding 9 to 13 g N m(-2) yr(-1) (the consistent amount used at all sites) increas ed ANPP in all communities by approximately 50% over control levels and red uced species density by approximately 30%. The magnitude of response of ANP P and species density to fertilization was independent of initial community productivity. There was as much variation in the magnitude of response amo ng communities within sites as among sites, suggesting community-specific m echanisms of response. Based on these results, we argue that even long-term fertilization experiments are not good predictors of the relationship betw een species richness and productivity because they are relatively small-sca le perturbations whereas the pattern of species richness over natural produ ctivity gradients is influenced by long-term ecological and evolutionary pr ocesses.