Patterns of species density and productivity at different spatial scales in herbaceous plant communities

A major challenge in evaluating patterns of species richness and productivi ty involves acquiring data to examine these relationships empirically acros s a range of ecologically significant spatial scales. In this paper, we use data from herb-dominated plant communities at six Long-Term Ecological Res earch (LTER) sites to examine how the relationship between plant species de nsity and above-ground net primary productivity (ANPP) differs when the spa tial scale of analysis is changed. We quantified this relationship at diffe rent spatial scales in which we varied the focus and extent of analysis: (1 ) among fields within communities, (2) among fields within biomes or biogeo graphic regions, and (3) among communities within biomes or biogeographic r egions. We used species density (D = number of species per m(2)) as our mea sure of diversity to have a comparable index across all sites and scales. A lthough we expected unimodal relationships at all spatial scales, we found that spatial scale influenced the form of the relationship. At the scale of fields within different grassland communities, we detected a significant r elationship at only one site (Minnesota old-fields), and it was negative li near. When we expanded the extent of analyses to biogeographic regions (gra sslands or North America), we found significant unimodal relationships in b oth cases. However, when we combined data to examine patterns among communi ty types within different biogeographic regions (grassland, alpine tundra, arctic tundra, or North America), we did not detect significant relationshi ps between species density and ANPP for any region. The results of our anal yses demonstrate that the spatial scale of analysis - how data are aggregat ed and patterns examined can influence the form of the relationship between species density and productivity. It also demonstrates the need for data s ets from a broad spectrum of sites sampled over a range of scales for exami ning challenging and controversial ecological hypotheses.