Scale dependence in the species-richness-productivity relationship: The role of species turnover

Recent research in aquatic systems suggests that productivity-richness relationships change with spatial scale and that species turnover (i.e., spatial and temporal variation in species composition) plays an important role in generating this scale dependence. The generality of such scale dependence and the effects of variation in temporal scale remain unknown. We examined the extent to which the richness-productivity relationship in terrestrial plant communities depends on spatial or temporal scale and evaluated how spatial and temporal turnover (i.e., species turnover in space and time) generates scale dependence in these relationships using data from two Long-Term Ecological Research (LTER) sites (Jornada and Konza). We found a weak hump-shaped relationship (Jornada) and no relationship (Konza) between richness and productivity at the smallest focal scale (I m(2) at Jornada and 50 m(2) at Konza) at each site, but strong hump-shaped relationships at the largest focal scale (49 m(2) at Jornada and 200 m(2) at Konza) for each site. Relationships between spatial turnover and productivity at each site mirrored the productivity-richness relationships that emerged at the larger spatial scale (i.e., a significant hump-shaped pattern). In contrast, temporal turnover was unrelated to productivity, and hence increasing temporal scale did not appreciably change the form of the productivity-richness relationship. Our study suggests that the way in which productivity-richness relationships change with spatial or temporal scale depends on the form and strength of the underlying relationship between species turnover and productivity. Moreover, we contend that a dominant effect of increasing productivity is the generation of dissimilarity in species composition among localities that comprise a region, rather than increasing the number of species that occur within local communities. Thus, understanding the mechanisms that cause species turnover to vary with productivity is critical to understanding scale dependence in richness-productivity relationships.