Dependency of diversity-productivity relationships on ecotypes of the dominant species across a precipitation gradient

Locally dominant and highly productive species affect diversity through competitive interactions with subordinate species. The relationship between diversity and productivity is commonly unimodal and referred to as the ‘humped-back model’ (HBM). The effect of local adaptation in dominant species on occurrence of the HBM is unknown, particularly among the different aspects of diversity. A reciprocal transplant experiment established across a precipitation gradient in the U.S. Great Plains was used to investigate the effect of variation among ecotypes of the dominant prairie grasses Andropogon gerardii (big bluestem) and Sorghastrum nutans (Indian grass) on expression of the HBM. The information from taxonomic, phylogenetic, and functional diversity metrics on diversity–productivity relationships was compared. The HBM was most common for taxonomic diversity (57% of tests), and when locally sourced (compared with distantly sourced) ecotypes were established in the transplant experiment, reflecting a home-site advantage (local adaptation), especially at the extreme ends of the precipitation gradient. Phylogenetic and functional trait diversity exhibited a less frequent humped-back relationship with productivity (26% and 13%, respectively). The HBM has value in providing insights into diversity-productivity relationships in grasslands restored with different ecotypes of a foundation species but is contingent on local environmental conditions and the ecotype of the dominant grass.