Diversity-productivity relationships are dependent upon location and ecotypes of the dominant species growing across a precipitation gradient

Locally dominant and highly productive species affect diversity through competitive interactions with subordinate species. Across environmental gradients the competitiveness of dominant species varies, and the relationship between diversity and productivity is commonly concave-down and unimodal, and referred to as the ‘humped-back model’ (HBM). The effect of local adaptation in dominant species and their resulting competitiveness on occurrence of diversity-productivity relationships is unknown. A reciprocal transplant experiment established across a precipitation gradient in the US Great Plains was used to investigate the effect of intraspecific variation in dominant prairie grasses on diversity-productivity relationships in assembled communities. Taxonomic diversity exhibiting a HBM relationship occurred in 47% of tests, most frequently in communities established with locally-sourced (compared with distantly sourced) dominant species, reflecting local adaptation, but only at the extreme wet end of the precipitation gradient. The HBM relationship was retained with experimental reduction of growing season precipitation using rainout shelters at the wet site as productivity was reduced and taxonomic diversity increased. Phylogenetic and functional trait diversity reflecting evolutionary-relevant relationships exhibited a HBM relationship in 33% and 31% of tests, respectively. However, the patterns of phylogenetic and functional diversity were less related to site or dominant species source than the patterns of taxonomic diversity. The relationship between diversity and productivity is influenced by intraspecific variation in dominant species, especially where dominant species exhibit local adaptation. Thus, grasslands restored with local ecotypes of a foundation species can have consequences for community processes through productivity effects on diversity.