COS 82-1 - Diversity-biomass relationships are dependent upon location and dominant species ecotypes.

Background/Question/Methods

Diversity influences ecosystem functioning, and diversity-biomass relationships can inform niche relationships among co-occurring species, especially the influence of dominant species. Andropogon gerardii is a dominant grass in U.S. Great Plains tallgrass prairies that exhibits ecotypic variation. We predicted that diversity-biomass relationships of tallgrass prairie would be differentially affected by ecotypes of A. gerardii and location across its range. We predicted a humped-back diversity-biomass relationship with highest subordinate species diversity at intermediate rainfall sites with the occurrence of the relationship to vary depending on local adaptation of ecotypes. Reciprocal gardens were established in 2009 at four sites spanning a 1,150 km, 500 to ~1200 mm year^-1 precipitation gradient from western Kansas to southern Illinois with dry, mesic, or wet ecotypes of A. gerardii, or a mix of the three sown into plots with a mix of subordinate species. Canopy cover of all species was visually estimated and peak-season total standing biomass sampled in late-summer in 2012 and 2021. Mixed effect models and locally-weighted regression with loess smoothing was used to test the relationship between Hill’s numbers of taxonomic and phylogenetic diversity and peak standing biomass across sites and among ecotypes.

Results/Conclusions

Site and year since establishment were the most significant determinants of diversity. Diversity metrics were positively correlated and diversity decreased from 2012 to 2021. Across sites, there was a humped-back diversity-biomass relationship for all diversity metrics only at the southern Illinois site in 2012, but not in 2021. Ecotype sources exhibited a humped-back diversity-biomass relationship in response to wet and mesic ecotypes, and in mixed plots, but not the dry ecotype source, and only in 2012. By 2021, diversity-biomass relationships were either flat or decreased at the highest levels of biomass. The occurrence of humped-back diversity-biomass relationships three years after establishment of experimental plots with wet and mesic ecotype sources reflect competitive dominance on subordinate species imposed by A. gerardii at the productive, wet end of the gradient. The absence of a humped-back relationship for the dry ecotype and at sites at the drier end of the gradient reflects a lack of competitive dominance in xeric conditions. The similarity between Hill’s numbers of taxonomic and phylogenetic diversity established concurrence of species diversity with evolutionary lineages among taxa. These findings inform understanding of the role of ecotypes in community assembly and adaptation and suitability for restorations under climate change.