Two important dominant species, Andropogon gerardii (big bluestem) and Sorghastrum nutans (Indiangrass) of the Great Plains of the United States have a wide regional distribution across the midwestern United States. This broad range of natural distribution of dominant species has resulted in local adaptation and genetic variation. Although genotypic and phenotypic differentiation among the ecotypes of dominant species have been studied, it is still uncertain: 1. if ecotypic diversity affects grassland community assembly processes equally, specifically taxonomic, phylogenetic, and functional diversity; 2. if grassland taxonomic, phylogenetic, and functional diversity show the same variation across the aridity gradient. Reciprocal common gardens were established in 2009 at four sites across a longitudinal aridity gradient in the North American tallgrass prairie: Colby, KS; Hays, KS; Manhattan, KS; and Carbondale, IL. Ecotypes of A. gerardii and S. nutans were seeded according to a randomized complete block design at each of the four sites. Species composition was measured in two 1 m2 quadrats per plot in 2012 and 2019, retaining the maximum percentage canopy cover obtained by each species from each survey for analyses. Linear mixed models were applied to analyze the effect of ecotypes, year, and sites on taxonomic, phylogenetic, and functional diversity.
Results/Conclusions
The generalized linear mixed model showed significant differences in an interaction between year, site, and ecotype ((Wald’s-χ2=26.27, P<0.01). Meanwhile, the linear mixed model found (1) significant effects on phylogenetic diversity (phylogenetic sesMPD) in interactions between year and ecotype (Wald’s-χ2=23.09, P<0.01), between site and ecotype (Wald’s-χ2=23.34, P<0.01), and between site and year (Wald’s-χ2=18.80, P<0.001); (2) significant effects on functional diversity (functional sesMPD) in interactions between ecotype and site (Wald’s-χ2=17.76, P=0.03) and between site and year (Wald’s-χ2=19.33, P<0.001). The results showed ecotypic differentiation has significant impacts on community taxonomic, phylogenetic, and functional diversity. Noticeably, the xeric ecotype dominated grassland communities showed an over-dispersed functional pattern which may indicate interspecific competition that can result in niche complementarity and diversification of functional traits. Also, grassland taxonomic, phylogenetic, and functional diversity all increased along the eastward decreasing aridity gradient which reflects the effect of strong environmental filtering on community assembly along this climate gradient.