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FIGURE 7.3 Compositional and phylogenetic similarity of Acidobacteria communities (A) and angiosperm communities (B), as a function of the elevation separating the communities. The compositional (hollow black triangle) and phylogenetic (solid black circles) similarity for both angiosperm and Acidobacteria communities significantly decrease with increasing elevational separation (Mantel test, P < 0.001). Lines represent best-fit regressions of similarity versus change in elevation (see Materials and Methods). The slope of the decay of phylogenetic similarity between Acidobacteria communities is significantly steeper than predicted by a null model constrained by the decrease in taxon turnover (solid gray circles) (P < 0.05). The slope of the decay in phylogenetic similarity across the angiosperm communities is not significantly different from the null prediction given species turnover.

FIGURE 7.3 Compositional and phylogenetic similarity of Acidobacteria communities (A) and angiosperm communities (B), as a function of the elevation separating the communities. The compositional (hollow black triangle) and phylogenetic (solid black circles) similarity for both angiosperm and Acidobacteria communities significantly decrease with increasing elevational separation (Mantel test, P < 0.001). Lines represent best-fit regressions of similarity versus change in elevation (see Materials and Methods). The slope of the decay of phylogenetic similarity between Acidobacteria communities is significantly steeper than predicted by a null model constrained by the decrease in taxon turnover (solid gray circles) (P < 0.05). The slope of the decay in phylogenetic similarity across the angiosperm communities is not significantly different from the null prediction given species turnover.

tently correlated with diversity in both plants and bacteria, and bacterial diversity was also strongly correlated with slope in a univariate analysis (Table 7.1). A multivariate analysis suggested that soil temperature was the major explanatory variable of taxon richness and phylogenetic diversity for both plants and bacteria (P < 0.001 in six of eight multivariate models). Turnover in taxon and phylogenetic composition of plant and bacterial communities was significantly correlated with changes in the majority of our measured environmental parameters (7.2); however, the combined influence of soil temperature, pH, and total nitrogen was the most important predictor for both groups. After controlling for these environmental parameters, geographic distance between samples significantly correlated with all turnover patterns (partial Mantel test, P < 0.001). These results imply that dispersal limitation could be occurring, but given the small geographic range of our gradient, they are more likely caused by effects of environmental heterogeneity that we did not characterize. The correlation of richness and turnover with temperature and pH is consistent with the



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