Temperature drives plant and soil microbial diversity patterns across an elevation gradient from the Andes to the Amazon

Climate strongly regulates plant community composition and diversity, exemplified by gradients in plant diversity and community structure with elevation. However, we do not know if soil bacteria and fungi, key drivers of terrestrial biogeochemical cycling, follow similar biogeographical patterns determined by the same climatic drivers. We studied an Andean tropical forest transect traversing 3.5 km in elevation. The species richness (α-diversity) and compositional dissimilarity of communities (β-diversity) were determined for plants, bacteria and fungi. We determined the environmental drivers of these patterns, using 31 environmental and edaphic predictor variables, and the relationship between microbial communities and soil organic matter cycling (extracellular enzymes). We found co-ordinated changes with elevation in the species richness and composition of plants, soil bacteria and fungi. Across all groups, α-diversity declined significantly as elevation increased, and β-diversity increased with increased elevation difference. Temperature was the dominant driver of these diversity gradients, with only weak influences of edaphic properties, including soil pH, which did not vary substantially across the study transect. The gradients in microbial diversity were strongly correlated with the activities of enzymes involved in soil organic matter cycling, and were accompanied by a transition in microbial traits, towards slower-growing, more oligotrophic taxa at higher elevations. We provide the first evidence of co-ordinated temperature-driven patterns in the diversity and distribution of plants, soil bacteria and fungi in tropical ecosystems. This finding suggest that, across landscape scales of relatively constant soil pH, shared patterns and environmental drivers of plant and microbial communities can occur, with large implications for tropical forest communities under future climate change.

We determined species richness (α-diversity) using Shannon diversity index, Community composition (β-diversity) was determined using Sorrenson indices for plants and Bray Curtis dissimilarity matrices for soil bacteria and fungi. We tested whether patterns in 1 9 7 bacterial and fungal diversity and community composition were explained by biotic 1 9 8 interactions with plant communities, by using Spearman's correlation (α-diversity) and 1 9 9 Mantel tests (β-diversity) among biotic groups and comparing the relationships between Principal Co-ordinates Analyses to explore differences in β -diversity with elevation or soil Results 2 2 0 The α -diversity of plants, soil bacteria and fungi declined as elevation increased along 2 2 1 our study transect (Fig. 1). The α -diversity of plants declined most steeply, followed by mineral horizon (Table S2). The composition of plant, bacterial and fungal communities also differed with contained a greater proportion of Acidobacteria and Archaea (Fig. 3). For fungi, increased 2 3 7 elevation was associated with increased dominance of Ascomycota (Archaeorhizomycetes,  Table S3). Bacteria exhibited the largest compositional dissimilarities of 2 4 1 communities with elevation (β-diversity) followed by plant and then fungal communities, and 2 4 2 the β -diversity patterns observed for bacteria and fungi were correlated with those observed 2 4 3 for plants (Fig. 4). Thus, plants and several major taxonomic groups of both bacteria and drivers in community structure.

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As with α -diversity, MAT was the strongest correlate of patterns in β -diversity. MAT in both organic and mineral horizons, and fungi in mineral horizons (Table 1). There were 2 4 9 additional significant correlations between the β -diversity of bacteria and fungi, and organic 2 5 0 nutrient concentrations and their ratios; these were stronger in the organic compared to 2 5 1 mineral horizons (Fig. S4). Nutrients other than nitrogen and phosphorus also influenced β -2 5 2 diversity, including potassium for plants and sodium for bacteria (Table 1). Soil pH was 2 5 3 correlated with bacterial β -diversity but not fungal β -diversity. To assess whether soil microbial distributions were related to differences in rates of community to shifts in substrate availability. For example, relative microbial investment into 2 6 0 different enzymes shifted with increased elevation, from enzymes that degrade phosphorus- between the differential activity of these seven enzymes and differences in β -diversity were these findings suggest that, in addition to temperature, differences in organic nutrient cycling 2 6 5 are related to the β -diversity of bacteria and fungi in organic soil horizons. Overall, our results demonstrate a fundamental role for environment, principally were strongly associated with variation in bacterial and fungal α -and β -diversity (Table 1).

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The role of temperature in determining microbial β -diversity is also illustrated by an increased elevation, but a decreased relative abundance of Actinobacteria and indicated that similar environmental factors, primarily temperature (Table 1), drive these 2 8 7 patterns across the three biotic groups. The high relative abundance of the Ascomycota, understanding of tropical montane forest habitats being energy-limited (Bruijnzeel et al. fungal biomass in these tropical montane forests.

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Temperature was also the principal correlate of plant β -diversity (Table 1).

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Temperature has previously been shown to be a major determinant of tree community between the α -diversity of plants and bacteria compared to fungi (Fig. 2), but a stronger There was a secondary role for edaphic properties in shaping these diversity patterns, with direct influences of nutrient ratios on microbial β -diversity and potassium on plant β -3 1 4 diversity (Table 1). Our data also suggest that this role of edaphic processes on microbial α -3 1 5 and β -diversity is more significant in organic horizons, where the community-wide Laboratory incubations of soils from this transect also support the link between differences in 3 2 5 microbial community composition and the rate at which different organic substrates undergo between the high soil microbial diversity in lowland forests and the diverse complexity and 3 2 8 stoichiometry of plant organic matter inputs to soil, through the high inter-and intra-species are apparent (Fig S6). While the importance of soil pH and rainfall variation in determining plants, bacteria and fungi, also suggesting that stronger interactions occur among these 3 5 0 groups than has been recognised previously. Our findings imply that, where other potential 3 5 1 influences such as soil pH and moisture remain relatively constrained, anticipated future 3 5 2 temperature change will have significant co-ordinated impacts on the identity and functioning 3 5 3 (above-and below-ground) of tropical biota. This study is a product of the Andes Biodiversity and Ecosystem Research Group consortium 3 5 7 (www.andesconservation.org) and was financed by the UK Natural Environment Research Australian Research Council grant FT110100457 to PM and a European Union Marie-Curie Fellowship FP7-2012-329360 to ATN. We thank the Asociacion para la Conservacion de la