Abiotic and biotic processes both play an important role in community assembly and the maintenance of biodiversity. However, fundamental gaps remain in our understanding of the contribution of biotic factors to spatial variation in plant functional composition (functional β-diversity). In a high-elevation landscape unit that extends from alpine tundra vegetation to near-barren subnival soils, we examined the joint influence of environmental conditions, spatial structure, and soil microbes on plant community assembly. Plant community composition, plant functional traits (plant height, specific leaf area leaf area, and chlorophyll content), soil microbial communities (16S, 18S, ITS marker-gene sequencing) and environmental characteristics (pH, moisture, water holding capacity, soil C and N) were measured in ninety-eight plots (1 m radius circles), spaced at 5 to 50 m intervals, along a 2 km long grid ranging in elevation from 3610 to 3940m. Using variance partitioning analysis we asked how microbial composition, environmental factors, and spatial structure contribute to variation in plant functional β-diversity. We then examined how the importance of those factors differed among lower density subnival plant communities and higher density tundra plant communities.
In general, we found that spatial structure, environmental conditions, and soil microbes jointly influence functional β-diversity in high elevation plant communities. In higher density tundra plant communities species-sorting of functional traits along local environmental gradients play a stronger role in influencing patterns of functional β-diversity than spatial processes or soil microbes. In contrast, in lower density subnival plant communities, soil microbes and spatial processes play a stronger role in influencing patterns of functional β-diversity. These patterns suggest that the upward range extension of tundra plants into previously un-vegetated areas is largely influenced by dispersal and soil microbes (cyanobacteria, fungi in Mortierrellales, and green algae) and the composition of intact tundra communities is predominantly driven by local environmental conditions. Our study highlights the importance of incorporating biotic factors into studies of β-diversity and suggests that soil microbes play important roles as drivers of the assembly of functional β-diversity in plant communities.