Climate change is one of the key factors driving range shifts of plants and animals all over the world. In high alpine ecosystems, snowfall extent, depth, and duration determine ecosystem structure and function, and rising snowline and shrinking snow pack have been associated with an upward migration of plants. At Niwot Ridge, Colorado, Front Range of the Rocky Mountains, these changing snow conditions occur across a range of habitats ranging from plant-free sites to those that are increasingly vegetated. We used this natural plant diversity/density gradient to examine how landscape level changes in plant communities affect soil microbial communities and associated soil biogeochemical characteristics. We hypothesized that aboveground-belowground interactions would be coupled. Plant composition and structure (count surveys) and soil (16S, 18S, ITS marker-gene sequencing) communities were evaluated in 98 high elevation (3610 – 3940 m.a.s.l.) plots (1 m radius circles), spaced at 5 to 50 m intervals, along a 2 km long grid. Biogeochemical soil characteristics included pH, moisture, water holding capacity, and different forms of C and N. Potential effects on soil processes were assessed by measuring activity of microbial enzymes.
Across this high-elevation gradient, plant and soil biotic communities were coupled such that soil community diversity (PD whole tree, Chao1) increased in conjunction with plant community diversity (a model including plant density and richness outperformed a density only model). This pattern was consistent across all major soil groups including bacteria (R2 = 0.556), fungi (R2 = 0.235), and non-fungal eukaryotes (e.g. algae, protists, nematodes) (R2= 0.456). Compositional shifts in the microbial community were also associated with plant cover. The contribution of symbiotic (e.g. bacteria in Rhizobiales and fungi in Glomerales) and pathogenic (e.g. nematodes in Dolichodoridae) components of the soil community, as well as many “free-living” taxa (e.g. bacteria in Verrucomicrobia, fungi in Agaricomycetes, or nematodes in Aphelenchida) increased with higher plant diversity/density, while the relative abundance of taxa dominant in unvegetated soil (e.g. bacteria in Cyanobacteria, fungi in Mortierrellales, or green algae in Chlorophyta) decreased . Biogeochemical properties (e.g. soil moisture, water holding capacity, or dissolved organic C) also increased in conjunction with plant and soil community diversity.
Overall, this study indicates the expansion of plants into previously un-vegetated areas may cause microbial community composition to shift away from cyanobacteria and algae-dominance and towards greater plant-dependence, resulting in soils that retain more moisture and nutrients, potentially affecting runoff to lower elevation ecosystems.