A soil fungal metacommunity perspective reveals stronger and more localised interactions above the tree line of an alpine/subalpine ecotone

Dispersal affects community structure by facilitating colonisation and homogenising local communities, while species sorting along environmental gradients contributes to divergent community assembly. Fungi often have widespread distributions and are assumed to be dispersed across the landscape, with environmental selection being a primary driver of community assembly. We characterised 143 communities of soil fungi along an altitudinal transect in Australia transitioning from subalpine to alpine vegetation (approximately 150m difference in elevation over a distance of 1200m). First, we inferred drivers of community assembly using canonical analyses including climate, edaphic properties, vegetation and spatial variables, all of which explained a statistically significant but very small amount of variation. We then employed an approach that defines the metacommunity with which each local community interacts via immigration and emigration and then estimates metacommunity characteristics. Using this approach, we inferred scales over which community interactions occurred along spatial, environmental and phylogenetic dimensions and the strength of community interactions based on the similarity of local communities to the metacommunity. Constructed metacommunities above the tree line consisted of local communities that were more clustered in space and more homogeneous than those at or below the tree line. Thus, mixing is likely occurring to a greater degree among fewer communities in the alpine environment. Comparisons of metacommunity estimates suggested that fungal evolutionary histories did not constrain community assembly as strongly as spatial proximity or environmental variation. This work suggests that differences exist in how fungal communities assembly along this altitudinal transect, despite site on its own explaining little compositional variation, and that the rate and the degree of species mixing among communities differs depending on the environmental context.