Linking hierarchical assembly rules and diversity patterns of soil fungi in primary succession

Ecological succession affects natural communities in numerous ways, including deterministic (e.g. environmental filtering and biotic interactions) and stochastic processes (e.g. ecological drift and homogeneous dispersal). These processes may act across different spatial scales to set local communities, including environmental drivers characterizing different successional patches and finer‐scale mechanisms such as biotic interactions, within a given successional stage. However, whether and, if so, how the hierarchical nature of assembly rules influences local species diversity in the successional system remains unclear. We addressed this knowledge gap by investigating the assembly process of soil fungi at the successional and local scales using a chronosequence approach in a well‐vegetated glacier foreland in the High Arctic. Fungal richness showed a unimodal pattern during primary succession and was the greatest in the mid‐successional stage (3300–2400 years). We found that this trend resulted from the relative intensity of the deterministic process decreasing once and then increasing for the assembly process at the successional scale, triggered by a decrease in soil pH and an increase in plant coverage along with primary succession. Notably, in the mid‐successional stage, with the highest fungal richness, the assembly process was dominated by stochastic processes at successional and local scales. Through a hierarchical framework, we demonstrated that primary succession limits fungal membership at each successional stage, particularly during the early and late stages, ultimately shaping patterns of local diversity.