Understanding how aridity shapes fungal communities is essential for predicting ecosystem responses to climate change. We investigated the richness, community composition, and spatial structure of ectomycorrhizal (ECM) and saprotrophic (SAP) fungi along a sharp aridity gradient. The mono-dominated pine forests along the steep precipitation gradient enabled us to test the effect of climate on distance decay patterns of soil fungi without the often-confounding effects of the dominating vegetation type. We conducted nested soil sampling in 4 Mediterranean climate and 5 semi-arid and arid forests along the precipitation gradient (250-800 mm annual precipitation) and examined the distance decay patterns of both SAP and ECM fungi at different spatial scales. High-throughput sequencing of soil DNA revealed that both fungal richness and diversity increased with precipitation. Fungal communities showed significant spatial autocorrelation at multiple scales, with stronger distance decay patterns in Mediterranean than arid forests. ECM and SAP communities in arid sites were largely subsets of the Mediterranean climate communities, and while stochastic assembly processes dominated under such mesic conditions, deterministic processes prevailed in arid regions, particularly for ECM fungi. Our results demonstrate that aridity reduces fungal richness and stochasticity, and that climate independently structures fungal communities even under uniform vegetation. This study highlights the need to consider scale-dependent ecological processes and emphasizes the role of climate, beyond vegetation, in shaping fungal community assembly.
