Richness, community structure and distance decay of soil fungi along a sharp aridity gradient in monodominant pine forests

Understanding how aridity shapes fungal communities is essential for predicting ecosystem responses to climate change. Monodominant forests of Aleppo pine (Pinus halepensis) occurring along a steep precipitation gradient, offer the opportunity to test the effect of aridity on distance decay patterns of soil fungi, without the confounding effects of vegetation. We conducted nested soil sampling in four Mediterranean, two semi-arid and three arid forests along an aridity gradient (250-800 mm annual precipitation) and examined distance decay patterns of saprotrophic (SAP) and ectomycorrhizal (ECM) fungi at different spatial scales. ITS2 soil metabarcoding 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. Stochastic assembly processes dominated under mesic conditions, while deterministic processes prevailed in arid regions, particularly for ECM fungi. Our results suggest that aridity can reduce fungal richness and stochasticity in community assembly, and that climate can structure fungal communities independently of 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 in forest soils.