Changes in the functional diversity and abundance of ectomycorrhizal fungi are decoupled from water uptake patterns in European beech forests

Temperate forests on their warm and dry distribution limit are expected to be most vulnerable to reductions in water availability. This prediction is mostly based on studies assessing single forest functions, mainly growth. Water and nutrient cycling are functions that rely on tree roots and their symbiotic association with ectomycorrhizal (ECM) fungi. Trees can compensate for seasonal reductions in water availability by shifting root water-uptake (RWU) towards deeper soil layers, but ECM fungi dwell in the upper soil, thus suffering from desiccation and compromising nutrient uptake. We hypothesised that drier sites should depict larger seasonal shifts in RWU, but at the expense of lower diversity and colonization of fine roots by ECM fungi. We selected three beech (Fagus sylvatica) forests in their warm distribution limit with contrasting geographic locations and mean annual precipitation: northern Atlantic (2500mm), intermediate transitional (1150mm) and southern Mediterranean (780mm). We collected soil, stem and root samples in spring (wet) and summer (dry) to quantify fine-root density and colonization by ECM fungi, to infer RWU from isotopic composition of plant and soil water, and to characterize ECM fungal diversity through DNA-metabarcoding. High moisture in the upper soil benefited the ECM community, but higher diversity and fine-root colonization by ECM fungi in the upper soil did not imply larger contributions of this soil layer to RWU. The prevailing climate and local abiotic conditions determined how ECM communities structured, more than seasonal variability. Across sites, ECM communities differed in their functional diversity: ECM fungi with long hyphae, more vulnerable to water scarcity, dominated at the site with the highest water availability. Our results suggest that transient reductions in soil water availability might not compromise RWU but could be detrimental for maintaining ECM-mediated nutrient uptake in beech forests experiencing longer and more severe drought periods under current climate change.