Elevated temperature may reduce functional but not taxonomic diversity of fungal assemblages on decomposing leaf litter in streams

Mounting evidence points to a linkage between biodiversity and ecosystem functioning (B‐EF). Global drivers, such as warming and nutrient enrichment, can alter species richness and composition of aquatic fungal assemblages associated with leaf‐litter decomposition, a key ecosystem process in headwater streams. However, effects of biodiversity changes on ecosystem functions might be countered by the presumed high functional redundancy of fungal species. Here, we examined how environmental variables and leaf‐litter traits (based on leaf chemistry) affect taxonomic and functional α‐ and β‐diversity of fungal decomposers. We analysed taxonomic diversity (DNA‐fingerprinting profiles) and functional diversity (community‐level physiological profiles) of fungal communities in four leaf‐litter species from four subregions differing in stream‐water characteristics and riparian vegetation. We hypothesized that increasing stream‐water temperature and nutrients would alter taxonomic diversity more than functional diversity due to the functional redundancy among aquatic fungi. Contrary to our expectations, fungal taxonomic diversity varied little with stream‐water characteristics across subregions, and instead taxon replacement occurred. Overall taxonomic β‐diversity was fourfold higher than functional diversity, suggesting a high degree of functional redundancy among aquatic fungi. Elevated temperature appeared to boost assemblage uniqueness by increasing β‐diversity while the increase in nutrient concentrations appeared to homogenize fungal assemblages. Functional richness showed a negative relationship with temperature. Nonetheless, a positive relationship between leaf‐litter decomposition and functional richness suggests higher carbon use efficiency of fungal communities in cold waters.