Urbanization alters fungal functional composition in boreal ecosystems by favouring larger-spore fungi and pathogenic fungi

Urbanization is a major threat to biodiversity, ultimately influencing ecosystem functions. Fungi, a highly taxonomically and functionally diverse group of organisms, underpin many of these ecosystem functions from nutrient cycling to symbiotic interactions. Yet, research on how urbanization impacts fungal diversity has primarily focused on detecting changes in fungal community structure, rather than understanding the functional implications of these changes. Here we examined shifts in fungal dispersal and resource-use traits due to urbanization in boreal ecosystems. We hypothesized that urban sites may favour large-spored taxa as they may be better suited to survive and germinate under harsher urban conditions. Conversely, small-spored taxa could dominate fragmented urban habitats, as their spores can disperse over longer distances. We hypothesized turnover of functional guilds due to altered substrates and hosts in urban versus natural settings. To test these hypotheses, we cross-referenced fungal community data from five Finnish cities and adjacent naturally-forested areas obtained via metabarcoding with databases on spore size and functional guilds. We found a consistent dominance of large-spored taxa in urban environments, suggesting a greater impact of abiotic stress on fungal communities rather than habitat fragmentation as large-spored taxa are believed to possess better survival structures for dispersal and germination under stressful conditions. In terms of functional groups, symbiotic taxa, such as pathogens affecting lichens, plants, and animals (including humans), showed an increase in abundance in urban settings. This pattern suggests a dual effect of urbanization, by introducing new susceptible hosts and creating stressful conditions for existing ones, favouring infections by pathogenic fungi. Our study highlights the utility of combining fungal metabarcoding community surveys with trait databases to detect fungal functional changes in urban environments. Our results show that spore size increases with urbanization, suggesting that larger spores may be beneficial to colonise heavily altered urbanised habitats. The thriving presence of fungal genera involved in pathogenic interactions suggests that urbanization may promote the prevalence and emergence of pathogenic fungi. Read the free Plain Language Summary for this article on the Journal blog.