Experimental species introductions influence fungal community succession through positive and negative effects on resident species.

Successional pathways of microbial communities are influenced by the complex interactive dynamics among the resident and immigrating species, along with the interactive feedback loops with their environment. Although studies on microbial communities have described patterns of microbial succession, quantitative evidence of how resident communities respond to immigrating species and how such relationships translate into successional changes remains limited, especially for species-rich communities under natural settings. Here, we carried out a field experiment to investigate how the identity of immigrating species influences the successional pathways of wood-inhabiting fungi. We simulated immigration through inoculations of nine selected wood-inhabiting fungal species and characterized resident fungal communities before and one and two years after the inoculations through DNA metabarcoding. The experiments included 275 naturally fallen and 185 artificially felled fresh logs of Norway spruce, with different log types hosting distinct initial resident communities of fungi and representing different abiotic conditions. While the resident community succession was mostly explained by the log-level abiotic characteristics, the identity of immigrating species also influenced the composition of resident communities, and consequently community succession. The immigrating species influenced resident species mostly negatively, suggesting competitive interactions to be important determinants of community succession. The responses of resident species to the immigrating species were phylogenetically correlated, suggesting that shared traits underlie species interactions in the species-rich wood-inhabiting fungal communities. This study advanced the understanding of community succession in species-rich natural systems by providing experimental evidence that the immigrating species influence community succession through the phylogenetically structured responses of resident species.