Loss of dominant tree‐mycorrhizal mutualism increases soil fungal diversity and alters community structure

Abstract Due to their mutualistic partnership with trees, mycorrhizal fungi are often important components of soil microbial communities. Mortality of tree hosts can have dramatic effects on mycorrhizal populations, but how these shifts impact the remaining soil microbial community and ecosystem function remains poorly understood. Using a long‐term oak girdling experiment at Black Rock Forest in New York, USA, we looked at changes in fungal community composition over 10 years following simulated large‐scale oak mortality to understand the interactions between canopy tree composition, soil fungal diversity and forest function. We collected soil samples in treatment plots of 100% oak girdling (OG 100 ), 50% oak girdling (OG 50 ), non‐oak girdling and a control without girdling. We sequenced the fungal community using the ITS1 forward and ITS2 reverse primers and classified the communities using the UNITE fungal reference and FUNguild databases. Fungal alpha diversity significantly increased by 32% and 17% in OG 100 and OG 50 , respectively, compared to the control, largely driven by decreased ectomycorrhizal fungi (ECM) and increased saprophytic fungal abundance in OG 100 and OG 50 . ECM showed a strong negative correlation with Archaeorhizomyetes (rho = −0.48) suggesting that these saprotrophic fungal taxa replaced ECM following the loss of oak trees. Girdling treatments, soil nitrogen, carbon: nitrogen ratios and soil pH explained over 20% of the soil fungal community variation. The removal of oaks led to a shift in the fungal community from ECM to more saprophytic fungi. Increases in soil pH with oak girdling, likely from tannic acid decreases from oak leaves, and the effect of pH on fungal diversity highlight the complex feedbacks and co‐determined successional dynamics of the tree and fungal communities. Synthesis . The removal of a dominant plant species, such as oaks in the northeastern US, can have cascading effects on the soil microbial community composition. Changes in the fungi community following disturbance can have significant impacts on ecosystem functions and properties such as decomposition, soil respiration, biodiversity and nutrient cycling. Together, this work improves our understanding of these forests' vulnerability to pathogen pressure and the impact these disturbances have not just on the focal species but on the whole ecosystem. Read the free Plain Language Summary for this article on the Journal blog.