Ectomycorrhizal fungal community responses to warming and rainfall reduction differ between pine host species but are similar across experimental cohorts

Given rapidly shifting climatic conditions, particularly in high latitude forests, understanding the responses of ectomycorrhizal (ECM) fungi and their tree hosts to warming and reduced soil water availability under realistic future climate scenarios is essential, yet few studies have investigated how combined global change stressors impact ECM fungal community richness and composition as well as host performance. Here we leveraged a long-term factorial warming (ambient, +1.7C, +3.3C) and rainfall reduction (ambient, 30% reduced rainfall) experiment in northern Minnesota, USA to investigate the responses of two congeneric hosts with varying drought tolerances and their associated ECM fungal communities to a gradient of soil moisture induced by a combination of increased temperature and reduced rainfall. Soil drying had host-specific effects; the less drought-tolerant Pinus strobus had decreased stem growth and a less species richness ECM fungal community, while the more drought-tolerant Pinus banksiana experienced no decline in stem growth, but had an altered ECM fungal community composition under drier soils. These responses were consistent across two cohorts in the same experimental system, demonstrating a repeatability of ECM community responses to experimental climate change stressors across time, even with temporal variation in background temperature and precipitation. Taken together, the results of this study suggest that the combined effects of warming and decreased precipitation will largely be additive, but also that drought-related responses above- and belowground will increasingly shape the dynamics of ECM communities at temperate-boreal forest ecotone.