Mycorrhizal fungal symbionts shape plant growth responses to experimental warming and drought

Climate change is creating locally novel environments for microbial symbioses in forests. Whether fungal symbionts can sustain tree growth under rapid warming and increasing drought has consequences for biodiversity, forestry, and global carbon (C) storage. A key way fungal partners may buffer trees is through extensive extraradical mycelium networks that enhance uptake of growth-limiting nutrients and water. Yet the role of these networks in shaping plant responses to climate stress has rarely been tested in the field, especially for ectomycorrhizal fungi (EMF), the dominant mycorrhizal type in European forests.

We tested how two dominant European trees (Fagus sylvatica and Pinus sylvestris) respond to simulated warming and drought, alone and combined, in relation to experimentally manipulated EMF mycelium networks. We established the Swiss Climate Change × Mycorrhizae experiment in summer 2023 across four long-term forest monitoring plots spanning large environmental gradients. Within this experiment, we focused on EMF shared by both hosts and used nylon meshes to restrict rhizomorph formation, specialized mycelium enabling long-distance resource transport. After two years of seedling growth, we destructively sampled 572 seedlings and quantified above- and belowground processes.

Drought reduced host growth more than warming, with P. sylvestris more sensitive than F. sylvatica. Foliar C isotope signatures corroborated this pattern, with increased δ¹³C values, reflecting reduced discrimination in primary carboxylation under drought. Allowing EMF to form rhizomorphs and extensive extraradical networks mitigated drought impacts on hosts by 10-25%. EMF communities were themselves drought-sensitive, showing lower biomass and respiration and respiring CO₂ that was less ¹³C-depleted. EMF growth was positively correlated with plant growth, indicating tight coupling and shared sensitivity to drought.

These aboveground effects extended belowground to carbon cycling. Where EMF networks were present, soil C storage declined relative to treatments limiting network formation, likely due to accelerated decomposition inferred from soil organic C isotopes.

Overall, we provide field experimental evidence that EMF mycelium networks help trees cope with climate stress, and that the magnitude of this “myco-support” tracks shifts in EMF growth and respiration. This is important because it demonstrates that fungal symbionts will play important roles in shaping future forest tree responses to climate change. However, this benefit may trade off against soil organic C storage, with implications for future forest carbon budgets.