Renewal application: How do ecological trade-offs drive ectomycorrhizal fungal community assembly? Fine- scale processes with large-scale implications

Nearly all plants are associated with mutualistic fungi which inhabit their roots. These mycorrhizal fungi provide the plants with a source of nutrients and in exchange, the plant provides the fungal partner with a source of carbon. Trees typically associate with ectomycorrhizal (ECM) fungi, which are critical to many processes within ecosystems including decomposition and nitrogen and carbon cycling. Different species of ECM fungi can differ in their ability to acquire nutrients or in the suite of functions that they carry out. Understanding what affects the species composition of ECM fungi is therefore important for a better overall understanding of nutrient cycling in soils, forest health, productivity and biodiversity. Individual species of ECM fungi compete for space on the roots of trees. Some fungi are likely to be better competitors for root space than others, but the best competitors are likely to perform less well in other important strategies, such as nutrient acquisition. These 'trade-offs', which mean the ability to perform optimally in one respect is associated with a decline in performance in another, are known to be a fundamental reason why so many species of macro-organisms can co-occur in a single habitat. However, the role that these trade-offs play in affecting mycorrhizal communities is poorly understood despite its probable importance in determining the species composition of these ecologically vital fungi. In the first part of this fellowship, I mechanistically tested the hypothesis that trade-offs between nutrient acquisition (the breadth of nitrogen that can be taken up) and carbon use (the ability to obtain plant carbon and persist when carbon supply is low) is a critical process driving the species of mycorrhizal fungi that can co-occur on individual plant roots. This was done using a series of experiments in the laboratory which allowed me to trace the quantities of carbon being supplied from the plant to individual specie of fungi, and in exchange how much nitrogen is being passed from the fungus to the plant. In the renewal phase of this fellowship, I will build upon the knowledge gained and test how the trade-offs demonstrated in the first phase can help explain the distribution and functioning of ECM fungal species in the field. I will do this by visiting intensively monitored forest plots across Europe, where the occurrence of key ECM fungal species is already known and can be linked to environmental conditions such as nitrogen availability. The UK's forests provide significant amenity, carbon-capture and timber value, whilst globally trees form hyper-diverse tropical rainforests, and hold significant stores of carbon in boreal ecosystems. As ECM fungi are a critical component of all tree-dominated ecosystems, the outputs from this project will provide high-quality insights into this key aspect of our natural environment, and help to develop future research, policy and forestry practice in the UK and beyond.