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

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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 this fellowship, I test the hypothesis that trade-offs between nutrient acquisition (the breadth of nitrogen that can be taken up) and competitiveness (the ability to win space on plant roots and obtain plant carbon) is a critical process driving the species of mycorrhizal fungi that can co-occur on individual plant roots, and that this ultimately affects the community composition of fungi observed across landscape-scales. I test this hypothesis using a series of experiments in the laboratory which allow me to trace the quantities of carbon being supplied from the plant to individual species of fungi, and in exchange how much nitrogen is being passed from the fungus to the plant. Field experiments in a forest which has received more than 30 years of experimental nitrogen fertilisation will allow me to test how these trade-offs operate in a more complex natural setting, where the availability of nitrogen may alter the dynamics of the trade-off and the exchange rate of carbon and nitrogen between plant and fungal partner. Finally, I will test how the results observed in the lab and field scale-up to patterns of ECM species occurrence across Europe, using a series of long-term monitoring forest plots.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.