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 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.

Outputs from this research will benefit scientists working across a broad range of disciplines, including those with a general interest in the processes that maintain soil condition, biodiversity and forest productivity. More specifically, the results will have high relevance for soil scientists in the fields of nutrient cycling and mycologists studying the factors maintaining fungal diversity. Evolutionary biologists working on the maintenance of mutualisms, and mathematical modellers with expertise in 'biological trade' will have an interest in the quantification of resource trade between fungus and plant, whilst scientists working towards the development of accurate carbon-cycling models with importance for future carbon budgets will be able to refine models based on these data. Ecologists and environmental scientists with expertise in global change factors will be interested in how changes in nitrogen availability may alter these trade balances and ultimately affect soil biodiversity and functioning. The results will be pertinent for restoration ecologists, interested in the identity of mycorrhizal fungi that will improve tree health under polluted conditions.

Outputs from this work will also be of relevance to government, environmental, charitable and industrial organisations with interests in forestry, soil health, sustainability and carbon storage. International organisations including The International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests), set up under the United Nations Economic Commission for Europe (UNECE) have a specific remit to study the effects of nitrogen deposition on forest condition across Europe, and produce reports with significant impact on international policy and forestry practices. Because of their interest in how ectomycorrhizal (ECM) fungi are affected by changes in nitrogen availability, they are a project partner in this fellowship, ensuring that outputs are fed directly into reports and thus impact policy decisions (see Pathways to Impacts). As a member organisation of ICP Forests, the UK will directly benefit from this route of knowledge transfer. ECM fungi are pivotal to nutrient cycling in forest soils and understanding factors that control their diversity and functioning across landscape-scales is of direct relevance to sustainable forestry practices, a major priority for organisations such as Forest Research in the UK who are a key government advisor on forest best-practices and environmental policy. I have both a mentor and a project partner in Forest Research, aiding data dissemination directly between my research and this key organisation. Regeneration and protection of native woodlands across the UK is a focus of charities such as the Woodland Trust, who will be interested in how changes in fungal communities affect forest functioning, and how global change factors such as nitrogen deposition might alter this. The expansion of woodlands in the UK is a major priority area for the UK government, as set out in the recent policy paper "A Green Future: Our 25 Year Plan to Improve the Environment". Woodlands are increasingly recognised for their importance in amenity and public well-being. My research will contribute to the body of knowledge supporting maintenance and growth of healthy forests, and as a result will be of benefit to the general public, as well as afforestation and restoration schemes such as the Northern Forest projects. Increasing the long-term supply of home-grown timber is another strategic priority which this project will be directly relevant for. Inoculation of trees with ECM fungi is already routinely used in forestry, due to increased tree growth and protection from pathogens - a deeper understanding of the mechanisms behind the ECM mutualism will improve our ability to utilise this symbiosis in productive, sustainable forestry practices.