What are the large-scale diversity, distributions and fates of forest mycorrhizal communities?

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Forests harbour a surprising secret. Although trees dominate these ecosystems, creating a complex habitat that shelters many species while shading out many others, this dominance relies on a hidden relationship with fungi. These fungi are rarely seen but their thread-like filaments, called hyphae, are intertwined among the roots of trees, forming a close symbiosis that helps both tree and fungus grow - a mutualistic interaction. Both organisms need carbohydrates and minerals to grow and, while trees excel at capturing and storing carbon from the air, the tiny hyphae of fungi excel at extracting water and nutrients, like nitrogen and phosphorus, from the soil. The amounts exchanged between trees and fungi is extremely high: all of the carbon in a fungus can be obtained directly from its partner trees, while most of the nitrogen in a tree comes directly from its partner fungi. The diverse groups of fungi involved are called ectomycorrhizal (or ECM) fungi, after the modified fine roots, or ectomycorrhizas, where the tree and fungus join each other.These symbiotic relationships are remarkably common: the majority of European trees depend on ECM fungi and there are many different fungi that form these relationships. Some of these fungi are well known for their sexual fruiting bodies, for example the poisonous fly agaric toadstools and the edible truffles and penny bun mushrooms. However, we know worryingly little about the diversity of ectomycorrhizal fungi. While some fungi may grow with a wide variety of trees and other plants, others may be specialists restricted to closely related tree species. And while around 8,000 species of ECM fungi have been described, there may be many more undiscovered. In addition, we largely ignore the geographic ranges of these fungi. Some may be widely distributed, but others may be restricted to a handful of small locations. We do know that at least some can invade in other continents and that others may be locally extinct. Most worryingly of all, we know that ectomycorrhizal fungi are sensitive to environmental change, particularly pollution and changes in rainfall and temperature. But we do not know how these observations translate to changes at large geographical scales.Here we propose to fill the gap in our understanding of the wider diversity and distribution of tree-fungi symbioses, and help focus our selection of experimental and genetic models. In order to do this, we put forward the use of one of the most extensive biomonitoring plot networks on Earth in which the effects of pollution and changes in forest soil quality have been closely monitored. We aim to use these plots to carry out the first precise mapping of mycorrhizal fungi across Europe's three major forest types: beech, Scots pine and Norway spruce. At each of 150 intensively-monitored plots, we will use optimised molecular ecology techniques to obtain DNA sequences from ectomycorrhizas. Once we know who and where are the dominant fungi in Europe, we intend to use the monitored data from the plots along with the latest environmental change predictions to understand the processes that control these hidden symbioses and their likely fate in our changing world.