Fungal community structure and dynamics: drivers of wood decay and carbon cycling

Globally, forests contain a vast reservoir of carbon, approximately 30% of that in the biosphere, much of which is in the form of woody plant tissues. Every year this is added to as plants photosynthesise, but in balanced systems a similar amount is broken down to CO2 and water, and nutrients are released. Understanding what controls this balance is crucial for understanding carbon cycling, and for predicting carbon cycle responses to global climate changes. Recycling of woody resources is almost exclusively confined to a narrow range of specialist fungi: basidiomycetes and a few ascomycetes. Thus, these fungi are central to carbon and nutrient cycling, and yet we still have relatively little understanding of how they grow in wood, how they interact with each other and how different community composition affects decay. Key objectives of this proposal are, therefore, to unravel these processes, and to obtain quantitative data on the way in which fungal communities influence wood decay rate to be able to incorporate these dynamics into global models of carbon cycling. The majority of decay takes place in fallen wood, but wood decay actually begins in standing dead parts of trunks and attached dead branches. Moreover, the fungi that start the process are already latently present while the tissues are still functional. When the wood dries, the latent fungi grow throughout the wood as mycelium and begin the decay process. Later, other fungi, arriving as spores, 'fight' with those already present. Preliminary evidence suggests that fungal community composition, when species become established, and how they interact with each other, have a dramatic effect on the rates of wood decay and thus carbon cycling. We have a general understanding of factors affecting the process built from studies on fungal communities developing in attached branches, and from felled wood, but felled logs do not reflect the situation in nature as they are not already well colonized. In this project we will for the first time investigate community development when naturally colonized wood falls to the forest floor. We will simulate naturally fallen wood by pre-colonising wood slices with fungi that are primary colonizers of attached beech branches. Firstly, we will determine whether certain species effectively 'select' which fungi follow them, by leaving colonized slices on the forest floor and collecting after different times, using new high throughput DNA sequencing technologies and traditional isolation onto agar. Secondly, we will quantify wood decay rate, by measuring loss of density of slices in the field experiment. Thus, we will relate the species mix of primary and later colonisers with decay rate. As decay in the field will also be affected by climatic variables etc., we will also perform lab experiments on the effect on decay rate of adding specific later colonisers to slices pre-colonised with specific primary colonisers, by measuring CO2 evolution and weight loss. Thirdly, we will study how antagonistic interactions between fungi affect decay rate. When fungi interact, the outcome can be deadlock in which neither species gains territory, or replacement of one species by the other. A preliminary study has indicated that decay rate actually changes during the course of replacement of one fungus by another. We will investigate this in detail and also ask whether the outcome of the interaction is related to decay rate, by following CO2 evolution during the interaction. Finally we want to know how different numbers of individuals/species affects decay rate. We will precolonize wood slices and then vary the number of individual strains added, and measure decay rate in the laboratory under standard conditions. This project will reveal how fungal communities alter, how communities affect decay rate, provide data for carbon cycling models, and possibly form the basis for future manipulations of fungal communities to optimise carbon cycling.