Climate change, fungus-invertebrate interactions and ecosystem processes

Saprotrophic fungi are the main agents of primary decomposition and nutrient cycling in woodland ecosystems. Powerful enzymatic capabilities enable then to break down the most recalcitrant components of wood and leaf litter, such as lignin and cellulose. Nutrients are retained by dynamic networks of mycelium, which are vulnerable to grazing by soil invertebrates. The studies reported in this thesis employed laboratory microcosm, mesocosm and field manipulations to further mechanistic understanding of climate change effects on basidiomycete fungal-dominated woodland decomposer community dynamics and ecosystem processes. Increased mycelial growth at elevated temperature can be prevented by collembola grazing in soil microcosms. The strength of this top-down effect varied with fungal palatability, which had a bottom-up effect on collembola populations and their responses to warming. A mesocosm multispecies collembola population was more strongly regulated by the bottom-up effect of inoculation with cord-forming fungi than climate change (warming, in combination with soil wetting or drying). Collembola can graze fungal cords, but thickness and chemical defences make them less palatable than soil microfungi, which are outcompeted by basidiomycete mycelia. In the absence of fungal biomass limitation by collembola, abiotic conditions regulated microbial community functioning. Warming stimulated fungal-mediated wood decomposition, particularly in drier soils. Moisture was the most important determinant of enzyme activity and displayed an interaction with temperature analogous to that for wood decay. Macro-invertebrates, such as woodlice, are better able to exploit nutritious, but thick and defensive, fungal cords. The consequences of macro-invertebrate grazing for fungal-dominated microbial community function were tested in a field manipulation of woodlouse (Oniscus asellus, Isopoda) population densities, predicted to increase due to climate warming. This provides the first evidence for bottom-up effects of fungal palatability on woodlouse populations. Body lipid analysis revealed fungi as a major component of the generalist woodlouse diet. Despite low population densities at the site, altered O. asellus abundance influenced aspects of microbial community functioning. The importance of biotic effects on decomposition may be more heterogeneous than abiotic influences, depending on microbial community dominance and the abundance of key macro-invertebrate taxa