The mycorrhizas of Europe’s pine forests in the context of nitrogen pollution

Abstract

Most trees in temperate and boreal ecosystems form obligate associations with ectomycorrhizal (ECM) fungi, but environmental change may alter ECM communities, potentially having major impacts on forest ecosystem functioning. Understanding the potential consequences of environmental change requires knowledge about current ECM fungal distributions, both to provide a baseline against which to measure changes in communities, and to establish the effects of current broad-scale environmental gradients on mycorrhizal biogeography. This study aims to assess the feasibility of a continental-scale survey of ECM fungal communities across a network of long-term forest monitoring plots, and provide insights into the community ecology of these important organisms. Using a spatially hierarchical sampling strategy and molecular identification of the fungi forming ECM root tips, the ECM fungal communities within 12 plots in the UK and Germany were characterised. In combination with measurements of a range of environmental variables at the 12 plots, the data indicate that nitrogen availability is a potentially important determinant of ECM fungal composition and richness across large spatial scales, but that other environmental variables may also act at large scales to influence ECM fungal communities. Evidence is also found to support a role of interspecific competition structuring ECM fungal communities within, but not across, plots, and a complex interplay between environmental gradients, co-occurrence patterns and phylogenetic relatedness is revealed. Within plots, microsatellite analysis indicates that pine roots from adjacent trees overlap, and do not have exclusive below-ground territories. Re-sampling plots, and reciprocal transplantation of pine seedlings between plots, indicates that pine mycorrhizas are stable both over time and on transplanted seedlings. This work increases our knowledge of the belowground ecology of ECM fungi, and highlights the value of identifying mycorrhizal abundance and distribution patterns - effects of global environmental change may already be significantly altering communities of this functionally critical group.