Effects of Soil Chemistry, Niche Partitioning, and Competitive Exclusion on the Ectomycorrhizal Community Composition of Red Alder (Alnus rubra)

There have been few studies of ectomycorrhizal fungal (EMF) communities in alder forests. The effects of spatial variability, competition, and environmental gradients on EMF community composition have seldom been investigated in sufficient depth to tease out more subtle impacts, particularly in field settings. In an exploratory field-based study, I analyzed how soil chemistry gradients influence EMF interspecific interactions and overall community composition of EMF species associated with an 85-year-old western Washington Alnus rubra stand. From May to September 2011, 150 root cores, soil variables (soil moisture, total C and N, available P, soil pH) and Frankia nodule presence data were assessed. Root tip morphology along with DNA analyses of root tip and sporocarp sequences revealed 22 EMF species, including 14 new associates with A. rubra. Spatial analysis of the community revealed a random pattern, indicating no spatial autocorrelation of samples. Compositional analysis identified two large groups of root cores due to the spatial separation of two dominant species: Cortinarius cf. alboviolaceous and Naucoria escharoides . Month of sampling and Frankia nodule presence (p<0.05) explained 16% of the variation of the EMF community. Soil chemistry variables (moisture, pH, phosphorous, total N) impacted community composition at finer scales; EMF species varied in abundance and tolerance for each variable. The use of multivariate techniques and intensive sampling effort provided the tools necessary to form a more complete picture of Alnus rubra ectomycorrhizal community composition than was previously described. Niche partitioning relative to soil chemistry gradients and competitive exclusion are inferred for species of the same genus and between Lactarius, Tomentella, Naucoria, and Cortinarius species. Further research incorporating ecological concepts of distribution in ectomycorrhizal community composition is essential for understanding the ecology of EMF in natural environments.