Ectomycorrhizal community responses to intensive forest management: thinning alters impacts of fertilization

Carbon and nutrient cycling are influenced by ectomycorrhizal (EM) fungi in forests. Factors altering carbon allocation in trees are likely to alter EM fungal community composition. We aimed to determine the effects of high fertilization, thinning, and their interaction on EM fungal communities and fine roots. Roots and EM fungi were sampled three years after establishment of fertilization and thinning plots in Pinus contorta forests of Canada. Ectomycorrhizas were identified using morphological and molecular techniques and changes to EM composition were detected by multivariate analyses. We recovered 77 EM fungal taxa with colonization levels up to 99 % in untreated plots; a 30 % decrease in colonization levels and similar declines in EM richness and diversity was found after fertilization. Thinning interacted with fertilization by decreasing the magnitude of shifts in EM abundance. Thinning tended to alleviatiate the effects of fertilization, suggesting there was sustained allocation of carbon to EM communities and greater root exploration. The fertilization treatment highlights an intimate and mechanistic relationship between tree and soil fungi and suggests that a reduced carbon allocation controls fungal proliferation and alters community composition. Intensive forest management treatments shift soil communities to an altered state, possibly impacting nutrient cycling in the interim.