Macrofungus communities correlate with moisture and nitrogen abundance in two old-growth conifer forests, Olympic National Park, Washington, USA

We characterized the epigeous macrofungus communities in two old-growth conifer forests by collecting sporocarps. Despite the similarity in dominant tree species in the two forests, the macrofungus communities were very different. At the drier, nitrogen-poor Deer Park area, the macrofungi were dominated by ectomycorrhizal species in the genera Cortinarius, Tricholoma, Hydnellum, Suillus, and Sarcodon. At the wetter, higher nitrogen Hoh Valley, the macrofungi were characterized by ectomycorrhizal species in different genera, such as Inocybe, Russula, Amanita, Boletus, and Phaeocollybia, and saprotrophic fungi accounted for a greater proportion of the community. Species richness was similar at the two areas, but sporocarp production was much higher at Deer Park. We propose that (i) these community differences developed over a long time; (ii) they are largely related to differences in ecosystem moisture and nitrogen abundance; and (iii) within the ectomycorrhizal fungi, possible causal mechanisms involve mycelial morphology and carbon allocation within the symbioses. The apparent response to relatively small but presumably long-term differences in nitrogen abundance suggests that sporocarp production by macrofungi could be an effective bioindicator and should be considered in determination of critical loads for atmospheric nitrogen deposition to temperate and boreal forests.Key words: critical nitrogen loads, ectomycorrhizal fungi, macrofungi, macrofungus communities, nitrogen, old-growth conifer forests.