Patterns of nitrogen and carbon stable isotope ratios in macrofungi, plants, and soils from two old-growth conifer forests, Olympic National Park, Washington, USA

摘要

Natural abundance stable isotope ratios represent a potentially valuable tool for studying the ecology of forest macrofungi. I measured delta 15N and delta13C in ectomycorrhizal and saprotrophic macrofungi from two old-growth conifer forests, and in associated plants and substrates. Different pools exhibited distinct delta15N/delta 13C signatures. Although overall patterns were similar at the two forests, there were differences in delta15N/delta 13C among genera and species of fungi, and the different species present in the two forests appears to be the dominant factor contributing to differences in overall delta15N. delta15N differences among ectomycorrhizal fungi appear to reflect ecophysiological traits that can be interpreted in terms of life-history strategies. Dual-isotope signatures of myco-heterotrophic plants (e.g., Allotropa virgata and Monotropa hypopitys) were most similar to ectomycorrhizal fungi, and least like those of green plants. Their delta15N values correlated strongly and positively with those of putative mycobionts. Thus, nitrogen and carbon stable isotope ratios can provide evidence of myco-heterotrophy and host-specificity in these plants. The large number of macrofungus collections allowed the epigeous macrofungus communities in the two forests to be characterized and compared. At the drier, nitrogen-poor Deer Park area, the macrofungi were dominated by ectomycorrhizal species in the genera Cortinarius, Tricholoma, Hydnellum, and Suillus. At the wetter, higher-nitrogen Hoh Valley ectomycorrhizal species in different genera, such as Inocybe, Russula, Amanita, and Phaeocollybia, and saprotrophic fungi were more abundant. Species richness was similar at the two areas, but sporocarp production was much higher at Deer Park. This suggests that: community differences developed over a long time; they are largely related to differences in ecosystem moisture and nitrogen abundance; and, within the ectomycorrhizal fungi, possible causal mechanisms involve mycelial morphology and carbon allocation within the symbioses. The apparent response to relatively small differences in nitrogen abundance suggests that sporocarp production could be an effective bioindicator and should be considered in determining critical loads for atmospheric nitrogen deposition to temperate forests. Thus, stable isotope ratios in macrofungi can provide important information about their ecology. However, more information on the ecophysiology of individual fungi will be required before the full value of isotope measurements can be realized.