Natural C-13 abundance reveals trophic status of fungi and host-origin of carbon in mycorrhizal fungi in mixed forests

Fungi play crucial roles in the biogeochemistry of terrestrial ecosystems, most notably as saprophytes decomposing organic matter and as mycorrhizal f ungi enhancing plant nutrient uptake. However, a recurrent problem in funga l ecology is to establish the trophic status of species in the field, Our i nterpretations and conclusions are too often based on extrapolations from l aboratory microcosm experiments or on anecdotal field evidence, Here, we us ed natural variations in stable carbon isotope ratios (delta(13)C) as an ap proach to distinguish between fungal decomposers and symbiotic mycorrhizal fungal species in the rich sporocarp flora (our Sample contains 135 species ) of temperate forests. We also demonstrated that host-specific mycorrhizal fungi that receive C from overstorey or understorey tree species differ in their delta(13)C. The many promiscuous mycorrhizal fungi, associated with and connecting several tree hosts, were calculated to receive 57-100% of th eir C from overstorey trees. Thus, overstorey trees also support, partly or wholly, the nutrient-absorbing mycelia of their alleged competitors, the u nderstorey trees.