Forest trees compete for light and soil resources, but photoassimilates, once produced in the foliage, are not considered to exchange between individuals. Applying stable carbon isotope labelling at canopy-scale, we were able to trace carbon allocation in and between mature individuals of Picea, Pinus, Larix and Fagus trees in the forest. Individual species of ectomycorrhizal fungi (EMF) are known to form mutualistic symbiosis with many different host species in temperate forests, and EMF form extensive mycelial networks in the soil connecting many mycorrhizal root tips to the same mycelium. As a consequence, unrelated neighbour trees may be connected to the same fungal network. Here we document the actual EM community for exactly these tree individuals by using molecular tools.
Results/Conclusions
We demonstrate that carbon assimilated by 40 m tall spruce is traded over to neighbouring beech, larch, and pine via overlapping root spheres. Among the nearly 2000 EM root tips examined, the only recently discovered, ancient basidiomycota genus Sebacina (that commonly produces no prominent sporocarps) accounted for more than 60% of EM associations common to more than two tree species and 40% of the associations common to the four tree species. Sebacina dimitica, the most abundant EM species, was identified on all 4 tree species, plus the additionally investigated Abies alba. While the resolution of our molecular markers does not enable us to confirm a common EMF genotype on ectomycorrhizas from the different tree species (i.e. a single common EMF genet), our data lend support to the existence of a common EMF network that facilitates carbon transfer. Our results also underline the importance of Sebacina and a number of other EMF taxa (from the genera Russula, Amphinema, Clavulina, Tomentella, Peziza, Trichphaea, Inocybe, Lactarius and Tuber) as potential agents of transfer. Of the 39 EM species of the 10 major genera found, 75% were not host-specific, indicating that a diverse, highly complex network of fungal hyphae is connecting those adult forest trees. Isotope mixing signals indicate that the interspecific, bidirectional transfer, assisted by common ectomycorrhiza networks, accounted for 40% of the fine root carbon (~280 kg ha-1 a-1 tree-tree transfer). While competition for resources is commonly considered as the dominant tree-tree interaction in forests, trees actually interact in more complex ways including a massive carbon exchange.