Belowground Carbon Trade among Tall Trees in a Temperate Forest

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 show that carbon assimilated by 40 m tall spruce is traded over to neighbouring beech, larch, and pine via overlapping root spheres. 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 may interact in more complex ways including substantial carbon exchange.