Net transfer of carbon between ectomycorrhizal tree species in the field

Different plant species can be compatible with the same species of mycorrhizal fungi1,2 and be connected to one another by a common mycelium3,4. Transfer of carbon3,4,5, nitrogen6,7 and phosphorus8,9 through interconnecting mycelia has been measured frequently in laboratory experiments, but it is not known whether transfer is bidirectional, whether there is a net gain by one plant over its connected partner, or whether transfer affects plant performance in the field10,11. Laboratory studies using isotope tracers show that the magnitude of one-way transfer can be influenced by shading of ‘receiver’ plants3,5, fertilization of ‘donor’ plants with phosphorus12, or use of nitrogen-fixing donor plants and non-nitrogen-fixing receiver plants13,14, indicating that movement may be governed by source–sink relationships. Here we use reciprocal isotope labelling in the field to demonstrate bidirectional carbon transfer between the ectomycorrhizal tree species Betula papyrifera and Pseudotsuga menziesii, resulting in net carbon gain by P. menziesii. Thuja plicata seedlings lacking ectomycorrhizae absorb small amounts of isotope, suggesting that carbon transfer between B. papyrifera and P. menziesii is primarily through the direct hyphal pathway. Net gain by P. menziesii seedlings represents on average 6% of carbon isotope uptake through photosynthesis. The magnitude of net transfer is influenced by shading of P. menziesii, indicating that source–sink relationships regulate such carbon transfer under field conditions.