Different plant species can be compatible with the same species of mycorrhizal fungi(1,2) and be connected to one another by a common mycelium(3,4). Transfer of carbon(3-5), nitrogen(6,7) and phosphorus(8,9) 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 field(10,11). Laboratory studies using isotope tracers show that the magnitude of one-way transfer can be influenced by shading of ’receiver’ plants(3,5), fertilization of ’donor’ plants with phosphorus(12), or use of nitrogen-fixing donor plants and non-nitrogen-fixing receiver plants(13,14), 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.