Different plant species can be compatible with the same species of myc
orrhizal fungi(1,2) and be connected to one another by a common myceli
um(3,4). Transfer of carbon(3-5), nitrogen(6,7) and phosphorus(8,9) th
rough interconnecting mycelia has been measured frequently in laborato
ry 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). Lab
oratory studies using isotope tracers show that the magnitude of one-w
ay transfer can be influenced by shading of 'receiver' plants(3,5), fe
rtilization of 'donor' plants with phosphorus(12), or use of nitrogen-
fixing donor plants and non-nitrogen-fixing receiver plants(13,14), in
dicating that movement may be governed by source-sink relationships. H
ere we use reciprocal isotope labelling in the field to demonstrate bi
directional carbon transfer between the ectomycorrhizal tree species B
etula papyrifera and Pseudotsuga menziesii, resulting in net carbon ga
in by P. menziesii. Thuja plicata seedlings lacking ectomycorrhizae ab
sorb 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 sour
ce-sink relationships regulate such carbon transfer under field condit
ions.