NET TRANSFER OF CARBON BETWEEN ECTOMYCORRHIZAL TREE SPECIES IN THE FIELD

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.