1. Mycorrhizal pathways are comprised of fungal hyphae that facilitate carbon transfer between plants. We determined whether net carbon transfer occurred between conspecific conifer seedlings in the field, and whether soil disturbance or access to mycorrhizal pathways affected transfer.
2. We established two soil disturbances and planted pairs of different sized Pseudotsuga menziesii var. glauca seedlings (naturally regenerated or planted) into one of four mesh treatments (0.5, 35, 250 μm or directly into soil) restricting mycorrhizal pathways. We pulse-labelled both seedlings, one with 13CO2 and the other with 14CO2, to quantify net carbon transfer. Ectomycorrhizas were identified using morphological and molecular techniques.
3. Net carbon transfers were detected and were not due to re-fixation of respired carbon. More transferred carbon accumulated in shoots than roots. In disturbed soil, there was greater net carbon transfer to natural seedlings than planted seedlings; the reverse pattern was observed in undisturbed soil. For planted seedlings only, the magnitude of net carbon gain was positively related to seedling size and height growth rate. Greater net accumulation of carbon occurred in Rhizopogon vinicolor, a long-distance ectomycorrhizal fungi exploration type (morphological character), than the two other most abundant ectomycorrhizal fungi with contact- and short-distance exploration types. Long-distance exploration types have the potential to form long-distance hyphal connections between plant roots, whereas contact- and short-distance are restricted to short-distance (c. 0–0.25 μm) connections.
4. Synthesis. These results confirm that net carbon transfer occurs through mycorrhizal pathways; however, the amount transferred was very small. Mycorrhizal pathways were facilitating net transfer of carbon to large, vigorous natural seedlings in disturbed soils, whereas smaller planted seedlings received more net carbon gain in undisturbed soils. The size variation within these planted seedlings was great enough to elicit a positive relationship between net carbon gain and seedling size and growth rate. These findings are relevant to regeneration of forests characterized by mixed severity disturbance regimes, which leave a suite of environmental conditions that may result in a greater magnitude of transfer.