RECIPROCAL TRANSFER OF CARBON ISOTOPES BETWEEN ECTOMYCORRHIZAL BETULA-PAPYRIFERA AND PSEUDOTSUGA-MENZIESII

Interspecific C transfer was studied in laboratory microcosms containi ng pairs of 6-month-old Betula papyrifera Marsh. and Pseudotsuga menzi esii (Mirb.) France seedlings growing in individual, root-restrictive (28 mu m pore size) pouches filled with held soil. Interspecific trans fer was examined by reciprocal labelling of seedlings with (CO2(gas)-C -13) and (CO2(gas)-C-14). At the time of labelling, the root zones of ectomycorrhizal (EM) B. papyrifera and P. menziesii were interconnecte d by an extensive network of EM mycelium. Carbon transferred through E M connections was distinguished from that through soil pathways by com paring microcosms where interconnecting hyphae were left intact vs. th ose where they were severed immediately before labelling. Transfer was bidirectional, and represented 5 % of total isotope uptake by both B. papyrifera and P. menziesii together. P. menziesii received on averag e: 50 % more C-14 and 66 % more C-13 from paper birch than vice versa, however, differences between species were not statistically significa nt. Neither net nor bidirectional transfer differed between severing t reatments, leaving in question the relative importance of EM hyphae ve rsus soil transfer pathways. The tendency for P. menziesii to receive more isotope than B. papyrifera corresponded with a 10-fold greater ne t photosynthetic rate per seedling and two-fold greater foliar N conce ntration of B. papyrifera than P. menziesii.