Interspecific C transfer was studied in laboratory microcosms
containing pairs of 6-month-old Betula papyrifera
Marsh. and Pseudotsuga menziesii (Mirb.) Franco seedlings growing
in individual, root-restrictive (28 μm pore
size) pouches filled with field soil. Interspecific transfer was examined
by
reciprocal labelling of seedlings with 13CO2(gas)
and
14CO2(gas). At the time of labelling, the root zones
of
ectomycorrhizal (EM) B. papyrifera and P.
menziesii were interconnected by an extensive network of EM mycelium.
Carbon
transferred through EM
connections was distinguished from that through soil pathways by comparing
microcosms where interconnecting
hyphae were left intact vs. those 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 average 50% more
14C and 66% more 13C from paper birch than vice versa,
however, differences between species were not statistically significant.
Neither net nor bidirectional transfer
differed between severing treatments, leaving in question the relative
importance of EM hyphae versus soil
transfer pathways. The tendency for P. menziesii to receive more
isotope than B. papyrifera corresponded with a
10-fold greater net photosynthetic rate per seedling and two-fold greater
foliar N concentration of B. papyrifera than P. menziesii.