Abstract
Little is known about the sources and age of C respired from tree roots.
Previous research in tree stems has identified two functional pools of
non-structural carbohydrates (NSC): an ‘active’ pool supplied directly
from canopy photo-assimilates that supports metabolism and a ‘stored’
pool used when fresh C supplies are limited. We compared the C isotope
composition of water soluble NSC and respired CO2 for
aspen roots (Populus tremula hybrids) that were cut off fresh C
supply via stem-girdling and prolonged incubation of excised roots. We
used bomb radiocarbon to estimate the time elapsed since C fixation for
respired CO2, water-soluble C, and structural
α-cellulose. While freshly excised roots respired CO2
with mean age <1 yr, within a week the age increased to
1.6-2.9 yr. Freshly excised roots from trees girdled ~3
months previously had similar respiration rates and NSC stocks as
un-girdled trees, but respired older C (~1.2 yr). We
estimate the NSC in girdled roots must be replaced 5-7 times by reserves
remobilized from root-external sources. Using a mixing model and
observed correlations between Δ14C of water-soluble C
and α-cellulose, we estimate ~30% of C is ‘active’
(~5 mg C g-1).