The size and the age of the metabolically active carbon in tree roots

Little is known about the sources and age of C respired by tree roots. Previous research in stems identified two functional pools of non‐structural carbohydrates (NSC): an “active” pool supplied directly from canopy photo‐assimilates supporting metabolism and a “stored” pool used when fresh C supplies are limited. We compared the C isotope composition of water‐soluble NSC and respired CO 2 for aspen roots ( Populus tremula hybrids) cut off from fresh C supply after stem‐girdling or prolonged incubation of excised roots. We used bomb radiocarbon to estimate the time elapsed since C fixation for respired CO 2 , water‐soluble NSC and structural α‐cellulose. While freshly excised roots (mostly <2.9 mm in diameter) respired CO 2 fixed <1 year previously, the age increased to 1.6–2.9 year within a week after root excision. Freshly excised roots from trees girdled ~3 months ago had respiration rates and NSC stocks similar to un‐girdled trees but respired older C (~1.2 year). We estimate that over 3 months 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 Δ 14 C of water‐soluble C and α‐cellulose, we estimate ~30% of C is “active” (~5 mg C g −1 ).