The size and the age of the metabolically active carbon in tree roots
Boaz Hilman, Jan Muhr, Juliane Helm, Iris Kuhlmann, Ernst‐Detlef Schulze, Susan Trumbore
Abstract
Abstract 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 ).