Numerous mechanisms may underlie drought legacies in tree growth, including hydraulic impairment, damaged needle cohorts, depletion of deep soil water, and/or exhaustion of non-structural carbohydrates (NSCs) following drought. However, it is unknown how availability (accessibility) of NSC pools contributes to this phenomenon. Incomplete availability of NSCs could explain past findings of failure to exhaust measureable NSCs during stress preceding mortality. Radiocarbon work has posited the existence of two NSC pools (“fast” and “slow”) in trees to explain “mixing” (radial transport) of NSCs into older xylem rings. It is possible that the “slow” pool is relatively inaccessible, and draw down is only triggered during extended stress (e.g., drought). We evaluated size and age of the NSC pool in aspen (Populus tremuloides) under varying water stress. We measured radiocarbon and δ13C of respired CO2 collected from live-incubated aspen sapwood rings and phloem, and quantified respiration rates and extractable NSC pool sizes (soluble sugars and starch). We addressed two questions: (Q1) do the ages of respired carbon differ between sites, representing different reliance on “slow” vs. “fast” pools at dry sites? And (Q2) Are δ13C and radiocarbon ages of respired carbon correlated, suggesting drought stress affects NSC mixing and utilization?
Results/Conclusions
NSC ages were younger than documented in other species: 50 year old rings respired CO2 that was only ~15 years old (largest Δ14C=67.9±2.95‰), suggesting deep mixing. Alternatively, younger NSCs could be preferentially respired, and older NSCs, though extractable, may not be metabolically available. Though radiocarbon values do not clearly differ between sites (Q1), results from both isotopic measurements suggest a boundary to mixing depth at both sites that is coincident with the 2011-2012 drought in the southwestern US. It is unknown whether this boundary to mixing also presents a boundary to mobilization and utilization for new wood growth. At the dry site, NSCs are younger relative to rings when δ13C is less negative (Q2). This suggests NSCs are nearly exhausted during/after drought years, and subsequently mixed in (recent) NSCs dominate the Δ14C signal, contributing to a younger (“fast”) NSC pool. These results suggest moisture stress effects NSC mixing and could underlie delayed recovery post-drought. Phloem NSC was ~3-6 years old, with ~2 times as much sugar and ~30 times as much starch per mass as sapwood. How sapwood and phloem NSC storage play complementary or redundant roles is unclear, particularly as aspen phloem NSC pools are potentially large.