Age and availability of bole nonstructural carbon across depths in Populus tremuloides

Drew Peltier, Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, James LeMoine and Mariah Carbone, Center for Ecosystem Science and Society, Northern Arizona University, Flagstaff, AZ, Chris Ebert, Center for Ecosystem Science & Society, Northern Arizona University, Flagstaff, AZ, Xiaomei Xu, Earth System Science, University of California, Irvine, Irvine, CA, Andrew D. Richardson, Center for Ecosystem Science and Society; School of informatics, Computing and Cyber Systems, Northern Arizona University, AZ

Background/Question/Methods
Woody plants store nonstructural carbon (NSC; mainly sugars and starch) for decades. Under stress, trees may remobilize stored NSC for metabolism, defense, and growth of new tissues. Large uncertainties remain, however, as to the availability of old NSC in large storage organs like bole sapwood. For example, it is unlikely that the NSC extracted from dead tissues like heartwood is metabolically available to trees. Similarly, trees always fail to exhaust 100% of NSC pools in mortality studies in both field and greenhouse settings. Methodologies to quantify stored NSC rely on destructive and labor intensive tissue extraction procedures that may overestimate the mass and mean age (quantified using radiocarbon, 14C) of available NSC. To better understand and quantify NSC amounts, mean age, and availability, we compared the traditional approach of extractions to incubations of live wood tissues in a ~30 year-old trembling aspen (Populus tremuloides). Using replicate 12 mm tree cores, we compared the NSC extracted mass and its ∆14C to the total CO2 respired and its ∆14C for three bole depths (shallow sapwood, deep sapwood, recent heartwood). Respiration samples were collected 24, 72, and 120 hours after sampling of live tissues.
Results/Conclusions
NSC concentrations generally decreased with sapwood depth; this pattern was most clear for starch, while sugar concentrations were more variable across rings (mean=0.75±0.51% by mass). Starch concentrations averaged 0.2±0.11% in the youngest 13 rings, corresponding roughly to the sapwood-heartwood boundary. No appreciable starch concentrations were detected in the heartwood (rings 14-18), whereas sugar concentrations remained appreciable (0.94±0.56%). Despite this, due to declining ring width from the pith to the bark, total non-structural carbon masses were around 5 mg for all three depths (shallow sapwood, deep sapwood, recent heartwood) in the tree cores. After 120 hours, wood tissue incubations exhausted ~100%, 82%, and ~3% of these carbon totals in the shallow sapwood, deep sapwood, and recent heartwood core subsamples respectively. Incomplete respiration of the total extractable NSC pools in deeper tree rings suggests not all of this NSC is available for respiration, with little to none available in dead heartwood. Comparison of the ∆14C from extractions and incubations will allow further insight on the age and homogeneity of these NSC pools, and the utility of old NSC for tree physiological metabolism.