Moisture stress limits radial mixing of non-structural carbohydrates in sapwood of trembling aspen

Dynamics in non-structural carbohydrate (NSC) pools may underlie observed drought legacies in tree growth. We assessed how aridity influences the dynamics of different-aged NSC pools in tree sapwood at two sites with differing climate conditions (wet vs dry), which also experienced widespread regional drought 5 years earlier. We used an incubation method to measure the radiocarbon (Delta C-14) in CO2 respired from Populus tremuloides Michx. (aspen) tree rings to evaluate NSC storage and mixing patterns, coupled with measurements of NSC (soluble sugars and starch) concentrations and respired delta C-13-CO2. At a wet site, CO2 respired from rings formed during 1962-67 was only similar to 11 years old, suggesting deep sapwood mixing of NSCs as starch. At a dry site, the total NSC was about one-third of wet-site totals, maximum ages in deep rings were lower and ages more rapidly increased in shallow rings and then plateaued. These results suggest historically shallower mixing and/or relatively higher consumption of NSCs under dry conditions. Both sites, however, had similar aged NSC (<1 year) in the most recent six rings, indicative of deep radial mixing following relatively wet conditions during the sampling year. We suggest that the significant differences in NSC mixing among sites are driven by moisture stress, where aridity reduces NSC reserves and restricts the depth of radial mixing. However, dynamic climate conditions in the south-western USA resulted in more complex radial patterns of sapwood NSC age than previously described. We suggest a novel conceptual framework to understand how moisture variability might influence the dynamics of NSC mixing in the sapwood.