Understanding how treeline and nontreeline trees allocate carbon at their upper elevation limits is key to forecasting species shifts under climate warming. We compared a treeline deciduous broadleaf, Betula ermanii, with a nontreeline evergreen conifer, Picea jezoensis, at their species-specific upper limits and lower sites on Changbai Mountain. We measured leaf gas exchange and traced recent photoassimilates using in situ 13CO2 pulse labeling. Within species, photosynthetic traits and carbon stocks did not differ between elevations, indicating no carbon-acquisition limitation. In contrast, allocation patterns diverged at the upper limits: B. ermanii retained a larger share of recent 13C aboveground and showed slower carbon flow with longer mean residence time in leaves, whereas P. jezoensis allocated more 13C belowground and exhibited faster turnover. These patterns indicate that aboveground carbon allocation is primarily determined by species-specific leaf habits (deciduous broadleaf vs. evergreen conifer), whereas belowground allocation is more strongly shaped by stress conditions associated with upper elevational limits. Patterns were consistent with the functional type driving aboveground allocation and elevation-related site context shaping belowground allocation. We infer that treeline B. ermanii prioritizes aboveground investment to maximize short-season carbon gain and support cold tolerance, while nontreeline P. jezoensis invests belowground to enhance resource uptake and cope with competition. Overall, contrasting sink-mediated allocation strategies, rather than source limitations, govern species responses at upper limits and inform predictions of composition, distribution, and upward migration under future climate change.
Dong et al. (Sun,) studied this question.