Tree carbon reserves and survival to drought: Important knowledge gaps

Background/Question/Methods

Despite the critical role of forests on the global C cycle and recent increases in drought-induced forest mortality, remarkable knowledge gaps exist to accurately predict tree growth and survival under climate change. In particular, storage of non-structural carbon compounds (NSCC) is thought to be critical for tree survival under drought but its regulation and function is the least understood of the tree’s C budget components. Our current understanding of the role and regulation of stored NSCC relies on several assumptions. First, stored NSCC is generally assumed to be a passive buffer between source and sink demand for growth and respiration and, therefore, is an integrator of the tree’s C balance. Second, most process-based models commonly assume that C availability drives growth and ignore storage and environmental regulation of sink activity. Third, trees under C deficits are assumed to rely on stored C until reserves are exhausted. For the most part, these assumptions have not been experimentally tested, and increasing evidence suggests that some of them are wrong.

Results/Conclusions

Here, I suggest that large NSCC pools in trees may indicate active C partitioning to storage at the expense of growth. If so, growth is not driven by carbon availability, and stored NSCC cannot be used as indicators of the C balance. Further, a fraction of the stored NSCC pool may become sequestered over time and no longer be available as a reserve. I propose that, in addition to a C reservoir for growth and respiration, an important function of the available NSCC pool is to serve as a C source to generate osmotic gradients and drive the movement of water within the plant. This function is not only important to maintain cell turgor but also to maintain xylem and phloem transport.  Tree size, growth form, physiology, ecology and life history likely influence the degree to which trees rely on stored C to maintain vascular integrity and, therefore, the interdependence between water transport and NSCC dynamics. Future research is needed to understand if and under what conditions NSCC storage is an active process, the fraction of stored NSCC that remains available at any given time, and ultimately, the specific role of stored NSCC for vascular integrity and resilience to drought in different species.