Saving for an emergency: how does carbon storage contribute to tree survival under long-term stress?

This scientific commentary refers to ‘Carbon dynamics in long-term starving poplar trees—the importance of older carbohydrates and a shift to lipids during survival’ by Helm et al. (doi: 10.1093/treephys/tpad135).As a fundamental element in plant structure and functioning, carbon (C) plays a primary role in tree growth, development and survival. Trees assimilate CO2 from the atmosphere via photosynthesis to produce carbohydrates (Figure 1a), which are then allocated to different functional pools and processes, such as growth, respiration, defense, symbionts and exudation, as well as storage (Kozlowski and Pallardy 1996, Dietze et al. 2014, Hartmann and Trumbore 2016). The storage of non-structural carbohydrates (NSC; here water-soluble sugars and starch as the main constituents) can be regulated via an imbalance between C supply and demand (i.e., passive process) and/or via a prioritized allocation of C to storage relative to other functions (i.e., active process) (Chapin et al. 1990, Körner 2003, Sala et al. 2012, Martínez-Vilalta et al. 2016, Resco de Dios and Gessler 2021). Generally, NSC storage can act as a buffer during times when C assimilation is lower than demand (Landhäusser and Lieffers 2002, Adams et al. 2013, Hartmann et al. 2013, Piper and Fajardo 2014, 2016, Sevanto et al. 2014, Weber et al. 2018, Piper and Paula 2020, Piper et al. 2022). Therefore, the capacity to mobilize and utilize stored NSC is a key mechanism for trees to survive biotic and abiotic stress (O’Brien et al. 2014, Huang et al. 2021, Hart et al. 2024, Ouyang et al. 2024). Despite some progress in quantifying the remobilization and use of previously stored NSC in large trees under normal or stressful conditions (e.g., Carbone et al. 2007, 2013, Richardson et al. 2015, Muhr et al. 2018, Peltier et al. 2023), it is still uncertain to what degree stored NSC are remobilized and utilized in trees to support their metabolic functions to survive extended periods of C limiting stress. In addition, it is not clear whether and how the storage of other potential secondary C compounds (e.g., lipids, phenolic glycosides) will or can contribute to energy requirements during longer-term environmental stress (Gessler and Treydte 2016, Hartmann and Trumbore 2016, Muhr et al. 2018, Hillabrand et al. 2023). Filling these knowledge gaps will help to understand how trees can regulate C metabolism to cope with unfavorable environmental conditions and more accurately predict forest C dynamics in the long run. In this issue of Tree Physiology, Helm et al. add interesting new insights into the existing knowledge of tree C dynamics when faced with a 3-year C limitation stress by linking the contribution of stored soluble sugars, starch and lipids to respiratory activities.