Woody plant species store nonstructural carbohydrates (NSCs) for many functions. While known to buffer against fluctuations in photosynthetic supply, such as at night, NSC stores are also thought to buffer against environmental extremes, such as drought or freezing temperatures by serving as either back-up energy reserves or osmolytes. However, a clear picture of how NSCs are shaped by climate is still lacking. Here, we update and leverage a unique global database of seasonal NSC storage measurements to examine whether maximum total NSC stores and the amount of soluble sugars are associated with clinal patterns in low temperatures or aridity, indicating they may confer a benefit under freezing or drought conditions. We examine patterns using the average climate at each study site and the unique climatic conditions at the time and place in which the sample was taken. Altogether, our results support the idea that NSC stores act as critical osmolytes. Soluble Sugars increase with both colder and drier conditions in aboveground tissues, indicating they can plastically increase a plants’ tolerance of cold or arid conditions. However, maximum total NSCs increased, rather than decreased, with average site temperature and had no relationship to average site aridity. This result suggests that the total amount of NSCs that a plant stores may be more strongly determined by its capacity to assimilate carbon than by environmental stress. Thus, NSCs are unlikely to serve as reservoirs of energy. This study is the most comprehensive synthesis to date of global NSC variation in relation to climate and supports the idea that NSC stores likely serve as buffers against environmental stress. By clarifying their role in cold and drought tolerance, we improve our ability to predict plant response to environment.

The data used for our analysis were initially collected in a database of NSC measurements, published in 2016 (Martínez-Vilalta et al., 2016), then updated by us to include articles through April 2020. To begin, we conducted a search on Web of Science (accessed April 6, 2020) for the following word combinations in any field of the articles “(NSC OR TNC OR starch OR carbohydrate*) AND (plant* OR shrub* OR tree* OR seedling * OR sapling*) AND (seasonal* OR temporal*). Our search yielded 1,040 articles, of which we reviewed the abstracts to further only include measurements taken on wild species (none under cultivation) under natural, field conditions. Where a paper reported the results of a natural or under-field conditions experiment, we only included the control samples in our analysis. We further only examined papers that took at least (1) three measurements over time of the same individuals or populations and (2) ran for at least four months or more. Finally, due to a lack of data in some categories and to keep comparisons consistent, we further limited studies to only those done on (3) woody land plants and (4) mature trees (as defined by the authors of each study), (5) where the tissue sampled was not bark, phloem, or cortex, and (6) the organ sampled was the branch, coarse root, or stem. The age of the tissue itself can also affect the amount of NSC stores (Carbone et al., 2013; Furze et al., 2019); in our database, all stem results included are the sapwood, typically the outer 2cm of the core, and only two studies that eventually made it to our final cut specified whether branches were current or previous years. Given these branch NSC concentration values of current versus older branches were all within 2–3 mg/g of each other, we averaged them together.

All data were taken from the text, tables, supplementary data repositories, or the figures. For figure data extraction, we employed the software WebPlotDigitizer (Rohatgi, 2021). NSC data were converted to standard units of milligrams NSC per gram dry weight (mg g^-1) and reported as soluble sugars and starch. The composition of soluble sugars may vary depending on method (Landhausser et al., 2018; Quentin et al., 2015), but they are largely composed of glucose, fructose, and sucrose (G. Hoch et al., 2003). Soluble sugars and starches were added to estimate total NSC (referred to throughout as NSC). For our following climate analyses, we examined NSC (SS + starch), SS, and % SS (SS / NSC).