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A sweet new study: tropical forest species use nonstructural carbohydrates in different ways during drought

Anthropogenic climate change results in more than just elevated temperatures. Precipitation regimes are also impacted, and these changes include altered rainfall amounts, seasonality and variability (Feng et al. 2013), and increases in the frequency and severity of droughts (Dai 2013). Climate change is likely to have profound consequences for forests (Brodribb et al. 2020). While the potential effects of droughts on forests through tree mortality have been acknowledged for some decades (Karnig and Wyford 1968), a concerted focus on trees and droughts in the scientific literature is relatively recent. As evidence in support of this claim, a Web of Science search for the key words ‘tree and mortality and drought’ for the years 1900 through 2020 reveals that 82% of articles on this topic have been published in the last decade. As such, there is still disagreement on the mechanisms through which individual trees, species and forest communities respond to drought (Brodribb et al. 2020). Part of this uncertainty arises because tree species have evolved a diversity of strategies to cope with drought (Bartlett et al. 2016; Griffin-Nolan et al. 2018; O’Brien et al. 2017). Understanding interspecific variation in different traits that render tree species vulnerable or tolerant to drought is an active area of research (Trugman et al. 2020) because such knowledge is critical for modeling forest dynamics (Xu et al. 2016), and managing or restoring forests for future climates (Brodribb et al. 2020). Of the different characteristics that have been linked to tree performance during drought, nonstructural carbohydrates (NSC) emerge as one of the most enigmatic traits that may have a large role in explaining tree function. NSC include molecules such as soluble sugars, lipids and starch, and they play many physiological roles in plant metabolism, function and defense (O’Brien et al. 2020; Sala et al. 2012). NSC are storage molecules; they may accumulate in plant tissues such as leaves, stems and roots and then are used for growth or respiration when needed. There are many questions regarding NSC in plants (Dietze et al. 2014; Palacio et al. 2014; Sala et al. 2012; Tomasella et al. 2020) including Do they passively accumulate in plants or are they actively regulated? How do plants allocate photosynthate to NSC storage versus growth, and how does this change with environmental conditions? Are older pools of NSC deployed to meet changing plant demand, or is some fraction of NSC sequestered and inaccessible within tissues? What role do they have in plant response to drought? Resolving these issues is complicated by methodological limitations. While it is relatively straightforward to measure pools or standing concentrations of NSC in plant tissues, it is more difficult to measure their dynamics or fluxes. By manipulating NSC pools, the vital role of NSC in maintaining osmoregulation and hydraulic function was clearly demonstrated in a study of seedlings from ten rainforest species in Borneo. In this study, NSC pools were altered by subjecting seedlings to growth in light or shade and then the converse light environment, which resulted in seedlings of the same species that were enriched or depleted in NSC (O’Brien et al. 2014). Across all 10 species and within species, seedlings that were enriched with NSC survived longer during the drought than those with initially lower concentrations (O’Brien et al. 2014). This study left unanswered questions, including how NSC alleviate drought stress and how a species’ reliance on NSC during drought is integrated within the larger suite of traits that define life-history strategies. Stable isotope studies that allow for precise tracking of C through the plant can help to resolve these ambiguities. The work by O’Brien et al. in this issue of the Journal of Plant Ecology (O’Brien et al. 2020) interjects nuance into the discussion of the role of NSC by applying a C tracer to track sugar and starch movement through leaves, stems and roots during experimental drought in two congeneric species included in the previously studied ten species that differed in vulnerabilities to drought.