Understanding the roles of nonstructural carbohydrates in forest trees – from what we can measure to what we want to know

Abstract

Summary: Carbohydrates provide the building blocks for plant structures as well as versatile resources for metabolic processes. The nonstructural carbohydrates (NSC), mainly sugars and starch, fulfil distinct functional roles, including transport, energy metabolism and osmoregulation, and provide substrates for the synthesis of defence compounds or exchange with symbionts involved in nutrient acquisition or defence. At the whole‐plant level, NSC storage buffers the asynchrony of supply and demand on diel, seasonal or decadal temporal scales and across plant organs. Despite its central role in plant function and in stand‐level carbon cycling, our understanding of storage dynamics, its controls and response to environmental stresses is very limited, even after a century of research. This reflects the fact that often storage is defined by what we can measure, that is, NSC concentrations, and the interpretation of these as a proxy for a single function, storage, rather than the outcome of a range of NSC source and sink functions. New isotopic tools allow direct quantification of timescales involved in NSC dynamics, and show that NSC‐C fixed years to decades previously is used to support tree functions. Here we review recent advances, with emphasis on the context of the interactions between NSC, drought and tree mortality. Contents Summary 386 I. Introduction 386 II. NSC in plant function: synthesis, classes, roles and responses to drought 388 III. Tools and approaches for quantifying NSC dynamics 392 IV. What is the spatial and temporal distribution of NSC in trees? 392 V. Studies on the use of NSC in plant functioning – progress towards answering longstanding questions 396 VI. Summary and conclusion 399 Acknowledgements 399 References 399