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.

A representation of the phosphorus cycle for ORCHIDEE (revision 4520)

“…During droughts the nitrogen reserves are filled with foliage nitrogen which is recycled prior to leaf fall, while the nitrogen reserves are depleted when water availability and subsequent growth is high. Defoliation experiments indicate that plants can rely on substantial amounts of internally stored reserves of carbohydrate and nutrients which allow them to survive multiple defoliation events (Hartmann and Trumbore, 2016); however, the extent to which plants rely on internal storages is strongly species dependent (Piper and Fajardo, 2014), and the role of nutrients is often overlooked (Hartmann and Trumbore, 2016). Nonetheless, Ichie and Nakagawa (2013) showed that in Dryobalanops aromatica stored phosphorus accounted for 67.7 % of the total phosphorus requirements for reproduction, while stored N accounted for only 19.7 %, indicating substantial nutrient reserves in tropical trees.…”

Compound-Specific Carbon Isotopes and Concentrations of Carbohydrates and Organic Acids as Indicators of Tree Decline in Mountain Pine

“…In recent debates on the possible limitation of carbohydrate reserves, it was stated that limitation rarely occurs [16], possibly only for trees exposed to long lasting drought periods [17]. The transport of photosynthetically derived carbon from leaves to belowground biomass is often reduced under drought hindering optimal development of a functional root system, which can lead to a lack of essential nutrients and to higher mortality rates in the long-term [17,18]. A better understanding of the metabolic processes in declining trees is of great interest for predicting forest response to future warming.…”

Inferring the effects of sink strength on plant carbon balance processes from experimental measurements

“…An understanding of the dynamics of storage is also essential to correctly represent the C balance in models (Hartmann and Trumbore, 2016). If, for example, a direct growth limitation is implemented into models, how should the surplus of accumulated photosynthates be treated?…”