• Energy Research

Summary The allocation of nonstructural carbon (NSC) to growth, metabolism and storage remains poorly understood, but is critical for the prediction of stress tolerance and mortality. We used the radiocarbon (14C) ‘bomb spike’ as a tracer of substrate and age of carbon in stemwood NSC, CO2 emitted by stems, tree ring cellulose and stump sprouts regenerated following harvesting in mature red maple trees. We addressed the following questions: which factors influence the age of stemwood NSC?; to what extent is stored vs new NSC used for metabolism and growth?; and, is older, stored NSC available for use? The mean age of extracted stemwood NSC was 10 yr. More vigorous trees had both larger and younger stemwood NSC pools. NSC used to support metabolism (stem CO2) was 1–2 yr old in spring before leaves emerged, but reflected current‐year photosynthetic products in late summer. The tree ring cellulose 14C age was 0.9 yr older than direct ring counts. Stump sprouts were formed from NSC up to 17 yr old. Thus, younger NSC is preferentially used for growth and day‐to‐day metabolic demands. More recently stored NSC contributes to annual ring growth and metabolism in the dormant season, yet decade‐old and older NSC is accessible for regrowth.

SummaryDespite the importance of nonstructural carbohydrates (NSC) for growth and survival in woody plants, we know little about whole‐treeNSCstorage. The conventional theory suggests thatNSCreserves will increase over the growing season and decrease over the dormant season. Here, we compare storage in five temperate tree species to determine the size and seasonal fluctuation of whole‐tree totalNSCpools as well as the contribution of individual organs.NSCconcentrations in the branches, stemwood, and roots of 24 trees were measured across 12 months. We then scaled up concentrations to the whole‐tree and ecosystem levels using allometric equations and forest stand inventory data.While whole‐tree totalNSCpools followed the conventional theory, sugar pools peaked in the dormant season and starch pools in the growing season. Seasonal depletion of totalNSCs was minimal at the whole‐tree level, but substantial at the organ level, particularly in branches. Surprisingly, roots were not the major storage organ as branches stored comparable amounts of starch throughout the year, and root reserves were not used to support springtime growth.Scaling upNSCconcentrations to the ecosystem level, we find that commonly used, process‐based ecosystem and land surface models all overpredictNSCstorage.