Mature beech and spruce trees under drought – Higher C investment in reproduction at the expense of whole-tree NSC stores

Under drought, potential plant death from depletion of carbon (C) stores, referred to as carbon starvation, is thought to result from negative carbon balance during (partial) stomatal closure. As evidence for C starvation is rare for mature trees, we investigated the C dynamics in mature beech and spruce under drought, focusing on non-structural carbohydrates (NSC) as an indicator of their C balance. Trees were exposed to complete exclusion of precipitation throughfall over two consecutive growing seasons. We assessed NSC concentrations during the early dormant season in leaves, twigs, stem phloem/xylem, coarse and fine roots. Up-scaling to whole-tree NSC pools was achieved using a process-based single-tree model (“BALANCE”), estimating tree biomass increment. While there were distinct differences in NSC concentration among different tissues in both beech and spruce (root < stem < twig < leaves and xylem < phloem), drought did not affect NSC concentrations. However, compared to controls, the whole-tree NSC pool size significantly decreased under drought in both beech (42 %) and spruce (36 %), in parallel to a significant growth decline of overall 52 % and 57 %, respectively. Nevertheless, drought-stressed beech and spruce invested almost twice as much C in reproductive structures relative to total C investment (i.e. 6.0 ± 3.3 and 52.3 ± 8.71 %) compared to control trees (3.1 ± 1.8 and 29.2 ± 7.8 %). This highlights the high priority of C investment into reproduction relative to growth under drought. Given that NSC concentrations are maintained even under severe drought over two growing seasons, NSC pool sizes appear to be a better proxy to assess whole-tree's carbon status in mature trees. Overall, trees maintained NSC availability, avoiding carbon starvation, by downregulating a major C sink (i.e. growth) while upholding reproduction.