Increased belowground tree carbon allocation in a mature mixed forest in a dry versus a wet year

Tree species differ in their carbon (C) allocation strategies during environmental change. Disentangling species-specific strategies and contribution to the C balance of mixed forests requires observations at the individual tree level. We measured a complete set of C pools and fluxes at the tree level in five tree species, conifers and broadleaves, co-existing in a mature evergreen mixed Mediterranean forest. Our study period included a drought year followed by an above-average wet year, offering an opportunity to test the effect of water availability on tree C allocation. We found that in comparison to the wet year, C uptake was lower in the dry year, C use was the same, and allocation to belowground sinks was higher. Among the five major C sinks, respiration was the largest (ca. 60%), while root exudation (ca. 10%) and reproduction (ca. 2%) were those that increased the most in the dry year. Most trees relied on stored starch for maintaining a stable soluble sugars balance, but no significant differences were detected in aboveground storage between dry and wet years. The detailed tree-level analysis of nonstructural carbohydrates and delta 13C dynamics suggest interspecific differences in C allocation among fluxes and tissues, specifically in response to the varying water availability. Overall, our findings shed light on mixed forest physiological responses to drought, an increasing phenomenon under the ongoing climate change. How does drought affect mature forest trees' carbon allocation dynamics? Using a monthly mass balance of carbon assimilation and investment above- and belowground, we ask how trees from different species in a mixed Mediterranean forest respond to dry versus wet year. We found that in comparison to the wet year, carbon uptake was lower in the dry year, carbon use was the same, and allocation to belowground sinks was higher. The detailed tree-level analysis suggests interspecific differences of carbon allocation dynamics, specifically in response to the varying water availability.image