Drought effect on tree biomass production and tree nutrient fluxes in a mature beech forest

Abstract Many temperate European forest stands are not well adapted to high drought levels and may endure significant decline of vitality under such conditions in the future. These declines may be partly due to perturbations of the biogeochemical nutrient cycles and their consequences on tree nutrition. This study was carried out in a mature beech forest subjected to three successive years of artificial drought. The objectives were to identify the drought effects on biomass production and the consequences on biogeochemical nutrient cycles. Monthly and annual monitoring allowed measurement of leaf, litterfall, stem and branches and fine-root production, tree nutritional status, and nutrient content (Ca, N, P, K, and Mg) in each biomass compartment and aboveground water fluxes. From these measurements, we calculated nutrient uptake, resorption, and foliar leaching. Droughts induced a 12% decrease in tree biomass production due to 36% and 20% decreases in perennial aboveground and leaf production, respectively, which was partially balanced by a 35% increase in fine-root production. This shift in biomass allocation could be an adaptation of trees to optimize water and nutrient uptake by roots. These changes in biomass growth allocation did not modify nutrient uptake due to the high nutrient allocation in fine roots. In addition, decreases in K resorption efficiency (from 44% to 17%) and K status in leaves were induced by drought. Our study highlights the importance of considering the belowground tree compartment to determine the impact of drought on tree biomass production and its related nutrient fluxes. The maintenance of nutrient uptake associated with lower nutrient resorption efficiency may be the origin of important nutritional issues under future climate scenarios.