Sacrificing growth and maintaining a dynamic carbohydrate storage are key processes for promoting beech survival under prolonged drought conditions

Abstract

Key message: In case of a prolonged drought, the stored carbohydrates in trees were remobilized to fuel survival functions until their nearly depletion at death stage. Abstract: Dynamic global vegetation models project forest tree mortality in response to the recurrent severe droughts likely in the future. However, these models should better take into account the physiological processes involved in tree mortality. Faced with severe drought, the Fagus sylvatica L. tree strongly limits its cambial growth. This suggests that readjustments in carbon (C) allocation among sink functions are taking place in response to the lack of water and this could allow tree’s survival. For 3 years, we induced a water shortage on 8-year-old beech trees in a rain exclusion system. During this period, we analysed the consequences of severe drought on survival rate, growth, and non-structural carbohydrate (NSC) dynamics in the aboveground and belowground compartments of control, water-stressed living, and dead trees. The survival rate after 3 years of drought was 87%, while primary and secondary growth was strongly reduced. The first 2 years, NSC concentrations increased in all tree compartments (stem, branches, and roots) in response to drought. However, during the third year, starch dropped markedly in water-stressed trees, while soluble sugar concentrations remained similar to control trees. All the compartments in dead trees were virtually empty of starch and soluble sugars. Maintaining an active C storage function at the expense of growth was certainly key to F. sylvatica survival under prolonged extreme drought conditions. Process-based models predicting mortality should better take into account C storage and remobilization processes in forest trees.