Relationships between non-structural carbohydrates reserves and growth resilience of Fagus sylvatica to extreme drought events

Following the 2018’s extreme drought summer in Central Europe and the recurrent episodes of drought in the summers 2019 and 2020, a massive dieback of European beech (Fagus sylvatica) has been observed over the following years in the core of its distribution range. This ecological and ecophysiological study aims at evaluating the role of carbon reserves in tree survival and resilience to extreme drought and recurrent soil water deficit using tree ring width analyses combined with measures of non-structural carbohydrates (NSC) concentrations from 2020 to 2022.

In 2019, 20 study plots were selected in North-Eastern France and 15 beech trees per plot were selected according to contrasted crown condition. The crown transparency of these 300 selected trees was then assessed annually from 2019 to 2023. Each autumn, from 2020 to 2022, trees were sampled at the base of the trunk with an increment borer to measure NSC concentrations. In 2022, an additional core was sampled from each tree at breast height and until the tree heart for a tree-ring growth analysis and to determine tree age, the resistance, recovery and resilience of radial stem growth to soil water deficit. The daily soil water balance was parameterized for each plot with the model Biljou© to compute retrospectively the soil water deficit of each plot from 1959 to 2022.

Our results show that NSC concentrations from 2020 to 2022 were negatively correlated with the crown transparency of the same year. Trees whose crown transparency increased during the monitoring presented lower NSC concentrations in 2021 and 2022.

Trees showing a better resistance, recovery and resilience of the radial growth to the 2018-2020 water shortage period had higher NSC concentrations and trees with higher mean basal area increment during the past 10 years exhibited higher NSC concentrations. During the monitoring, 31 trees died and their NSC reserves were significantly depleted after 3 successive years of soil water deficit. Moreover, 59 % of the trees whose NSC concentrations were below 10  in 2020 died in the three next years while only 2 % of the trees showing NSC concentrations higher than 10  in 2020 died in the next three years. A possible carbon starvation threshold has been identified for the Fagus sylvatica species.

Our results suggest that NSC storage is important for the survival and the resilience of beech radial growth to soil water deficit. The depletion of NSC reserves will be discussed as a key component in the process of beech tree mortality.