Recent extreme drought events in Central Europe have caused widespread forest dieback with detrimental effects on forest functioning and carbon and water balance. This impact has been notable on European beech (Fagus sylvatica L.), particularly at the core of its distribution, causing concern among forest stakeholders and questions about the resilience capacity of beech trees. The objective of this study is to investigate the physiological processes linked to water and carbon constraints involved in the resilience of beech cambial growth to drought. We selected 56 beech trees distributed in four plots in north‐eastern France with different soil water deficits characterized retrospectively by a water balance model. Functional traits including tree ring width, wood anatomical traits and stable isotopes (e.g. δ¹³C and δ¹⁸O) were measured to retrospectively assess the effect of recent recurrent drought in 2015, 2018–2020, and 2022. Decreased tree growth and increased δ¹⁸O and intrinsic water use efficiency (iWUE) were observed due to soil water shortage, whereas xylem vessel size and theoretical specific xylem hydraulic conductivity (K_th) did not show obvious changes. Vessel density was negatively correlated with annual ring width index and was highly sensitive to drought. δ¹³C, δ¹⁸O and iWUE were not significantly related to tree ring width index. The plot that experienced the most severe drought intensity in 2018–2020 showed a significant decrease in tree growth resistance and resilience compared to its resistance and resilience during the 2015 drought event. Surprisingly, growth resilience was not associated with tree anatomical and isotopic traits. Synthesis. Our results demonstrate that beech xylem structure responds to drought by adjusting the radial growth of tree rings with a relatively stable vessel diameter. Our study also highlights the impact of consecutive or recurrent drought in reducing beech tree resistance and resilience, particularly at sites with higher drought intensity. Tree resilience does not seem to involve changes in traits that would promote the hydraulic functioning to better cope with future soil water shortages.

Forest trees adopt effective strategies to optimize nitrogen (N) use through internal N recycling. In the context of more recurrent environmental stresses due to climate change, the question remains whether increased frequency of drought or defoliation threatens this internal nitrogen recycling strategy. We submitted 8-year-old beech trees to two years of either severe drought (Dro) or manual defoliation (Def) to create a state of N starvation. At the end of the 2nd year before leaf senescence, we labeled the foliage of the Dro and Def trees, as well as that of control (Co) trees, with ¹⁵N-urea. Leaf N resorption, winter tree N storage (total N, ¹⁵N, amino acids, soluble proteins) and N remobilization in spring were evaluated for the three treatments. Defoliation and drought did not significantly impact foliar N resorption or N concentrations in organs in winter. Total N amounts in Def tree remained close to those in Co tree, but winter N was stored more in the branches than in the trunk and roots. Total N amount in Dro trees was drastically reduced (−55%), especially at the trunk level, but soluble protein concentrations increased in the trunk and fine roots compared to Co trees. During spring, ¹⁵N was mobilized from the trunk, branches and twigs of both Co and Def trees to support leaf growth. It was only provided through twig ¹⁵N remobilization in the Dro trees, thus resulting in extremely reduced Dro leaf N amounts. Our results suggest that stress-induced changes occur in N metabolism but with varying severity depending on the constraints: within-tree ¹⁵N transport and storage strategy changed in response to defoliation whereas a soil water deficit induced a drastic reduction of the N amounts in all the tree organs. Consequently, N dysfunction could be involved in drought-induced beech tree mortality under the future climate.

Key message: Winter reddening of young Douglas-fir (Pseudotsuga menziesii Mirb. Franco), triggered by large thermal fluctuations in late winter, is a critical problem for European forestry. A literature review identified certain climatic conditions that are characteristic of ‘reddening’ years, including warm daily temperatures, high daily temperature amplitude, low relative humidity, moderate wind speeds, as well as the occurrence of freeze-thaw cycles with cold night temperatures. By describing the triggering environmental and stand factors, we propose three hypotheses for the physiological processes leading to winter reddening, namely (i) hydraulic failure due to winter drought stress, (ii) photo-oxidative stress in shade-acclimated trees, and (iii) early cold deacclimation during warm periods. i) Low soil temperature, by reducing root water uptake, combined with anticyclonic conditions, by increasing water losses, can induce hydraulic failure in the xylem. Hydraulic failure may be further accelerated by night frosts. ii) Winter reddening can occur when low temperature and high irradiance coincide, disrupting photostasis. Overwhelming of winter photo-protection may lead to photodamage and subsequent reddening. iii) Warm periods, by inducing cold deacclimation, make trees susceptible to frost damage. Finally, the three processes may interact under atypical anticyclonic conditions in late winter (e.g. cold or dry soils, warm days, high irradiance and/or freezing nights). Indeed, trees under water stress would develop a higher sensitivity to freezing night and photooxidative stress. We therefore proposed mitigation actions to avoid exposing trees to stressful conditions based on e.g. stand characteristics, understorey vegetation and planting.