Analysing resilience of European beech tree to recurrent extreme drought events through ring growth, wood anatomy and stable isotopes

Abstract

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 (Kth) 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.