Guangqi Zhang, UMR SILVA – INRAE, Grand-Est Nancy Research Center, discusses his article: Analyzing resilience of European beech tree to recurrent extreme drought events through ring growth, wood anatomy and stable isotopes

As the climate warms, extreme weather events like droughts are expected to become more frequent and intense. A recent extreme and recurrent drought occurred in 2018–2020 in Central Europe and pushed forest trees to their limits, particularly Fagus sylvatica species. Following this event, many beech trees showed visible signs of distress, such as early leaf discoloration and premature leaf drop, and their health declined (branch mortality, defoliation, and reduction of growth) at large scale. Faced with this crisis, foresters were wondering about the fate of declining trees and the resilience of their radial growth to drought, i.e the capacity of trees to return to pre-drought growth levels. The physiological processes involved in forest tree growth resilience to drought are little understood. One of the key goals of this research was to understand how wood anatomy and isotopic traits are linked to tree growth resilience after drought stress.

European beech is one of the most important deciduous trees in temperate forests, and it is widespread in Central Europe. Beech is typically adapted to moderate drought conditions, but recent climate extremes have pushed the species beyond what it can tolerate. While beech has a high degree of intraspecific plasticity, it may not be well-equipped to handle the intensified drought conditions projected for the future. By studying tree ring width in four plots with varying soil water deficits across northeastern France, we assessed how trunk radial growth responded during the drought and afterwards, when conditions were more favorable.

Location of the four study sites in northeastern France. The background map represents the altitude distribution in the zone.

We aimed to uncover whether the anatomical and physiological responses of beech trees during and after extreme drought were involved in resistance and resilience of growth to drought. We assessed i) wood anatomy to know whether beech tree modifies the size of the vessels that transport water within the tree to increase performance and ii) the stable isotopic composition of tree rings (carbon and oxygen isotopes) as indicators of water use efficiency, to assess how efficiently trees used water to produce biomass.

Our results confirmed that extreme droughts reduced the growth of beech trees. This growth decline was especially notable in 2018 and 2020, both years of severe water shortage. In addition, there was a decline in tree growth resilience in areas that had experienced consecutive droughts. Trees in sites exposed to the harshest drought conditions in 2018–2020 showed significantly lower growth resistance and resilience compared to earlier drought events (e.g., 2015). Our results suggest that beech trees, especially in the driest regions, may struggle to recover fully after recurrent droughts, pointing to a long-term decline in forest health if droughts continue to worsen. Moreover, one of the more surprising findings was the stability of xylem vessel sizes. Vessel diameters remained relatively constant throughout the study, even during severe droughts. Only vessel density increased with drought due to narrow tree-rings. Water use efficiency also increased under drought. We found that despite changes in water use efficiency and vessel density, beech trees do not appear to modify their hydraulic architecture in ways that would help them better cope with repeated droughts. In simpler terms, trees are not making the structural adjustments that might improve their ability to conduct water under drought stress. This raises important questions: if xylem structure does not change significantly, how do trees survive multiple droughts? It seems that their resilience lies not in hydraulic adjustments, but in other strategies, like stomatal regulation, which helps them conserve water. However, this does not appear to be enough to maintain growth over time.

Drought’s toll on beech forests creates a canopy of contrast. Photo by Nathalie Breda.

Our findings have important implications for forest management, particularly as climate change increases the frequency and severity of droughts. Beech trees, once considered resilient to moderate drought, may no longer be able to survive in their traditional habitats, especially under the repeated stresses of climate extremes. If the current trend of increasing drought intensity continues, species like beech may begin to decline, leading to changes in forest composition and productivity. Forest managers must consider alternative strategies for maintaining forest health, such as introducing drought-tolerant species or improving management practices that reduce the competition among trees for resources.