Re-Appearing Legacy Effect in Timing of Autumnal Leaf Senescence and Compensation Growth After Severe Drought in Fagus sylvatica L.

Drought events are expected to induce both immediate and delayed effects on tree functioning, yet the persistence and dynamics of these legacy effects remain poorly understood. In an experimental set-up, European beech (Fagus sylvatica L.) saplings have been subjected to two consecutive water-withholding treatments during the 2022 growing season, in a factorial design, resulting in five drought severity categories. We monitored these categories over the following three years to assess drought legacy effects on phenology and growth. In the first post-drought year, bud burst proceeded more slowly in all drought categories, with the plants exposed to severe summer drought displaying the longest duration. No to minor differences between drought categories and control plants were observed in timing of autumn leaf senescence in the first post-drought year and in the following bud burst, suggesting recovery in these phenological responses. Unexpectedly, differences re-emerged in autumn leaf senescence in the second post-drought year, when all drought categories displayed delayed senescence. This differentiation persisted into the following spring, with delayed bud burst in four drought categories, and two severe spring-drought categories maintaining delayed senescence in the third post-drought year. Diameter and height growth were strongly reduced in the first year following drought. Although growth differences largely disappeared in the second post-drought year, two severe summer-drought categories exhibited enhanced diameter growth in the third year, indicating compensatory growth. Together, these results demonstrate that drought legacy effects in F. sylvatica are dynamically expressed over multiple years, and that such temporal variability complicates predictions of long-term forest responses to increasingly frequent episodic drought events under climate change.