Kristine Vander Mijnsbrugge

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 were subjected to two consecutive water-withholding treatments during the 2022 growing season, 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. 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 the severe spring- and summer-drought categories. The severe spring-drought categories maintained 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, the 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.

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.

Background: Understanding the mechanisms by which woody perennials adapt to extreme water deficits is important in regions experiencing increasingly frequent and intense droughts. Methods: We investigated the effects of drought severity in the shrubs Corylus avellana L., C. maxima Mill., and their morphological intermediate forms, all from local Belgian origin, and C. avellana from a Spanish-Pyrenean origin. Potted saplings in a common garden were not receiving any water for a duration of 30 days in July 2021 and developed a range of visual stress symptoms. We assessed responses across the various symptom categories. Results: Droughted plants senesced later than the controls (up to 6 days). The most severely affected plants disproportionately displayed the longest delay (21 days). The delayed leaf senescence was reflected in the subsequent bud burst which was delayed for the droughted plants, with again the largest delay observed for the most severely affected plants. Interestingly, radial growth shifted towards the autumn among the drought-treated plants, suggesting compensation growth after growing conditions normalized. The Spanish-Pyrenean provenance, characterized by smaller plants with smaller leaves, developed visual drought symptoms later than the local provenance during the drought. Conclusions: The results indicate that severe early summer drought, followed by rewatering, not only diminishes radial growth but also prolongs the growth period, and delays leaf senescence. A prolonged time frame for radial growth and a delayed leaf senescence indicate a longer period in which carbon is incorporated in woody tissue or in non-structural carbohydrates. This can help the fine tuning of carbon sequestration modeling. The Pyrenean provenance, adapted to high altitude, holds an advantage under water-limited conditions.

Understanding how woody plants cope with severe water shortages is critical, especially for regions where droughts are becoming more frequent and intense. We studied the effects of drought intensity, focusing on post-drought resprouting, autumn leaf senescence and the subsequent spring bud burst. Furthermore, we aimed to study population differentiation in the drought and post-drought responses. We performed a summer dry-out experiment in a common garden of potted Prunus spinosa L. (Rosaceae) saplings. We analysed responses across different visual stress symptom categories and examined differentiation between provenances from a local origin (Western Europe, Belgium), a lower latitude (Spain) and a higher latitude (Sweden). The chance of post-drought resprouting was greater for the more severely affected plants than for the less severely affected ones, and it occurred earlier. The plants that displayed wilting of the leaves during the drought had a leaf senescence 2.7 days earlier than the controls, whereas that of plants with 25 to 75% and more than 75% of desiccated leaves was 7 and 15 days later, respectively. During the drought, the local provenance was the first to develop visual symptoms compared to the other two provenances. However, among plants that exhibited no or only mild symptoms, this provenance also had a higher likelihood of post-drought resprouting. Among the control plants, the higher-latitude provenance displayed leaf senescence earlier, while the lower-latitude provenance senesced later compared to the local provenance. However, these differences in the timing of leaf senescence among the three provenances disappeared in treated plants with more than 25% of desiccated leaves due to the drought. Whereas leaf senescence could be earlier or later depending on the developed drought symptoms, the timing of bud burst was only delayed. Results indicate that resprouting and timing of leaf senescence are responsive to the severity of the experienced drought in a provenance-dependent way.

Understanding how woody plants cope with severe water shortages is critical, espe-cially for regions where droughts are becoming more frequent and intense. We studied the effects of drought intensity, focusing on post-drought resprouting, autumn leaf se-nescence and the subsequent spring bud burst. Furthermore, we aimed to study popu-lation differentiation in the drought and post-drought responses. We performed a summer dry-out experiment in a common garden of potted Prunus spinosa L. (Rosaceae) saplings. We looked at the responses in different visual stress symptom categories and at provenance differentiation between a local (Western Europe), a more southern and a more northern provenance. The chance on post-drought resprouting was larger for the more severely affected plants, and the timing of resprouting was earlier than for the less severely affected ones. The plants that displayed wilting of the leaves during the drought had a leaf senescence 2.7 days earlier than the controls, whereas plants with 25 to 75% and more than 75% of desiccated leaves were 7 and 15 days later, respectively. During the drought, the local provenance was the first to de-velop visual symptoms. Still, for the plants with no to mild symptoms due to the drought, this provenance displayed a larger chance for post-drought resprouting. Among the control plants, the northern provenance was earlier, and the southern was later in leaf senescence compared to the local provenance. This differentiation disap-peared for the plants with more than 25% of desiccated leaves due to the drought. Whereas leaf senescence could be earlier or later depending on the developed drought symptoms, the timing of bud burst was only delayed. Results indicate that resprouting and timing of leaf senescence are responsive to the severity of the experienced drought in a provenance dependent way.

Increased water limitations due to climate change will pose severe challenges to forest ecosystems in Europe. We investigated the response of potted saplings of Fagus sylvatica L., one of the major European tree species, to a spring and a summer water-withholding period with control–control (C-C), control–drought (C-D), drought–control (D-C) and drought–drought (D-D) treatments. We focused on recovery capacity and phenological and growth traits and questioned the extent to which an earlier drought influenced the response to a second drought in the same growing season. To examine the impact of the level of drought stress, a distinction was made between saplings with less or more than half of their leaves desiccated due to the spring drought (D50). The timing of the drought influenced the immediate post-drought response: saplings severely affected by the spring drought (D>50) resprouted, whereas saplings severely affected by the summer drought (C-D and D50-C and D>50-D), which may enable the repair of damaged tissues. The saplings that were less affected by the spring drought (DF. sylvatica saplings in response to (repeated) drought.

Water limitations will pose significant challenges to forest ecosystems across Europe. To gain a deeper understanding of the potential impacts, we investigated the response of the common shrub Crataegus monogyna to two summer droughts, each followed by rewatering. The experimental design consisted of a common garden with potted saplings from a local Belgian (n = 48), a Swedish (n = 47), and a Spanish-Pyrenean provenance (n = 48). We quantified the effects on growth and leaf phenology, focusing on the legacies in the year following the droughts. Responses were influenced by the severity of the drought and by its timing. Most strikingly, height increment was enhanced by 24% (p = 0.046) in comparison to the controls in the year following the droughts in the group of plants that endured the two drought treatments, each time without developing visible stress symptoms. Only one such mild drought, whether early or late summer, did not lead to this response, suggesting stress memory acting as a growth promoter. A late summer drought that resulted in visible drought symptoms led to a reduced diameter increment in the year following the droughts, independent of the preceding treatment (severe, mild, or no drought), whereas this was not the case for a similar drought in early summer. Minor leaf phenological responses were detected in the year following the droughts. Finally, the non-local provenances did not respond in a deviating way to the droughts compared to the local provenance. Our findings contribute to the prediction of carbon sequestration in forests and other woody vegetations in the temperate regions of Europe.