Floodplain Forests Are Sensitive to Salt-Intrusion During Summer Droughts When Dominated by Salix

Climate change and human activities are increasing salt intrusion into estuaries worldwide. This is threatening the last remaining temperate freshwater floodplain forests, which are vulnerable to sea level rise and more extreme river discharges. In temperate regions, salt intrusion events typically occur during summer droughts and/or winter storm surges. While such shift in timing of salt intrusion events could influence vegetation differently based on their phenological cycle, the seasonal impact on freshwater vegetation remains poorly understood. In this study, we exposed two dominant freshwater floodplain-forest tree species, Salix alba and Alnus glutinosa, to saltwater stress for one week during each temperate season: spring, summer, autumn, and winter. Our results show that Salix alba is highly sensitive to salt stress in spring and summer, growing up to 96% less in hight and up to 95% less in diameter, and reduced recovery the next spring (Leaf Health Index of -1 compared to 1 in the control treatment). On the other hand, exposure in autumn and winter had little effect. In contrast, Alnus glutinosa showed consistent resistance and recovery across all seasons. These findings highlight that assessments of flood-plain forest vulnerability must consider not only species-specific salt tolerance, but also the timing of salt intrusion, as events during the spring or summer growing season pose a particular risk.