Historic Human‐Induced Species Shift Increases Climate Sensitivity of Today’s Western European Floodplain Forests: Restoring Past Conditions for Future Resilience

In many estuaries worldwide, climate change and human interventions are driving saltwater upstream. This increase in salinity can threaten temperate tidal floodplain forests, generally dominated by (cultivated) willow species, by reducing tree growth, increasing mortality and reducing key ecosystem services such as flood protection. Knowledge on how temperate tidal floodplain forests respond to saltwater intrusion is largely lacking. This study analysed two common freshwater trees: Salix alba and Alnus glutinosa. Salix alba has been widely cultivated in tidal wetlands for centuries for harvesting its flexible twigs and branches, while Alnus glutinosa was historically dominant but is currently a minor inhabitant of many semi-natural tidal forests. We subjected both species to a combination of two flooding regimes, three different salt concentrations and two durations of salt stress. Surprisingly, our results indicate that the historically dominant Alnus glutinosa exhibited a higher height growth and number of leaves in salt and tidal conditions compared to Salix alba. While tidal conditions enhanced the growth and survival of Salix alba, they had no significant effect on Alnus glutinosa, which consistently outperformed Salix alba regardless of tidal influence. Our results underscore the adaptability of Alnus glutinosa to increasing salt intrusion and inundation expected with rising sea levels. They also highlight that conservation efforts aimed solely at preserving willow stands that became dominant due to historical horticultural practices may inadvertently diminish wetland resilience to climate change.