Morphological and biomechanical responses of floodplain willows to tidal flooding and salinity

Willow floodplain plantations have been proposed as part of plans to create tidal wetlands for ecosystem-based flooding defence of estuaries. Forests characterised by willows (genus Salix) are found in riparian floodplains in Europe up to the river mouth, e.g. up to tidal wetlands along the Scheldt and Elbe. However, sea level rise accompanied by accelerated tidal flooding and salt-water intrusion may limit the effectiveness of willows for flooding defence of floodplains located at estuaries near their junction with the sea. We studied juvenile floodplain willows (Salix alba and Salix viminalis) in a mesocosm experiment with a combined tidal flooding and salinity treatment in a climate chamber. Permanent and semi-permanent flooding of roots and periodic flooding of shoots reduced shoot length, shoot biomass, bending capacity, and breaking resistance. However, partial submergence did not affect shoot morphology or biomechanical traits. In S. viminalis, shoot diameter was generally larger compared to S. alba and this larger diameter resulted in a higher maximum breaking force. However, S. alba showed more consistent results in diameter size with lower variation than S. viminalis. The applied salinity treatments of up to 2 parts per thousand did not have significant effects on willow shoot morphology or biomechanical traits. We conclude that juveniles of both willow species are resilient to periodic tidal flooding and salinity values of up to 2 parts per thousand with respect to shoot traits. However, the reduction in shoot growth and biomechanical properties with permanent flooding suggests that juvenile willow establishment will be sensitive to sea level rise and increased flooding, and will result in changes in the vegetation of tidal freshwater wetlands. Salix alba and S. viminalis may serve in innovative capacities as supplementary features for estuarine flooding defence in tidal wetlands and tools for ecological restoration at appropriate sites. However, studies addressing whole ecosystems at a large scale are recommended before using S. alba and S. viminalis in floodplain plantations to protect river estuaries