Willow species vary in elevational occurrence and morphological characteristics on the tidal freshwater section of the Elbe estuary

Abstract

Willows are regarded as globally occurring pioneer species providing ecosystem functions like bank stabilization and water purification, and serving as shelterbelts to prevent degradation of the environment. Some species are adapted to physical disturbance like wind, waves and currents due to their fast growth and vigorous resprouting capacity, and are widely used in short rotation coppices for biofuel production. Knowledge on willow river floodplain-forest ecology exists and Salix flooding tolerance is documented. However, information on floodplain-forests along river mouths where the estuarine tide affects willows is scarce. In the Anthropocene, tidal amplitudes are increasing in many estuaries due to the deepening of the navigational channel for harbour accessibility. With regard to tidal forest restoration, the Elbe estuary close to the Hamburg harbour served as a model system to study the effect of increasing tidal flooding on Salix alba and Salix viminalis. Our results show occurrence of both species at sites up to 60cm below mean high water and their tolerance to twice daily flooding. Experimental findings on cuttings that were placed on flooding stairways at the tidal freshwater stretch showed the ability to resprout and grow up to more than 1m below mean high water for Salix viminalis but not for Salix alba. Willows may maintain their habitat when tidal amplitudes are increasing but repeated physical flood damage prevents Salix alba in tidal floodplain-forests from rejuvenation. These forests may be restored along protected sites to ensure the ecosystem functions and services they provide in the estuarine environment.