|Willow trees as Nature Based Flood Defense|
Slegt, M. (2021). Willow trees as Nature Based Flood Defense. MSc Thesis. NIOZ Royal Netherlands Institute for Sea Research: Yerseke. 44 pp.
Willow forests can be used in a nature-based flood defense system. The biodiversity of willow forests is affected by forest management through changes in the forest structure. This research determined how different management strategies of willow forests influence the biodiversity in a pollard, wild-grown and man-made willow forest. The man-made forest has small stem pollard willow trees for flood protection. Forest structure and biodiversity were examined by measuring tree density, tree height, dead-wood presence, litter layer, abundance- and variance of ground-dwelling invertebrates. The highest biodiversity was found in the unmanaged wild-grown forest, which scored H’ = 2.05 on the Shannon-Wiener Biodiversity Index and had the most dead trees, thickest litter layer and had the highest tree density. The man-made forest and pollard forest had a lower biodiversity (H’ =1.45 and H’ = 1.22, respectively). The low biodiversity in the man-made forest for flood protection can be explained by the high tree density, lack of dead wood dead wood and no height differences, which are signs of low complexity and variety caused by forest management. The lower-than-expected biodiversity in the pollard forest of this research can be explained by local factors such as tidal fluctuations and trap placement. To improve biodiversity in nature-based flood defense systems it may be needed to increase forest complexity through forest management. Forest management can lead to more openness, height differences, dead-wood presence, or plant variety. Additionally, further research into the ecological value of pollard willow forests is recommended, for instance by using other insect traps and additional methods to catch flying insects and other organisms.