DIRECT AND INDIRECT EFFECTS OF WAVE ATTACK ON HELOPHYTES

Along the shorelines of larger water bodies, wave action affects littoral emergent vegetation directly through mechanical action on plants, and indirectly by interference with the plant's environment. A study was conducted to reveal the effects of wave exposure on four helophyte species, Phragmites australis (Cav.) Trin. ex Steudel, Typha angustifolia L., Scirpus lacustris L. and Scripus maritimus L. in the northern Delta area in the Netherlands. The species differed with respect to the wave exposure of their locations. Typha angustifolia was restricted to sheltered sites, whereas the other species were present on sheltered and moderately exposed shores. In a gradient from sheltered to exposed sites, wave exposure appeared to be related to soil texture and organic matter content. In a cultivation experiment with the four species on transplanted soil originating from three sites (exposed, intermediate and sheltered), only the growth of T. angustifolia was affected by the soil types. At the sites, transplanted rhizomes of the helophyte species studied did not survive the wave attack in the growing season at the exposed site, while at the completely sheltered site these plants produced the highest biomass. Wave attack on the sites within the growing season was determined by means of the wave-forecasting model HISWA (Hindcast Shallow Water Waves). Maximal wave heights and orbital velocities were concluded to be key factors in the decreased growth rates of plants at exposed sites.