DISTRIBUTION AND GROWTH OF THE HELOPHYTE SPECIES PHRAGMITES-AUSTRALIS AND SCIRPUS-LACUSTRIS IN WATER DEPTH GRADIENTS IN RELATION TO WAVE EXPOSURE

Interaction effects of water depth and exposure to wind-generated waves on the growth of two helophyte species, Phragmites australis (Cav.) Trin. ex Steudel and Scirpus lacustris L. were investigated. The colonization by P. australis of the shoreline of Lake Volkerak, a former estuarine branch transformed into a freshwater lake, correlated strongly with wave exposure. A transplantation experiment was performed to measure growth responses of P. australis and Scirpus lacustris to water depth and exposure gradients. The experiment showed that the wave action influenced biomass production, tiller formation and tiller height for both species in the lower part of the shoreline elevational gradient. Both species performed well at three positions along the sheltered gradient, but growth in deeper water was strongly reduced in the exposed gradient. This reduction was greater for Scirpus lacustris than for P. australis. The importance of differences in stem stiffness of the species and consequences for the shoreline vegetation zonation are discussed.