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

Interaction effects of water depth and exposure to wind-generated wave s on the growth of two helophyte species, Phragmites australis (Cav.) Trin. ex Steudel and Scirpus lacustris L. were investigated. The colon ization by P. australis of the shoreline of Lake Volkerak, a former es tuarine branch transformed into a freshwater lake, correlated strongly with wave exposure. A transplantation experiment was performed to mea sure growth responses of P. australis and Scirpus lacustris to water d epth and exposure gradients. The experiment showed that the wave actio n influenced biomass production, tiller formation and tiller height fo r both species in the lower part of the shoreline elevational gradient . Both species performed well at three positions along the sheltered g radient, but growth in deeper water was strongly reduced in the expose d gradient. This reduction was greater for Scirpus lacustris than for P. australis. The importance of differences in stem stiffness of the s pecies and consequences for the shoreline vegetation zonation are disc ussed.