EFFECTS OF WAVES ON HELOPHYTE STANDS - MECHANICAL CHARACTERISTICS OF STEMS OF PHRAGMITES-AUSTRALIS AND SCIRPUS-LACUSTRIS

The effects of wave attack on stands of the two species Scirpus lacust ris L. and Phragmites australis (Cav.) Trin. ex Steudel were studied i n a wave tank, and related to bending and breaking characteristics of the stems. Stands of the two species, growing at a water depth of 0.5 m, were exposed to uniform waves with a height of 0.23 m high and a wa ve period of 1.9 s. Mean stem height and density were reduced strongly in the S. lacustris stands after wave attack. The effects of wave att ack on mean stem height and density of P. australis stands were much l ess pronounced. In both species, critical breaking force and flexural stiffness of stems increased with individual stem height. Bending stif fness was much lower in S. lacustris stems (mean E = 0.25 GPa) than in P. australis stems (mean E = 4.93 GPa), Seasonal variation in stiffne ss was observed in P. australis stems but not in S. lacustris; for P. australis, E was lower in February (dead standing stalks) and June (yo ung shoots), than in August, Stands of P. australis withstand exposure to waves better than S. lacustris stands as the result of a higher be nding stiffness of the stems and a lower susceptibility to breaking un der mechanical stress. Therefore, S. lacustris can grow in waterside h elophyte fringes when wave exposure is moderate or absent, but cannot survive at highly wave-exposed shores.