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 lacustris L. and Phragmites australis (Cav.) Trin. ex Steudel were studied in 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 wave 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 attack on mean stem height and density of P. australis stands were much less pronounced. In both species, critical breaking force and flexural stiffness of stems increased with individual stem height. Bending stiffness 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 stiffness was observed in P. australis stems but not in S. lacustris; for P. australis, E was lower in February (dead standing stalks) and June (young shoots), than in August, Stands of P. australis withstand exposure to waves better than S. lacustris stands as the result of a higher bending stiffness of the stems and a lower susceptibility to breaking under mechanical stress. Therefore, S. lacustris can grow in waterside helophyte fringes when wave exposure is moderate or absent, but cannot survive at highly wave-exposed shores.