EFFECTS OF SUBSTRATE PARAMETERS ON GROWTH-RESPONSES OF 8 HELOPHYTE SPECIES IN RELATION TO FLOODING

The interactive effects of substrate parameters (redox potential, orga nic matter content) and flooding depths on growth responses and field distribution of eight helophyte species, common in European floodplain s, were studied. These species (Acorus calamus L., Carer acuta L., Equ isetum fluviatile L., Sagittaria sagittifolia L., Scirpus lacustris L. , Sparganium erectum L., Typha angustifolia L. and Typha latifolia L.) showed distributional differences, which could be related to the hydr ological regime and to the redox potential and organic matter content of the submerged sediment in the littoral of floodplain lakes. The gro wth responses of these helophyte species were studied in relation to f looding ( plants partially flooded versus totally submerged) and sedim ent characteristics (mineral, moderately reductive sediment versus org anic, highly reductive sediment) for 8 weeks during the growth season in outdoor artificial ponds under ambient climatic conditions. Floodin g conditions were found to exert a clear impact on helophyte growth re sponses. In response to total submergence, most species showed shoot l ength extensions and restored contact between leaves and air. Shoot fr actions were generally lower under submerged conditions, as a result o f growth reduction at prolonged submergence. The impact of sediment ch aracteristics on growth responses of the species under study depended partly on the flooding conditions. All species under study tolerated p rolonged anoxic and toxic sediments, provided the plants grow emergent . When growing emergent on organic, highly reductive sediments, these species increased their growth rate and allocated biomass to above-gro und plant parts, which suggests a general adaptation mechanism to hypo xic and toxic conditions. Prolonged submergence on organic, highly red uctive substrates resulted in a growth reduction of all species select ed. Our results indicate that the decline of the helophyte vegetation in moderately dynamic floodplain lakes along the Lower Rhine and Meuse can be related to the combination of the organic, highly reductive se diments and the increased incidence of summer floods there in particul ar. Besides shared growth responses, species-specific differences in f looding tolerance and field distribution were observed between the sel ected helophyte species. The experimentally established flooding toler ances and tolerances to highly reductive substrates were generally lar ger than predicted by the observations on the distribution of the spec ies in the field, suggesting that additional factors which restrict th e occurrence of these species are involved.