Effects of substrate parameters on growth responses of eight helophyte species in relation to flooding

Abstract

The interactive effects of substrate parameters (redox potential, organic matter content) and flooding depths on growth responses and field distribution of eight helophyte species, common in European floodplains, were studied. These species ( Acorus calamus L., Carex acuta L., Equisetum 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 hydrological regime and to the redox potential and organic matter content of the submerged sediment in the littoral of floodplain lakes. The growth responses of these helophyte species were studied in relation to flooding (plants partially flooded versus totally submerged) and sediment characteristics (mineral, moderately reductive sediment versus organic, highly reductive sediment) for 8 weeks during the growth season in outdoor artificial ponds under ambient climatic conditions. Flooding conditions were found to exert a clear impact on helophyte growth responses. In response to total submergence, most species showed shoot length extensions and restored contact between leaves and air. Shoot fractions were generally lower under submerged conditions, as a result of growth reduction at prolonged submergence. The impact of sediment characteristics on growth responses of the species under study depended partly on the flooding conditions. All species under study tolerated prolonged 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-ground plant parts, which suggests a general adaptation mechanism to hypoxic and toxic conditions. Prolonged submergence on organic, highly reductive substrates resulted in a growth reduction of all species selected. 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 sediments and the increased incidence of summer floods there in particular. Besides shared growth responses, species-specific differences in flooding tolerance and field distribution were observed between the selected helophyte species. The experimentally established flooding tolerances and tolerances to highly reductive substrates were generally larger than predicted by the observations on the distribution of the species in the field, suggesting that additional factors which restrict the occurrence of these species are involved.