Preference of wet dune species for waterlogged conditions can be explained by adaptations and specific recruitment requirements

The preference of wetland angiosperms for waterlogged soils has been explained by several hypotheses: (1) wetland species are adapted to waterlogging and sensitive to drought; (2) wetland species are tolerant to drought, but inferior competitors at drier conditions; (3) wetland species have narrow moisture optima for recruitment. We tested these hypotheses by the application of permanent and fluctuating water levels to experimental wet dune plant communities with four species that frequently occur in wet dune slacks (Carex flacca, Molinia caerulea, Samolus valerandi and Schoenus nigricans) and one competitive species of drier conditions (Calamagrostis epigejos). After 3 years, fluctuating water levels had led to lower total biomass production than permanent water levels, indicating that switching from aerated to anoxic soil conditions involved physiological costs. The collective biomass of wetland species was highest at permanently waterlogged conditions with interspecific variation in the biomass optima. At the nutrient-poor growth conditions of this long-term experiment, biomass of Calamagrostis was independent of water level treatment, thus the hypothesized competitive superiority of this species at drier conditions could not explain the biomass responses of the wetland species in this study. Instead, this is evidence for an ecological preference of adult wet dune plants for waterlogged to moist conditions. Recruitment of most wet dune slack species occurred in a narrower range of water levels than adult growth, indicating that recruitment requirements also pose a limitation to the distribution of these wet dune species. Incorporation of recruitment into nature management support models may improve their predictions.