Responses of fen plant species to groundwater level and light intensity

Characteristic species of sedge-moss fen communities occur in constantly wet, nutrient-poor sites with a high penetration of light through the vegetation canopy. We studied the effects of water table depth and differences in light intensity on the performance of fen species. Three fen species (Carex curta, Viola palustris, Hydrocotyle vulgaris) and one species with a wide range of occurrence (Poa trivialis) were grown for 10 weeks in a sedge-moss peat substrate at 4 different water levels and 3 light intensities. In all species differences in light availability had a larger effect on biomass production than differences in water level. Under a light availability reduced to only 10% the root weight ratio of all the species decreased while leaf weight ratio increased. The biomass allocation ratios were hardly affected by differences in water level. For Viol a and Hydrocotyle an interaction between the two factors was observed. Poa did not show particular differences compared to the other species. We discuss the results in the context of the establishment of fen species in riparian vegetation. It is suggested that the occurrence of fen species in the landscape is directly related to the availability of light, whereas the relationship between fen species occurrence and hydrological conditions seems to be an indirect one.