Questions: 1. To what extent does light availability differ among fen plant communities? 2. To what extent does light coincide with productivity and moisture gradients? 3. Does light act as an important environmental filter in natural and transformed riparian landscapes?

Location: Current data from the Biebrza Valley, NE Poland; literature data from the Třeboň area, Czech Republic and four sites in the western and southern Netherlands.

Methods: Relative light intensity (RLI) was measured in vertical profiles, next to vegetation relevés accompanied by measurements of above-ground biomass, summer groundwater level, N and P content in vegetation, pH and soil redox potential. Data derived from literature included profiles of RLI, biomass and vegetation records. Relationships between RLI and biomass and between species distribution, RLI and other variables were examined by regression analysis and CCA. Four traits were analysed: ability to spread clonally, seed weight, maximum height of adults and time of commencement of flowering.

Results: RLI at ground level varied from < 1% in reed beds and tall herb meadows to > 60% in sedge-moss communities and litter meadows. RLI was largely determined by the standing crop and explained a large part of variation in species occurrence. The combinations of analysed functional traits were constrained by the communities' light profiles.

Conclusion: Light availability is related more closely to site fertility than to hydrological regime. This confirms that hydrological regime and productivity should be analysed separately with regard to their effect on species distribution in wetlands. Limited light availability seems the major environmental control of the distribution of low growing and late flowering species.

Nomenclature: Mirek et al. (1995) for species; Schaminée et al. (1996–1999) for Dutch and Czech and Pałczyński (1984) for Polish plant communities.

Abbreviations: LAI = Leaf area index; RLI = Relative light intensity.