Effects of litter removal and mowing on germination and establishment of two fen-grassland species along a productivity gradient

To study the factors that limit the occurrence of species in fen grasslands, the effects of mowing and litter removal on germination, growth and establishment of two common species (Silene flos-cuculi and Lotus pedunculatus) were analyzed along a productivity gradient at seven sites in Northern Germany. The sites differed in nutrient availability, vegetation composition and standing crop. In autumn 2002 a field experiment was set up at each site, including factorial combinations of two treatments (litter removal, mowing). Seeds and juvenile individuals of both species were transferred within the treatment combinations. The number of germinated and transplanted individuals was recorded during the 2003 and 2004 seasons and biomass of survived plants weighed in late summer 2004. Mowing and litter removal had positive effects on the number of germinated seeds of both species. The effect of litter removal was mostly significant at sites with a comparatively low nutrient availability and standing crop (350 g/m(2)), while the effect of mowing was more important at the site with the highest standing crop (1000 g/m(2)). In some cases the presence of litter had a positive effect on the survival of individuals, which may be caused by the extremely dry weather conditions in 2003. The experiment showed that plant species reaction on mowing and litter removal differed both according to their life stage and the site conditions. These differences could be interpreted in context with Shifting Limitation Hypothesis because the abolition of seed limitation by sowing and creation of additional gaps for germination was most efficient at low productive sites, while the high standing crop reduced first of all the establishment probability, which could be increased by mowing.