Effects of litter on establishment of grassland plant species: the role of seed size and successional status

Litter layers have a negative effect on seedling recruitment in many ecosystems. Successional fen grasslands are characterized by the built-up of litter layers which might be a key-factor influencing species composition and diversity during succession. Seed size is thought to be an important feature for the regeneration niche of species. Large seed size has been shown to be of advantage for germination and establishment under shaded conditions and beneath litter layers and thus for late-successional species. We investigated the effects of litter (control, litter layer of 3 cm and 8 cm) on seedling recruitment in 35 grassland species varying in seed size and successional status and on the abiotic conditions temperature, light quantity and light quality. Furthermore, we analyzed the species' light requirement for germination and its relation to seed size, successional status and establishment beneath litter. Light quantity was significantly lower beneath litter (reduction of 78% and 91% beneath 3 cm and 8 cm litter, respectively). The R/FR-ratio decreased from 1.2 without litter to 0.8 and 0.6 in presence of 3 cm and 8 cm litter. Seedling recruitment of 33 species was hampered by a litter layer. Establishment in presence of litter was positively related to seed size (r = 0.62, p < 0.001 and r = 0.46, p < 0.01 for 3 cm and 8 cm litter, respectively). In addition, establishment of early-successional species was hampered by litter to a significantly greater extent than recruitment of late-successional species independent of seed mass (ANCOVA: F = 7.6, p < 0.01). We found a positive relation between seed mass and the proportion of germination in darkness (r = 0.57, p < 0.001) indicating that small-seeded species had a higher light requirement for germination. We also found positive relationships between the proportion of germination in darkness and establishment in the 3 cm and the 8 cm litter treatment (r = 0.4, p < 0.05 and r = 0.3, p < 0.05 for 3 cm and 8 cm litter, respectively). We conclude that species-specific effects on seedling establishment are mainly due to differences in seed mass, successional status, and light requirement for germination and that these factors thus influence species composition and diversity during abandoned fen grassland succession.