Herbivores, seed banks and seedling recruitment in mesic grassland

An experiment was carried out in a species-poor acid grassland to determine the effect of insect, mollusc and rabbit herbivory on the size and composition of the seed bank and on seedling recruitment from the seed bank and seed rain. From 1991 to 1997, insects and molluscs were excluded with pesticides, and rabbits with fences. Seedling recruitment was monitored over 22 months in gaps established in the vegetation in summer 1995. The most common species recorded from the seed bank in early summer 1995 were dicots (17 species), but perennial grasses (five species) were numerically the most abundant (65% of total). There was no relationship between the species composition of the seed bank and the established vegetation. The size of the seed bank of eight species was greater on fenced plots, a result that reflected increased seed rain where rabbits were excluded. Insects and molluscs had no effect on the size of the seed bank of any species. The number of species in the seed bank was not affected by any of the herbivore exclusions. A comparison of seedling emergence in gaps formed over the original soil with gaps where the soil had been sterilized indicated that only Galium saxatile and Cytisus scoparius recruited from the seed bank. Seedling recruitment was almost entirely derived from the recent seed rain, was dominated by the most abundant perennial grasses in the vegetation (Festuca rubra and Holcus lanatus), and had a species composition that resembled the established vegetation. Results highlight that the potential for seedling establishment in gaps to bring about vegetation change in this grassland is low. Six species had higher seedling densities on rabbit-fenced plots, but the significant effect of fencing disappeared by plant maturity for most species. Survival of seedlings was lower on fenced plots where non-grazed biomass accumulated, so that after 22 months Agrostis capillaris was the only species with more plants present where rabbits were excluded. Rumex acetosa and Stellaria qraminea showed higher seedling emergence where molluscs were excluded. More seedlings of Rumex acetosa were also found where insects were excluded. These invertebrate effects were still evident at plant maturity.