Herbivory, litter and soil disturbance as determinants of vegetation dynamics during early old-field succession under set-aside.

Early-successional old fields are a major component of the European landscape. While a range of factors governing vegetation development in old fields has been identified, empirical and theoretical studies have tended to concentrate on plant competition as the dominant driving force behind succession. We studied the influence of three little researched, yet inter-related, factors on the early stages of an old-field succession: litter cover, soil disturbance and herbivory. Physical and chemical techniques were used to exclude large vertebrates and insects from experimental plots. These treatments had little effect on plant recruitment. A litter-removal experiment, nested within the exclusion treatments, revealed a significant inhibition of forb seedling germination by litter cover. However, the majority of seedlings died during the first month following emergence, whether or not litter was removed. A second experiment, involving the factorial combination of mollusc exclusion and soil disturbance, revealed that the response to disturbance was dependent on life-history characteristics of the plants. However, the dominant factor regulating community composition was seedling herbivory by molluscs. Molluscs caused high rates of forb seedling mortality and promoted the transition from a forb-dominated, to a grass-dominated community. Herbivory is often assumed to influence plant community dynamics through effects on competitive interactions. However, direct effects of herbivory, on the survival of seedlings, may be a significant factor structuring plant communities in ruderal, or other annual dominated systems.