Low recruitment across life stages partly accounts for the slow colonization of forest herbs

1. Former land use has long-lasting effects on the distribution and abundance of forest herbs. Previous studies mainly focussed on limited dispersal capacities of forest herbs to explain these patterns and few studies have experimentally evaluated the relative importance of recruitment. Introduction experiments offer a direct test of recruitment limitation, but are generally only monitored until the germination stage. 2. We examined recruitment of 10 forest herbs during five growing seasons by means of a seed sowing experiment in two contrasting forest types (valley and plateau) established on former agricultural land. Effects of seed density and clearing of vegetation and litter (disturbed microsites) were tested in a factorial design. The data were analysed in two successive steps: germination 2 years after seed introduction and subsequent recruitment into the adult life stage. We focussed on both the species and the community-level. 3. Although adding more seeds resulted in more recruits, only a small fraction of seeds (between 1% and 20%) germinated after two growing seasons in both forest types. The need for disturbed microsites was species-specific and differed between the two forest types. Evergreen hemicryptophytes benefited from clearing; vernal geophytes were not affected. Post-seedling mortality further reduced recruitment success into the adult life stage. Results at the species and community-level were largely analogous. 4. Because only a fraction of seeds effectively recruited into the adult life stage, forest herbs need high seed densities for colonization. Seed availability is, however, severely limited by the low dispersal capacity of many forest herbs. Disturbed microsites promoted recruitment of some, but not all forest herbs. The effect was most clear in valley forest, probably because the established vegetation had high cover and was dominated by competitive herbs. 5. Synthesis. Overall, the experimental results indicate that community assembly can be considered a two-stage process in which restricted seed availability followed by low recruitment limits colonization. The need for disturbed microsites can be crucial, but depends on species-specific life histories and the prevailing site conditions.