Environmental factors and seed abundance influence seedling emergence of a perennial forest herb

Seedlings are an important, but vulnerable stage in the life cycle of plants. The identification of factors affecting their recruitment is therefore fundamental for understanding basic plant population processes as well as plant distribution and abundance. In this study, we used a combined experimental and observational approach to examine how microsite quality and quantity as well as seed supply affect different processes of seedling establishment, using the perennial forest herb Phyteuma spicatum (Campanulaceae) as model species. This species reproduces exclusively by seed, and seedling emergence, growth and survival are therefore critical stages in its life cycle. Seedlings were frequent in microsites with bare soil and overall high light intensities, and were less common in sites with deep litter layers as well as dense and low vegetation. Seed addition, via experimental sowings or via the natural seed rain, consistently enhanced seedling emergence. Seed density effects, however, were variable among years; seedling emergence rates decreased at high seed densities in one of two seed cohorts. Seedling emergence time, size and survival were largely not affected by microhabitat type or seed density. In summary, our findings suggest that environmental factors and seed abundance determine recruitment success of P. spicatum and that effects on early processes of seedling establishment (emergence) are stronger than effects on late processes of establishment (growth and survival). Our work thereby contributes to a better understanding of the mechanisms underlying recruitment patterns of this species and other perennial herbs.