Seedling recruitment in the invasive clonal shrub, Mahonia aquifolium Pursh (Nutt)

The clonal shrub, Mahonia aquifolium, is an aggressive invader of some forests in central Germany. We analysed the importance of seedling recruitment for the local dynamics of these invasive populations. In contrast to many other clonal plants, repeated seedling recruitment takes place in M. aquifolium, contributing to the colonization of available space within populations. Thus, sexual reproduction is not only important for dispersal to new sites but also for the local invasion process. In situations with grass competition (Calamagrostis epigejos) the number of successful recruits is reduced, and clonal growth is the predominant mode of reproduction. Since seedling recruitment results in an increasing number of genets as well as ramets, the investigated populations are still below their carrying capacities. Seedling densities are higher beneath the canopy of adults than outside adult patches, and increase with increasing cover of adult ramets. The spatially clumped pattern of seedling emergence results in density-dependent mortality especially in the most crowded seedling clusters. However, adult neighbourhood imposes a much higher mortality. Therefore, we expect that the importance of seedling recruitment for the dynamics of local M. aquifolium populations will decline as the invasion process continues.