Fitness and survival in fragmented populations of Narthecium ossifragum at the species' range margin

Abstract

Decreasing population size and increasing isolation may lead to reduced plant fitness. These effects of habitat fragmentation may especially apply to populations close to the margin of their geographical range, where populations generally are often smaller and more isolated, and the environment is less favourable than at the range centre. In this context we studied north-west German range-margin populations of Narthecium ossifragum (L.) HUDS., a clonal, perennial herb typical for acidic and nutrient-poor wet heathlands. We asked whether plant fitness and reproduction of the range-margin populations were affected by population size and habitat quality, and whether any changes in population size over the past 10–20 years were related to past population size and habitat quality. Population size varied between 60 and 100.000 individuals and was highly positively related to shoot density. Larger (and denser) populations formed more flowers and fruits, but fewer seeds. Soil water content had a strong positive effect on seed production and seed mass, and the latter also increased with increasing soil phosphorus content. Relative light intensity showed a positive impact on shoot density and, hence, population size. Compared to British range centre populations, the populations at the German range margin formed more, but smaller seeds that failed to germinate both in the field and in laboratory experiments. Despite the lack of sexual reproduction, population sizes generally had not decreased during the past two decades, most likely due to successful clonal growth. The change in population size of Narthecium, however, was negatively related to the present soil phosphorus content, indicating that an increased supply of nutrients and a decrease in light availability may have a negative effect on population dynamics. The results imply that fragmented and small range-margin populations not necessarily are doomed to extinction, provided that the habitat quality remains suitable for growth and vegetative reproduction.