Fumana procumbens on Oland--population dynamics of a disjunct species at the northern limit of its range

Demographic processes were followed over seven successive years in an outlying population of Fumana procumbens on its northern range-limit on the Baltic island of Oland. Comparisons were made between three sites which represented different types of habitat, vegetation and grazing regime. Demographic variation and reproductive success were related to variation in local weather conditions and microclimate. Fumana procumbens was patchily distributed within the study sites, with the spatial scale of the patchiness varying between the three sites. Patch structure and density distribution within sites were related to grazing intensity, vegetation type and the availability of fissures in the underlying bedrock. Seed production and seed quality varied in space and time. The best overall reproductive success was found at the occasionally grazed site with thin soil, low cover of vascular plants, high cover of bryophytes and a low degree of shading. Low temperatures and drought in early summer delayed the onset of flowering and caused a decrease in seed set in all sites. Stage-based transition population matrix analyses showed that life-stage distribution and projected population growth rates varied between years, reflecting demographic succession in the population and temporal variation in local climate. Elasticity analyses of the matrices showed that the life-history transition probabilities that had the greatest influence on the predicted population growth rates were those concerned with survival, particularly in the reproductive stages. Extended periods with low precipitation or low temperatures, which increase adult mortality and lower seed production, may thus have dramatic effects on population sizes. Reproduction, establishment and survival in Fumana procumbens was found to depend to a large extent on local climatic variation. The long-term existence of the Baltic range-margin populations of F. procumbens is likely to depend ultimately on future climatic trends. On a shorter time-scale, the survival of the species on Oland is likely to depend on the continued maintenance of open habitats by grazing.