FUMANA-PROCUMBENS ON OLAND - POPULATION-DYNAMICS OF A DISJUNCT SPECIES AT THE NORTHERN LIMIT OF ITS RANGE

1 Demographic processes were followed over seven successive years in a n outlying population of Fumana procumbens on its northern range-limit on the Baltic island of Oland. Comparisons were made between three si tes which represented different types of habitat, vegetation and grazi ng regime. Demographic variation and reproductive success were related to variation in local weather conditions and microclimate. 2 Fumana p rocumbens was patchily distributed within the study sites, with the sp atial scale of the patchiness varying between the three sites. Patch s tructure and density distribution within sites were related to grazing intensity, vegetation type and the availability of fissures in the un derlying bedrock. 3 Seed production and seed quality varied in space a nd time. The best overall reproductive success was found at the occasi onally 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 de crease in seed set in all sites. 4 Stage-based transition population m atrix analyses showed that life-stage distribution and projected popul ation growth rates (lambda) varied between years, reflecting demograph ic succession in the population and temporal variation in local climat e. 5 Elasticity analyses of the matrices showed that the life-history transition probabilities that had the greatest influence on the predic ted population growth rates (lambda) were those concerned with surviva l, 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 si zes. 6 Reproduction, establishment and survival in Fumana procumbens w as found to depend to a large extent on local climatic variation. The long-term existence of the Baltic range-margin populations of F. procu mbens is likely to depend ultimately on future climatic trends. On a s horter time-scale, the survival of the species on Oland is likely to d epend on the continued maintenance of open habitats by grazing.