Spatial variability in the structure of a population and in the reproductive traits of the narrow endemic coastal plant thorny knapweed (Centaurea horrida; Asteraceae) was estimated. Variations in the distribution of individuals and reproductive effort were described and quantified at the per-site scale (3 sites, less than 30 kilometers apart) for two habitats (cliff face and cliff plateau) to provide a basis for further investigations of the factors affecting C. horrida performance and to provide information necessary for an effective conservation of this species. Centaurea horrida was considerably more abundant at one site, especially in the cliff plateau, in the contribution of adults. Estimates of spatial variance for each life stage at both habitats revealed that, on the cliff plateau, a much greater variability was found at the per-site scale for classes of saplings and adults of middle size, whereas in the cliff face local (among replicates), spatial variance was higher for all comparisons. For all the reproductive traits considered, variation at the scale of the site in the two habitats was extremely low, whereas it was much more important among replicates, indicating that other determinants rather than recruitment are likely responsible for the heterogeneity in the abundance of adults across sites. Overall, these results suggested that, on cliff plateaus, different processes operate on the C. horrida population structure at the three sites. Consequently, site-specific management needs to be addressed. However, we believe that effective management strategies can only be proposed based on manipulative experiments that highlight the role of the major determinants of spatial patterns.
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Vol. 28 • No. 1A