Populations of the orchid Dactylorhiza sambucina are declining in number and size on the island of Bornholm, Denmark. To study the reasons for this trend, we compiled previously unpublished estimates of population sizes from 1987–2010 and found complicated fluctuations, as population sizes generally increased in the early 1990s but declined after that. We could find no effect of vegetation management, neither were the fluctuations related to suggested climatic factors. High proportions of plants were lost to grazing in the study year; same could be deduced for previous years when more than one census was available. Grazing in spring and early summer may therefore contribute to the decline of D. sambucina, as suggested by others. Populations seemed not to be affected by lack of pollination or population inbreeding, as fruit set and reproductive size was as high as in large and interconnected D. sambucina populations on Öland, Sweden, and as reported in other studies.
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Vol. 51 • No. 6