Untersuchungen zum Lebenszyklus der hochalpinen Polsterpflanze Eritrichium nanum (L.) Schrad. ex Gaudin Teil I: Blütenbiologie

Abstract

This paper represents the first of seven parts dealing with the ecology of Eritrichium nanum and reports findings on the pollination biology of this attractive and well-known alpine plant species. In contrast to statements in the literature that flowers are protogynous we show that they are distinctly protandrous. Various insects from 12 families frequently visit E. nanum flowers. Flies from the families Anthomyiidae and Muscidae were the most frequent visitors. Under optimal weather conditions (maximum solar radiation, minimum wind speed) visitation rates of 200 simultaneously observed flowers reached 32.5–46.7 insects per hour, i. e. 0.16–0.24 insects per flower and hour. However, Anthomyiidae and Muscidae clearly preferred the white-yellowish flowers of Saxifraga exarata and Saxifraga bryoides which offered plenty of nectar when they were in bloom at the study sites. These two species, certainly also pollinated by Anthomyiidae and Muscidae, may compete for pollinators with E. nanum. Thus, fly pollination in E. nanum may rather be caused by the unfavorable ecological conditions of its habitats than by its particular floral characteristics. In contrast, various other insects such as Eristalis tenax and closely related hoverflies showed a higher degree of flower constancy to E. nanum, often flying from one Eritrichium cushion to the next and hence causing outcrossing. Bagging experiments showed that outcrossing and geitonogamy are the prevailing pollination modes, and that autogamy, although possible, plays only a minor role. The extended flowering period from the beginning of June to the end of August seems to prevent pollinator limitation in spite of potential competition by other, white flowering plant species. We also recorded that most of the nutlets are spread before mid-September. This contradicts again old statements in the literature that they remain in the calyx until spring.