Population history of the rare fern species Dryopteris cristata in Switzerland.

Dryopteris cristata, a lowland plant of wet bugs and alder swamp forests, h as become a rare and threatened species in south-western Central Europe. In Europe the Alps form the southern border of the distribution area of this nordic-subarctic fern species. The historic distribution and former populat ion sizes of D. cristata in Switzerland and Liechtenstein were reconstructe d based on extensive herbarium and literature studies. Present distribution and population sizes were determined in a field survey in which the actual existence of 14(38% of all described) populations was confirmed. Twenty-tw o populations are now extinct mainly due to habitat destruction till 1945, resulting in a broad distribution gap between the remaining sites in the we stern and the eastern parts of the study area. For eleven of the present po pulations a classification of the former population sizes and their changes was possible. Five of the remaining populations suffered a bottleneck situ ation with a population size of less than 25 individuals at least once duri ng the last 120 years. At present, three of the four remaining populations in western Switzerland are small remnants of formerly large populations, wh ereas at most of the eastern sites, a tendency towards increasing populatio n sizes was observed. The presented data will serve as a basis for further investigations into D. cristata as well as fur future monitoring and conser vation biology or genetics.