Ökologie und Entwicklung des Zwerg-Rohrkolbens (Typha minima) dargestellt am Beispiel der wieder eingebürgerten Population an der Oberen Drau (Österreich)

The dwarf bulrush (Typha minima Funck ex Hoppe) is a characteristic alpine pioneer herb that oc-curs primarily along wild rivers. However, since the 1970s this indicator species has become locally extinct in Germany, and nearly so in Austria. Ongoing population declines are expected to be the result of wide spread river regulation and resulting alteration to key demographic processes. Thanks to the effort of reintroduction programs, T. minima can be found again along the Upper Drava River, Austria. In this paper we report on seed germination, growth, reproduction and the environmental preferences of T. minima. Germination experiments revealed a very low mean germination rate of 15.6% with a range of 0–90%. Germination rates increased with higher temperatures, increased seed maturity and shorter seed bank periods. After seed storage for 480 hours no germination was observed. In monitored field populations at the Upper Drava, seedlings (height < 5 cm) were rarely found. Individuals at the young stage (height > 15 cm, only sterile shoots) accounted for the majority of basal area at about 62%. Individuals up to three years old developed only sterile shoots. Reproductive shoots developed after 9 years and the number of inflorescences increased with increasing age.An analysis of environmental conditions showed that T. minima was especially dependent on high soil moisture content, with an average of 39 Vol-% humidity. In addition, light conditions were also important for growth - declines in shoot density were found at > 50% canopy coverage by Salix shrubs. We conclude that T. minima populations are demographically sensitive during seed germination, and that modern range retractions are predominantly the result of river regulation and reduced morphodynamics that would normally create appropriate open patches for germinating seedlings.