Patterns and processes of Heracleum mantegazzianum invasion into German cultural landscapes on the local, landscape and regional scale
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The present dissertation analyses the invasion of the Giant Hogweed (Heracleum mantegazzianum Somm. et Lev.) into cultural landscapes of Germany on three spatial scales. At the habitat scale, I studied the dominance of Giant Hogweed with respect to environmental gradients and anthropogenic disturbances. An analysis of current invasion patterns at the landscape scale indicated strong effects of landscape structure on the dispersal process. Further, a germany-wide survey showed marked regional differences in invasion intensity. The Giant Hogweed is a neophytic tall herb introduced to central Europe from the Western Greater Caucasus in the 19th century. The species shows a mass increase in several european countries since the middle of the 20th century. It is regarded as a problem plant because it is capable of forming dominant stands and elicits photo-dermatitis. During this study, I conducted a germany-wide questionnaire survey of invasion intensity in counties. Further, I carried out field studies of invaded habitats in 20 study areas (each 1 qkm), that represented the most-heavily invaded landscape sections of Germany. Additionally, I mapped time series of aerial images of the study areas to calculate area balances of invaded habitats and assess historical landscape dynamics. In Germany, the Giant Hogweed occurs prevalently in abandoned grasslands, on riverbanks, and on road verges, where it invades successional or ruderalised areas. Dominant stands, that account for ca. a third of all incidences, can mostly be found in young successional stages, whereas older stages feature mixed stands with native tall herbs. Here, the abundances are apparently controlled by inter-specific competition. The invasion of the Giant Hogweed is facilitated by disturbances of the vegetation and land-use changes, that lead to secondary successions at productive sites. Young successional stages are "windows of opportunity" for native and invasive tall herbs, because competitors are often missing due to preceding land use or disturbance, and so the first colonizing competitive species have an advantage. Such "priority effects" may explain a large part of the high variation in abundances of tall herbs in secondary successional series. The spread of Giant Hogweed at the landscapes scale depends largely on the landscape structure, particularly the connectivity of invasible habitats and corridors for migration and seed dispersal. Here I found evidence for such corridor function of rivers and roads. If corridors are lacking and invasible habitats are isolated, the dispersal success of Giant Hogweed is strongly limited. This disperal limitation is expressed in surprisingly moderate invasion rates up to now. For the most favourable habitats abandoned grasslands and tall-herb stands in the most heavily invaded landscape sections of Germany, the proportion of invaded area was 18.5% and the area covered by Giant Hogweed was 8.7%. The germany-wide assessment of county-based invasion intensities indicated that some, comparatively peripheral areas face a real "invasion", while in intensive agricultural areas there was hardly any invasive spread until today. The Giant Hogweed is often supposed to reduce species numbers of the invaded vegetation and to exclude native species. A detailed account of species numbers, cover percentages and vegetation types showed, however, a general decline of species numbers in the course of secondary successions, largely independent of the cover percentages of Giant Hogweed. The findings of this study, suggest that negative impacts of Giant Hogweed on native biodiversity have been moderate until now. It does not appear likely that it will threaten native species and communities in the future.Verknüpfung zu Publikationen oder weiteren Datensätzen
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Phytocoenologia 36/2 (2006), S. 281-320