Analysis of habitats and communities invaded by Heracleum mantegazzianum Somm. et Lev. (Giant Hogweed) in Germany
Thiele, Jan; Otte, Annette
The aim of the present study was to analyse Heracleum mantegazzianum's habitat preference and to identify recipient communities in its invasive range in Central Europe with regard to the species' effects on resident vegetation and potential implications for nature conservation. Field investigations were carried out in 20 study areas (each 1 × 1 km2) in Germany. In all encountered stands of H. mantegazzianum the vegetation composition and various site parameters were sampled. Additionally, time series of aerial photographs of study areas were analysed to reconstruct the history of invaded sites.H. mantegazzianum occurs in a variety of different habitat types, such as grasslands, roadsides, riverbanks, woodland margins etc. Stand densities of the species vary widely from scattered individuals to dominant stands. Primary constraining factors for H. mantegazzianum densities are land use, shading and low-productive site conditions. Site conditions of preferred habitats are more or less uniform, and are characterised by high productivity in combination with lack of land use and recent or historic disturbances or habitat changes. H. mantegazzianum is a successful invader and a potentially dominant species only if these particular habitat requirements are met. However, even then most stands of the species are not dominant.The majority of invaded sites have been subject to human caused habitat changes within the last 50 years which have enabled or facilitated invasion. The most important process here is land-use decline, especially abandonment of grasslands.The prevailing vegetation types with H. mantegazzianum are ruderal Arrhenatherion grasslands and Galio-Urticetea tall-forb stands which represent stages of secondary successions from grasslands to woodlands after abandonment of land use. Successional age seems to play a role with respect to stand densities of H. mantegazzianum as maximum densities occur prevalently at sites which represent young successional stages.The results of the present study suggest that high densities of H. mantegazzianum can decrease native diversity of invaded stands, especially in abandoned grasslands. However, a loss in diversity is a typical effect of the processes that facilitate the invasion of H. mantegazzianum, i.e. abandonment of grassland management and severe disturbances or even habitat destruction (e.g. due to mining), and can be brought about by native species, such as Urtica dioica, as well. From this point of view H. mantegazzianum can be seen rather as a symptom of diversity loss than the cause of it.Further, the results suggest that H. mantegazzianum does not seriously threaten to conflict with nature conservation as preferred habitats and plant communities are very common today and habitats which are of special conservation interest present no favourable conditions for this species.