Conclusions

H. mantegazzianum invades a variety of habitat types in Europe. While roadsides, riverbanks, ruderal places and woodland fringes have previously been documented as preferred habitats, results from our field study in Germany emphasize the importance of abandoned grasslands as habitats of H. mantegazzianum in European landscapes.

H. mantegazzianum can occur under a wide spectrum of environmental conditions. However, preferred habitats are rather similar, characterized by rich resource supply and disturbance but lack of regular management. Primary environmental factors constraining invasion of H. mantegazzianum are

m AGflfcOFT O FORËSTsOPT

m AGflfcOFT O FORËSTsOPT

M OPT>AGffl

M OPT>AGffl

■ AGRtSUB O FOREST>SUB

Fig. 8.7. Origin and fate of optimal and sub-optimal habitats of H. mantegazzianum that existed in the 1970s in 20 study areas in Germany. Habitats were mapped from multi-temporal time series of study areas for three dates: 1950s, 1970s and approx. 2000. Habitats were classified into optimal and sub-optimal based on electivity indices (see Table 8.3). Optimal habitats are open habitat types, whereas sub-optimal habitats contain woody components (>10% tree or shrub cover). A and B: origin of 1970s habitats, i.e. land-cover types from which they had developed since the 1950s. C and D: fate of 1970s habitats, i.e. land-cover types into which they have developed until approximately 2000. Abbreviations of land-cover and habitat types: OPT = optimal habitats; SUB = sub-optimal habitats; AGRI = agricultural land (arable fields, managed grasslands); FOREST; OTHER = all other landcover types which are not habitats of H. mantegazzianum.

regular land use and shading by trees. Furthermore, it appears that low soil nutrient status and/or wetness constrain invasion.

From a phytosociological perspective, abandoned or neglected semi-natural grasslands are a major plant community type of H. mantegazzianum, although the sociological centre of the species is on nitrophilous tall-herb communities. Plant communities with H. mantegazzianum form a successional gradient from grasslands, or subordinately severely disturbed sites, to tall-herb communities and woodlands. The successional age of the sites affects the cover of H. mantegazzianum. In old successional stages, cover is constrained by interspecific competition from other tall herbs and woody species.

Habitats of H. mantegazzianum in German study areas have been very dynamic during the last 50 years. On the whole, the area available for invasion has increased considerably, which is mainly due to abandonment of agricultural land use. However, optimal (i.e. open) habitats are characterized by a substantial turnover. Abandonment creates new habitat patches, while secondary succession resulting in forests eliminates habitats. Altogether it appears that abandonment of land use is the primary driver of habitat dynamics and may well have enhanced invasion of H. mantegazzianum during recent decades.

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