Die unbeachtete, aber trotzdem spektakuläre Ausbreitung des Neophyten Rubus armeniacus in Städten – das Beispiel von Braunschweig
Intra-urban woodlands are not a popular topic for investigations due to their floristic and ecological heterogeneity as well as to their mixture of planted, running wild and indigenous species which are difficult to unravel. It seems too complicated to place them within the conventional system of plant communities and they are somewhat like the urchins within the woodlands and shrubberies. Rubus armeniacus, the Himalayan [precisely: Armenian] blackberry, originates from the Caucasus region and was introduced to Germany first in 1837. For a long time the species was the most frequently grown bramble in our gardens and in some regions it is still like this. Running wild plants established quite unnoticed shrubberies of different plot sizes. Actually publicity is increasingly interested in the spreading of neophytes whereas the assessment changes remarkably. The heavy discussions sometimes may take xenophobic attributes. To generate an authoritative basis distribution, habitats and establishing of Rubus armeniacus were investigated within the city of Braunschweig. Besides this possible vectors of dispersal and the potential suppression of indigenous species were in the focus of our investigations. Within the area of Braunschweig (192 km²) 1,703 locations with stands of Rubus armeniacus where documented during winter 2017. The area covered by Rubus armeniacus was at least 104,589 m². 175 of the locations are plantings, 464 are juvenescences near plantings (distance < 15 m), whereas 1,064 locations are not near to plantings. This results in a rejuvenation index (Verjüngungswert) of 72.6 % as well as in a value of spreading (Ausbreitungswert) of 69.6 % in Braunschweig. Random remapping in summer 2017 showed that 90 % of the stands have been covered by the investigations during the winter 2017. The mapping shows a clear accumulation of stands with Rubus armeniacus along railways and with allotments. Within intensively maintained gardens of detached houses Rubus armeniacus was found only seldomly. The association of the species was determined by 23 relevés. The species composition is very heterogeneous: Of the 70 accessory species only Urtica dioica and Galium aparine reach medium constancy. At the edges of the bramble stands and in their neighbourhood only trivial ruderal species and trivial grassland species have been found. A punctual suppression of such species by Rubus armeniacus seems not to be critical. Furthermore a control measure in the district of Lüneburg was evaluated. It shows that a singly cut directly at the soil surface is not sufficient to repress a stand of Rubus armeniacus. Depositing the chopped shoots at edges of woods speeds up the spreading of the species in the open landscape. A suppression of Rubus armeniacus will be successful only by long lasting maintenance. The costs however will not be within a reasonable correlation to its success. The probability is high that Rubus armeniacus will not be eliminated from our flora, especially because you can still buy it as a garden plant.