A fundamental feature of the urban species pool is the prevalence of nonnative species that were introduced accidentally to urban habitats or that escaped from cultivation (Kowarik 1995). As a consequence, non-native species play a large role in reforestation processes on urban abandoned areas, but are significantly less important on peri-urban sites, for example, during succession on surface mines (Prach and Pysek 1994; Tischew and Lorenz 2005). Table 4 illustrates the significance of non-native species with the example of woody species that are dispersed in different habitats of Berlin.
The dispersal processes that lead to the colonization of abandoned areas are determined culturally as the choice of species planted in the surroundings of the site that then serve as sources for further dispersal was horticulturally determined. In Berlin’s inner-city abandoned areas this has led to a heterogeneous pattern in the distribution of stand-building tree species. In the colonization processes of peri – and non-urban abandoned areas, it is usually native early-successional trees such as European white birch (Betula pendula) and European aspen (Populus tremula) that dominate. These are found in inner city areas as well. When competitive non-native species have been cultivated in the vicinity, however, their dispersion can lead to new types of urban-industrial woodlands, as the dominance of the North American Robinia pseudoacacia in the former Diplomatenviertel (Diplomatic Quarter) of Berlin demonstrates (Kowarik 1992a; Fig. 1). In the Ruhr, the influence of urban dispersion sources is revealed in the widespread establishment of the ornamental Buddleja davidii in early succes – sional woodlands that are dominated by native birches (Dettmar 1992).
A few native forest species may colonize even isolated urban sites by natural means, such as some ferns (Keil et al. 2002). When forest remnants lie in the vicinity of the succession area, however, typical forest species can establish themselves over time (Tischew and Lorenz 2005). Most forest species, however, can not overcome the distance between remnants of pristine forests and urban-industrial sites. In city centers, forest development on abandoned areas is thus mostly determined by species that migrate from the urban surroundings.
When non-native species colonize derelict land, the connection to horticultural dispersion sources in the surrounding areas and also to the relics of earlier plantings on the site is often very clearly readable as a culturally based process. Less obvious is the parallel case of native species. Because native species are also mostly introduced from other areas via horticultural distribution and partly demonstrate features of domestication, they form a component of the species pool that deviates genetically from the original species composition (Kowarik 2005). Native escapees from cultivation, like non-native ones, can therefore change the species composition or strongly influence the stand development of urban-industrial woodlands. Both cases lead to altered biodiversity patterns in comparison to the other woodland types described in Table 2.
Table 4. Woody species (trees, shrubs, woody vines) escaped from cultivation and occurring in different urban ecosystems of Berlin. All non-native species are escapees. For native species, the number of species descended from natural populations or from cultivated populations is unknown (after Kowarik 1992a). Column 1
Fig. 1-5. Cultural layers and natural processes in urban woodlands. The Diplo – matenviertel in the center of Berlin was heavily damaged during World War II and was then cleared, leaving only a few ruins. The woodlands that mainly resulted from natural colonization processes illustrate, directly and indirectly, the influences of previous uses, particularly earlier horticulture. The dead black locust (Robinia pseudoacacia) in the upper left came from a former embassy garden. It was a dispersion source for the expansive wild woodlands, now dominated by Robinia. A few relics of old plantings point directly to the 19th – century garden history of the site (Fagus sylvatica, Catalpa bignonioides). In the
herbaceous vegetation, another North American species, Parietaria pensylvanica, occurs, whose German distribution was limited to Berlin for much of the post-War period. According to Sukopp and Scholz (1964), the seeds of these species that escaped from the Botanical Garden may have been widely dispersed throughout the city by the turbulence caused by bombing raids. Through informal uses, open areas are still maintained. The picture below shows the changes in the soil morphology from jumping areas for mountain bikers. Here pioneer species, such as the eastern Mediterranean Chenopodium botrys, that were typical of the new rubble fields of the early post-War period can survive. The naturalness of the woodlands, from a retrospective perspective is low. The naturalness of the site from the second, prospective approach is, in contrast, relatively high, because the leeway for natural processes in these urban woodlands is large.