These types of vegetation have been identified in many cities around the world, often on derelict land in transition from one use to another. They generally involve a mix of opportunistic native and exotic species. The resulting vegetation often reflects local variations in land use, cultural practices and climate. As a result, these spontaneous plant communities often differ substantially even between different cities within a relatively small geographical area (Gilbert 1992) and are of considerable ecological interest (Table 6.7).
Spontaneous urban vegetation is generally associated with the subsoils, crushed building rubbles, and shallow layers of transported topsoils used as a surface covering. As many of the plants involved often have strong ruderal tendencies, there is often dramatic change in the vegetation during the first five years after the colonisation event. Community composition is often strongly related to soil productivity, with nitrogenfixing legumes, such as clovers, medics and naturalised garden escapes such as Galega officinalis, dominating initially on the most infertile materials. These often decline in importance as they contribute to increasing soil nitrogen, and are replaced by taller nonnitrogen fixing forbs and grasses. In Sheffield, for example, there is a gradual increase in the standing biomass
Table 6.7. Typical forbs of synanthropic plant communities in maritime versus continental climates in Europe (adapted from Gilbert 1992;
Kuhn 2000; plus the observations of the author)
Oenothera biennis Saponaria officinalis Senecio squalidus Solidago canadensis Tanacetum vulgare Taraxacum officinale agg.
of perennial long-lived species such as Chamaenerion angustifolium, Solidago gigantea, Saponaria officinalis and Centaurea nigra. Many of these species will have been established as part of the initial wave of colonists, but are initially held in check by low soil nitrogen levels. High rates of seed production and dispersal are common characteristics of synanthropic species.
Although the public may consider these plant communities to be weedy, they are often unique assemblages closely associated with human environments. Some of these communities are extremely attractive, and, given that they are clearly well-fitted to urban conditions, in some cases they provide an attractive basis for developing truly sustainable naturalistic vegetation. In Berlin, Ktihn (2000) is working on creating new urban plant communities based on the combination of spontaneous local species supplemented with nonnaturalised species to add additional colour and interest. A potential disadvantage of these types of communities is that they may be viewed negatively by the public because of the visual associations of some species with derelict land and the notion of urban decay.