The retrospective approaches, which evaluate the naturalness of areas by comparing them with conditions that haven’t been influenced by humans, have a long tradition. This is due to the fact that the specific natural development of urban-industrial sites was recognized in ecological research relatively late. There are, however, a few authors, little-recognized today, that early on suggested a second perspective for evaluating naturalness. Bernatsky (1904) wrote that anthropogenically shaped vegetation types could regenerate themselves to an “Urformation” (a virginal or primeval condition), but that a development to a “natural condition” was also possible when the earlier cultural influences continued to have an effect. The forestry scientist von Hornstein (1950, 1954) demonstrated that a new type of forest, not previously existing, could grow on severely altered sites, namely a “tertiary type.”
As a continuation of such an approach, a second type of naturalness can be identified beyond a historic naturalness. For this, the reference point is not an original condition of a natural landscape, but rather a condition is defined based on the current site potential and the greatest possible degree of self-regulation (Kowarik 1988, 1999). From this perspective, therefore, the natural capacity for process is the central point, not a particular, retrospectively determined and often idealized, picture of nature. This perspective can be described as contemporaneous, because it begins from the existing potential of the site without regard for whether this has been exclusively naturally or anthropogenically shaped. It can also be described as prospective in that the naturalness of a site is evaluated according to how far, based on current uses for example, the stand is from a future condition of self-regulation without direct human influence. The hemeroby system is used to evaluate the degree of this second kind of naturalness; this system was created by Jalas (1955) and more precisely defined by Kowarik (1988) with regard to its contemporaneous orientation (Table 5).
Remnants of pristine woodlands are evaluated in this second, prospective view of naturalness as just as “natural” as they are in the retrospective view because they are determined by self-regulation of abiotic and biotic processes to the fullest extent possible. Distinctions are possible, however, when non-native species such as black cherry (Prunus serotina) can become established in the stands in the long term. From the retrospective view, this would stand in the way of a ondition; this is not true in the prospective approach (for a description of the scientific conflicts that arise from this, in setting goals for national parks and in process conservation, see Kowarik 2003, Korner 2005).
In the evaluation of the horticulturally shaped stands of urban green spaces, diverging assessments are probable. Woodland stands of horticulturally introduced non-native plants are always considered “artificial” in the retrospective perspective, even when other species have become established and the original planted species have propagated and spread through
natural regeneration processes and have, over the course of time, replaced the planted individuals. In the prospective perspective, the naturalness degree of such stands would increase during the course of such processes.
Clearly, the two perspectives on naturalness will result in diverging assessments for woodlands resulting from succession on severely altered urban-industrial sites. Profound changes to the sites as well as the dominance of species from the urban species pool lead to significant deviations from historic vegetation types. A convergent development to pristine vegetation types is definitely possible, for example, on mining slagheaps, where mixed-hardwood forests may grow on acid soils. Studies in Berlin have made clear that non-native species such as Robinia pseudoacacia may result in divergent succession trends even in the long term (Kowarik 2003). For this second, prospective perspective of naturalness, however, the origin of the species is irrelevant as is an irreversible, anthropogenic transformation of the morphology, soils and moisture regime. Urban-industrial woodlands are assessed as natural to the extent that they are free from direct, reversible human impacts and that they have reached a high level of self-regulation, for example, in the population dynamics of the plant species. This is, in principle, characteristic of pristine natural woodlands as well.
On urban-industrial sites, the prospective approach to classifying naturalness allows, for instance, for the differentiation between planted woodlands (e. g. for ornamental or reclamation purposes) and woodlands resulting from natural colonization processes. It further makes possible a differentiation between woodlands that are shaped by human uses to different degrees. This may lead, in certain urban agglomerations, to the surprising view that, of the ecosystems in the impact area of cities, it is the new types of urban-industrial woodlands that reach the highest level of naturalness when other sites are very heavily influenced by urban, forestry, or agricultural uses.