For about one hundred years, numerous approaches for scientifically classifying the naturalness of vegetation types and ecosystems have been developed. They share the fact that they evaluate varying degrees of naturalness or, reciprocally, they evaluate the extent of human influence. They share the further characteristic that a defined reference point of maximum naturalness is often missing or at least remains undetermined. Two perspectives on naturalness can be fundamentally differentiated: naturalness from a retrospective or a prospective perspective (Kowarik 1988, 1999). Table 5 illustrates this concept through the assignment of existing classification approaches to both perspectives. The deciding factor is the reference point. It may lie in the past or it may be adopted from the present or the future.
Naturalness from a retrospective perspective
The retrospectively (i. e. vegetation historically) oriented perspective analyzes the extent to which a current woodland corresponds to an earlier stand that grew in the same region and whose structure, species composition and site factors were not influenced by human activities. The point of reference is therefore, pristine vegetation uninfluenced by humans. Based on the cultural history of the relevant area, the reference period may lie decades or a few millennia in the past. Such a historically based comparison is fundamentally made possible with the help of vegetation-history knowledge. Different scales have been developed to estimate the different grades of “historic” naturalness; the scale of Ellenberg (1963), with seven degrees from “untouched” to “artificial,” is the most widely used (for more details see Kowarik 1999).
In the retrospective perspective of naturalness, remnants of pristine woodlands are most natural and woodlands used for forestry are at least semi-natural. Should the silvicultural use of these woodlands, such as the Sihlwald near Zurich and the Saarkohlenwald near Saarbrucken, be reduced, forest development will lead to a convergence to pristine vegetation and therefore back to a “retrospectively” natural stand.
In contrast, horticulturally planted and maintained woodlands are, from a vegetation history perspective, artificial, as the species composition, stand structure and frequently the site conditions deviate substantially from pristine woodland types. How are new types of urban-industrial woodlands assessed? In the traditional, retrospectively oriented analysis, these woodlands, based on their cultural shaping, must be considered “artificial” when, under anthropogenic site configurations, stands grow that are dominated by species from the urban species pool rather than the historic species pool. This has the consequence that urban-industrial woodlands are assessed, in the retrospective approach, as just as artificial as horticulturally shaped stands, even when they are profoundly shaped by the natural processes described above.
In summary this means that from the retrospective perspective the development back to nearly natural or natural woodlands can be analyzed well. With the evaluation of new development of “wild” urban-industrial woodlands, however, the traditional concept of naturalness oriented toward historical comparisons runs aground. The retrospective approach does not recognize the clear wilderness character of such stands and therefore can not be used to differentiate these woodlands from other stands and allow for a differentiated evaluation.