Long-Term Observations of Secondary Forests Growing on Hard-Coal Mining Spoils in the Industrial Ruhr Region of Germany

The Ruhr region is a large industrial region in northwest Germany with very dense settlements and other man-made environments, resulting from very intense hard-coal mining of earlier times. This region was the former “black country” in Germany in the sense that Nerys Jones describes (see Chapter 8). This black material consists of hard-coal mining waste, a substrate that becomes dry and hot in the summer, up to 80°C on the surface (personal observation), but very wet in winter and spring. This waste has been piled into hills, the largest of which are 22 to 100 m high, upon a landscape that is otherwise relatively flat. Most of these waste heaps are reafforesting, with only a very few still actively being formed. Because of their close association with highly developed areas, we consider these emerging forests to be one type of urban forest that we are calling “industrial forests.” We consider this forest type to be important in this region, since it covers an area of almost 10,000 hectares (ha).

Nearly all of the 22 large heaps had been planted by landscape managers (not ecologists), and therefore the plant species consisted of exotic ornamental woody plants, nearly all being species of shrubs and trees available in garden centers and tree nurseries (species lists are found in Schwiederowski, 1994). Because these waste piles exhibit extreme soil conditions of a substrate type called “Berge” (Wiggering and Kerth, 1991; Weiss et al., 2005), nearly all planted trees and shrubs, exotic as well as native ones, exhibit a high rate of die-back after transplantation. So the purpose of the project we describe here is to understand the ecology of the piles that are becoming naturally vegetated to learn how we can accelerate the process of secondary succession with species that can persist on these disturbed sites.