Tree species in German forests

The Forest Strategy 2020 (Federal Ministry of Food and Agriculture, BMELV) and the National Strategy on Biological Diversity (Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety, BMUB) ascribe natural and semi-natural woodland communities an important role in adapting forests to climate change and in promoting biodiversity. As there are no natural woodlands in Germany nowadays, "semi-natural", as a substitute reference, describes the deviation of the current condition of an ecosystem from the natural status. This natural condition which is no longer present is conceived with the help of "potentially natural vegetation" (PNV). PNV maps and their explanatory volumes in turn serve as a reference for planning and measures in nature and landscape, and as a reference for compensation and replacement measures which require the planting of a "natural tree species composition based on the PNV". The forestry authorities of many federal states are also guided by a "diversity of natural reference woodlands" in the context of managing semi-natural forests. The criterion of semi-naturalness is therefore of key importance for environmental and nature conservation policy. In view of the growing use of biomass in forests as part of the energy transition, there is an increasing urgency to develop meaningful criteria and concepts to assess the impacts of management operations in comparison to the natural reference status of the woodland.

However this "diversity of natural reference woodlands" has no standard derivation and there is no information on how widely the methods and assumptions used for this differ in their results. Consequently a comparison of the "semi-natural condition" determined by a range of methods is practically impossible and the meaning and representativeness of the concept cannot be justified. For environmental policy purposes and practical relevance for landscape it is therefore important to analyse the processes leading to the derivation of semi-natural. The main aim of this project is to compare and quantify the effect of these different methods.

Methods

The project works with the "map of potentially natural vegetation of Germany" published by the Federal Agency for Nature Conservation (Bohn et al. 2003, Suck und Bushart 2011). The basis for the modelling of this work is the results of the third federal woodland inventory (BWI) and in particular the tree species composition of the stands. These BWI data are compared by means of optimum curves with the species composition of the PNV at various hierarchical levels of varying detail and different meaningfulness for assessing the semi-natural status. The effect on the current semi-natural evaluation of introducing an early succession stage with pioneer tree species, climate-related changes in tree species, and the number of selected semi-natural levels also needs to be quantified and discussed. The findings of this should enable future assessments of semi-naturalness to be evaluated better and compared in terms of their significance and representativeness.