Admixing broadleaved to coniferous tree species: a review on yield, ecological stability and economics

For several reasons the conversion of mono-species into mixed-species forests is presently a major concern of forest management and policy in Central Europe. Although it is possible to show a clear trend in favour of mixed-species forests, private forest owners and some forest economists have often not favoured mixed forests, assuming that they are less profitable. The trend towards mixed forests seems mainly for ecological reasons, while sound economic analysis of mixed forests is still rare. Based on this background the objective of the study is to answer the following four questions: (1) Does the yield of mixed-species forests differ from that of pure forests? (2) Does the mixing of tree species influence the ecological stability of forests? (3) Is the economic value of a mixed forest less than that of a monoculture? (4) How do forest economic models integrate the findings on yield and ecological stability of mixed forests? To answer these questions a literature review was conducted on the possible impacts of mixed-species forests. In comparison to pure stands a greater yield is not necessarily given in mixed stands. Yet, mixed-species stands are better able to compensate disturbances than monocultures. Moreover, there is substantial evidence that mixed-species stands are more resistant against biotic and abiotic disturbances. Applying an extended forest economic model, it was possible to demonstrate that mixing large blocks of native broadleaf species into pure conifer forests may lead to a significant reduction of financial risk. From a risk-averse perspective the economic value of a mixed-species forest may thus be greater than that of a mono-species forest. Yet, it became clear that forest economists do not often integrate the research findings on yield and ecological stability of mixed stands in modelling, but rather apply simple bioeconomic modelling. Moreover, in the context of mixed forests economists also largely ignore even classical financial approaches, which consider risk and risk preferences. We concluded that forest economics has to close substantial research gaps. Firstly, the knowledge of how to integrate biophysical properties of mixed forests in bioeconomic modelling is still an open question. Secondly, forest economists have to adopt the modern approaches of financial theory and management science to value mixed forests.