A growth model for mixed forest stands

The area of mixed forest stands in western Europe has significantly increased during recent decades and is still increasing today. Tools are needed to support the forest management decision taking in these mixed forests. However, though pure stands have been modelled extensively and rather successfully for decades, relatively few models for mixed-species stands have been developed. In this study, a mechanistic tree-based model of mixed forest growth is presented and applied. The aims of the study were (1) to present the modelling concept, (2) to compare growth and yield of mixed stands and monospecies stands, and (3) to demonstrate potential model application by simulating effects of management on mixed Douglas-fir/beech stands. The effects of stand composition and management on mixed stand productivity were investigated by simulating a replacement series. Simulation results showed that in general the yield of the mixture was somewhere in between the yield levels of the two monocultures. In stands with a low proportion of basal area of Douglas-fir, the productivity of Douglas-fir was lower than could be expected from its basal area share in the mixture; in stands with more than 40% of Douglas-fir, the opposite was the case. The productivity of beech in stands with few Douglas-fir was generally higher than expected from its proportion of basal area in the mixture, but much lower in mixtures with a higher Douglas-fir proportion. Thinning had a strong impact on productivity of the mixed stands. Yield of the mixtures was not linearly related to the basal area fraction of Douglas-fir present at the start of the simulations; simply combining monospecific yield tables will thus result in biased estimates of the yield of a mixed stand. (C) 2000 Elsevier Science B.V. All rights reserved.