Simulation of growth and competition in mixed stands of Douglas-fir and beech

For a long time, the emphasis in silviculture in Western Europe was solely on even-aged, monospecific stands; many empirical stand-level growth models were developed and successfully used for managing such stands. In contrast, no generally accepted growth and yield approach has emerged so far for mixed forests. Moreover, the inexhaustible number of species combinations, management regimes, and site-dependent interactions make an empirical approach less suitable.In the present study, a mechanistic model was developed that simulates growth and yield in mixed forest stands. Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) and beech ( Fagus sylvatica L.) were used in this research. In the model, tree growth is dependent on radiation availability. Stand development is largely driven by competition for radiation. A spatial module was developed to investigate the effects of tree and stand characteristics on radiation interception. The study showed that in heterogeneous stands a spatial approach is needed to account for competition between trees.Growth of the trees was estimated using the radiation-use efficiency concept (RUE). Results revealed that detailed process models can be used to estimate RUE and that it is a suitable tool for (mixed) forest modelling.To describe the distribution of the dry matter growth, a separate module was developed using functional relationships between tree components: the dry matter distribution is driven by the aim to maintain structural balances within the tree. The study showed that this approach is able to reproduce the development of an individual forest tree. The approach was thus considered very suitable for modelling the effects of between-tree competition for resources on growth and development of mixed forest stands.The overall growth model, COMMIX, was applied to investigate the effects of stand composition on mixed stand productivity, using a replacement series. Analysis showed that the productivity of mixed forest stands is generally somewhere in between the yield levels of the monocultures of the less productive and the most productive species. It will only be possible to achieve higher yields in mixed stands if these stands have a relatively small proportion of the sub-dominant species. In the case of Douglas-fir and beech, the maintenance of a mixed stand appeared to conflict with the maximization of the wood production.Insufficient data are available on mixed stands to directly support decision taking in forest management. New research tools capable of providing forest managers with information on possible management scenarios and on the consequences of certain management regimes are therefore urgently required. The present modelling approach is part of an ongoing development of models for mixed stands. The infinite variety of possible species mixtures coupled with the range of environmental conditions under which mixtures might be grown, necessitates a mechanistic approach and emphasises the potential use of such models.