Effects of stand composition and thinning in mixed-species forests: a modeling approach applied to Douglas-fir and...

Models estimating growth and yield of forest stands provide important tools for forest management. Pure stands have been modeled extensively and successfully for decades; however, relatively few models for mixed-species stands have been developed. A spatially explicit, mechanistic model (COMMIX) is presented that simulates growth of mixed-species forest stands, and takes account of the effects of management on stand dynamics. Previously, it was shown that COMMIX satisfactorily reproduced the development of monospecific stands. In the present study, the model was used to simulate growth and yield in mixed stands differing in the proportions of species present. The concept of a "replacement series" was used to compare productivities of the mixed stands. The model was also used to analyze effects of thinning regimes and stand composition on productivity. Model simulations indicate that productivity of a mixed stand will generally be intermediate between the productivities of monospecific stands of the contributing species. However, stand composition, and especially thinning regime, will strongly affect stand productivity. The simulations are discussed with reference to the effects of resource partitioning, canopy stratification, complementarity, spatial pattern, crown dynamics, and phenology on the growth and yield of mixed stands. The study highlights the value of using mechanistic approaches to predict mixed stand development in relation to management regime.