The wind stability of different silvicultural systems for Douglas-fir in the Netherlands: a model-based approach

The aim of this study was to evaluate different silvicultural systems for Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in the Netherlands in terms of timber production and wind stability over a full rotation. This was done using the forest genetics, ecology, management and wind model (ForGEM-W), which combines a distant dependent tree growth simulator with a mechanical-empirical wind damage module. Six different silvicultural systems were evaluated: normal yield table management, free thinning from above in a monospecies and a mixed stand (50 per cent mixture of beech (Fagus sylvatica L.)), uneven-aged system, no thinning regime with low initial stand density of Douglas-fir with and without admixture of beech. Silvicultural systems leading to low height-diameter (h/d) ratios were most successful in avoiding damage. Low h/d ratios were obtained in the system with low stand density and no thinning and in the uneven-aged system by systematically removing trees with the highest ratios during thinning. In particular, the uneven-aged system combined a high timber production with low risk. The use of Douglas-fir-beech mixtures changed the competition pressure on Douglas-fir, and thus the h/d ratio and the wind risk. Results from this study indicate that the current trend towards more nature-oriented management could lead to lower wind risks and even to an increase in overall productivity.