Optimising the yield of Douglas-fir with an appropriate thinning regime

Aim of this study was to determine the long- term effects of thinning regime with different intensities on the stand and tree social classes development, particularly regarding the old question of compensative effect between stocking and girth improvement using the example of Douglas-fir. Data provide from a thinning experiment in one site of Swiss Central Plateau, from age 11 until 41 years containing six treatment variants (three planting densities and two level of stand density) repeatedly mea- sured six times, as well as from seven permanent yield plots times series aged between 17 and 127 years. We present as well the real observed characteristics in term of stem numbers distribution, diameter increment per d.b.h. categories, as results from simulation over the whole pro- duction time until 105 years with an appropriate growth simulator, in terms of net value increment (MAIv). Our results show that the effect of thinning works mainly on the social medium categories, in terms of number and girth improvement. Social dominant trees are more or less untouched by thinning, demonstrating that there are in every stand a quite important number of self-dominating trees which do not need particular silvicultural help to maintain their dominant status in the upper storey. The expected compensation between stem number reduction and girth improvement shows that the remaining stand density is determinant on the net value level. The so-called mass effect seems really more effective. The best way to improve a stand value appears to be artificial pruning. Because a thinning regime should not consider only the economic productivity but take in account risk, stability and resilience and particularly the way of renewal, we suggest a way for optimising the thinning regime com- bining effect of biological rationalisation and risks.