Impact of spacing and pruning on quantity, quality and economics of Douglas-fir sawn timber
scenario and sensitivity analysis
Andreas Rais (Technische Universität München)
Werner Poschenrieder (Technische Universität München)
Jan Willem G van de Kuilen (TU Delft - Bio-based Structures & Materials, Holzforschung München)
Hans Pretzsch (Technische Universität München)
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Abstract
Controlling the long-term effect of management on the quantity and properties of individual boards is a fundamental challenge for silviculture. Within this basic study on Douglas-fir, we have investigated the sensitivity of the net present value (NPV) to three most common planting densities and a prominent pruning strategy. We therefore have applied an individual tree growth model, which represents intrinsic stem structure as a result of crown competition. The model extrapolated board strength development to the rotational age of 70 years, starting from real and comprehensive data recorded from experimental Douglas-fir plots at the age of 20 years. Total volume production increased from about 1600 m3 ha−1 for 1000 and 2000 trees ha−1 to 1800 m3 ha−1 for 4000 trees ha−1. The economic superiority of the lowest density stands increased considering the NPV at inflation-adjusted interest rates of 0%, 2% and 4%: Given an interest rate of 2% and no pruning, the NPV at 2000 was at about 50% of the one at 1000 trees ha−1. The NPV at 4000 trees ha−1 was even negative. Generally, artificial pruning was not effective. The revealed financial trade-off between growth and timber quality in young stands underlines the importance of silvicultural guidelines, which quantify the effect of management on yield per strength class and financial outcome.
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