Silviculture of oak for high-quality wood production : effects of thinning on crown size, volume growth and stem quality in even-aged stands of pedunculate oak (Quercus robur L.) in Northern Europe
Abstract: Oak is one of the most valuable noble hardwood species in Europe. The production of high-quality wood is associated with long rotations and high labour costs. The aim of this thesis was to quantify and model the effects of some silvicultural practices, mainly thinning, on crown size, volume growth and stem quality on pedunculate oak (Quercus robur L.), to provide a scientific basis to modify contemporary oak silviculture. The response pattern of specific silvicultural practices was tested in nine long-term thinning experiments and one operational stand located in Northern Europe. Results showed that pruning led to an overall increase in the total production of new epicormic shoots, which decreased with increasing height along the stem and with decreasing stand density. This suggests that early, heavy thinning combined with high pruning at regular intervals may help shorten the rotation length. A set of models for predicting crown radius was presented, based on different levels of predictor variables that offer flexibility in terms of data availability. The models can be used, for example, to calculate the average area potentially available for final crop trees and therefore the potential number of trees at the end of the rotation. Models for individual tree volume were fitted separately for predicting total and stem volume, whilst accounting for thinning practice. Results showed an increase in prediction accuracy in comparison to similar models available in the literature. Stand volume growth was analysed and modelled in relation to age, thinning practice and site. The highest growth was for unthinned stands, decreasing with increasing thinning grade. Volume growth of 50 potential future crop trees ha-1 increased with increasing thinning grade at all ages, demonstrating a positive effect of thinning on the growth of selected crop trees. The results lend support to the concept of crop tree silviculture based on early initiated, heavy thinning for the 'best' trees at regular intervals.
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