Analysing effects of thinning on stand volume growth in relation to site conditions: A case study for even-aged Sitka spruce (Picea sitchensis (Bong.) Carr.)

This study challenges all-else-equal assumptions in the analysis of thinning effects and more specifically those regarding local variation in forest site productivity. It is demonstrated that considerable variation may occur in site productivity in apparently homogeneous forest stands, whether thinned or unthinned, and that this may not be reflected in height, but rather in basal area growth. To account for such variation, the suggestion is to include an unbiased pre-treatment measure of site productivity, such as total volume production prior to first thinning, as a covariate in the analysis. In a wider perspective, the study further challenges two general site productivity hypothesis, namely, the general assumption that stand volume growth correlates well with stand height (the site index hypothesis, i.e. that site productivity can be estimated based on stand height) and the assumption that, for a given species, total volume production at a given stand height is identical for all site classes (Eichhorn's rule). The thinning response pattern for even-aged Sitka spruce is quantified for different site types based on two thinning experiments in Denmark and six comparable experiments in Great Britain, Norway and Sweden. The effect of thinning from below on stand volume growth is strongly site dependent, but heavy thinning usually leads to a reduction in volume growth as compared with an unthinned stand growing under similar site conditions. For some sites, typically those with stable and ample water supply and a deeply developed root system, stand volume growth is maximized through light thinning. For some other sites, typically those with a high ground water table and a shallow root system, thinning reduces stand volume growth and considerably so with increasing thinning grade. There seems to be a gradient in the reduction, so that it is lessened with increasing rooting depth and with increasing stability of water supply. Due to site-dependent variation in the relation between height growth and stand volume growth as well as site-dependent variation in thinning response pattern, it is recommended to establish and maintain unthinned observation plots in managed stands to ensure reliable growth and yield prognoses. Such plots can serve as a reference for thinning practices as well as a standard for potential volume growth. This recommendation could also hold for other species and in other parts of the world.