More timber from fewer trees – determining what tree density optimises silver beech merchantable yield based upon a long-term thinning trial

Abstract

Background: The tree stem density which optimises merchantable timber yield (volume per unit area) is unknown for most of New Zealand’s indigenous tree species. While moderate thinning of even-aged stands can promote yield, intense thinning may decrease yield by creating space that cannot be filled by residual trees, increasing tree mortality or reducing tree height. We quantified the effects of density on silver beech (Lophozonia menziesii (Hook.f.) Heenan & Smissen) tree growth, height and mortality, identified the density leading to optimal merchantable yield and assessed if this density varied with stand age. Methods: Tree stem diameter growth, height, and mortality responses to density were determined using tagged individuals monitored over time on a long-term thinning trial combined with flexible, multilevel, non-linear models. Empirical stand yield responses to density were determined and compared to yield–density relationships in simulated stands. The stand simulations projected beyond the monitored stand ages using the tree-level responses fitted to empirical data. Results: Low densities (≤400 stems ha-1) sustained fast tree growth for longer than high densities (≥700 stems ha-1) after thinning, but density did not consistently affect merchantable tree heights. The probability of tree mortality increased after intense thinning, but only temporarily, and never exceeding c. 0.01 year−1. A regression of yield–density relationships identified an empirical optimum of c. 570 stems ha–1 for stand ages of 48 and 58 years. At this density, merchantable yield at 58 years was seven-fold greater than that in unthinned stands. The simulations suggested moderately higher densities for optimal yield than our empirical optimum, a moderate increase in optimal densities with stand age, and that c. 90 % of potential cumulative yield was attained at 80 years. Conclusions: Because thinning increased tree growth, but had minimal effect on tree mortality, our results alleviate concerns about the stability and productivity of thinned stands. Densities that optimise yield are about two-fold greater than those previously recommended for silver beech and they remain relatively stable as stands age. This suggests that a single density will be adequate for a range of harvest ages, although harvest should take place before a stand age of 80 years. Such conclusions are relevant to managing regeneration within coupes harvested under existing legislation and to areas planted with silver beech.