Modelling stand growth of radiata pine thinned to varying densities

Growth trends in a radiata pine thinning experiment established 18 years ago in Kaingaroa Forest, New Zealand, are portrayed and analyzed. The six thinnings, to 200, 300, 400, 500, 600, and 700/ha, were carried out at age 7 years; four replicates of each thinning level were laid out in 0.2 ha plots in a randomized complete block design. The dbh outside bark of all trees and the heights of a sample of 12–15 trees per 0.1 ha inner plot were remeasured at least once each year up to age 20 and then at 2-year intervals thereafter. Mean top-height development after age 7 years was little affected by thinning intensity, although initial negative selection may have reduced the subsequent vigour of the most heavily thinned plots. Analysis of a Gompertz projection model for net basal area per hectare and a similar yield formulation indicated that thinning down to 300/ha progressively lowered basal-area production and the maximum attainable yield but did not impair the ability to capture fully the available growing space. The 200/ha treatment, however, was much impaired, and its asymptote was lowered to such an extent that its yield trajectory was continuing to fall away relative to the others. The analysis quantifies this reduction and could possibly allow estimates of how much production would be lost between 200 and 300/ha, so that due allowance could be made when predicting yields for routine thinning prescriptions within that range.