Stocking and intensity of thinning affect log size and value in Eucalyptus pilularis

Stocking at and after thinning forest plantations influences the site resources available to each tree, thereby affecting the size of logs and the value of the stand at harvest. Log prices generally indicate increasing value with increasing log size. A Eucalyptus pilularis Correlated Curve Trend (CCT) trial was used to examine distribution of tree size for stands established at 1495 trees ha⁻¹ and progressively thinned to final stocking densities of 700, 450, 250, 125 and 87 stems ha⁻¹. Diameter at breast height over bark and total height measurements, collected periodically over 36 years, were used to define stem taper. Smalian’s sectional volume formula was used to calculate log volume at 1-m intervals. Stems were then merchandised to current log size and pricing to determine optimum dollar values. Establishment and early maintenance costs were used to calculate net present value (NPV) at a discount rate of 6% and internal rate of return (IRR). The relationship between value over time and final stocking was irregular. The control (1495 stems ha⁻¹) had the highest volume ha⁻¹ at age 36 years. Final stocking densities of 125 stems ha⁻¹ and 700 stems ha⁻¹ achieved the highest (8.13%) and lowest (6.19%) IRR, respectively.