Thinning is a key silvicultural practice, applied widely throughout New Zealand’s Pinus radiata plantation forests. The timing of thinning has often been a topic for debate, with selection ages varying depending on factors such as site, regime, and managers’ preference. A paper by Piers Maclaren (1995) analysed the effect of selection age on final crop acceptability. He concluded that there is “… insufficient information present at young ages to make informed decisions around the most valuable trees at harvest.”

In 1998, forest managers at Carter Holt Harvey Ltd established a trial within Hamptons Forest in the Hawkes Bay. They were interested in taking Maclaren’s research a step further. The treatments in the trial were variations in the timing of thinning, at ages 3, 6, 9, 11, and an unthinned control. In 2021, the trial was decommissioned and clear-felled. A total of 15 measurements took place over the life of the trial, from ages 3 – 26. Using the data from this trial, this study investigated how the timing of thinning influences tree and stand characteristics that affect value.

The results indicated that by age 26, later thinning had significantly improved: the number of acceptable crop trees at harvest age; the proportion of trees that met the maximum branch diameter requirement for small-branch sawlogs (S grade) in New Zealand of 7 cm; and stand uniformity. Analysis of Rieneke’s (1933) stand density index (SDI), revealed that thinning at age 6 did the best job optimizing stand dynamics. Despite differences in these factors, the timing of thinning had no significant impact on LEV.

Forest managers may also wish to consider site, thinning intensity, safety, carbon sequestration, and log quality when determining the optimal timing of thinning. Growth rate variability across New Zealand makes it difficult for managers to optimize thinning schedules on different sites. Theoretically, a better alternative to standardized selection ages may be to use SDI as an indicator of competition between trees. This can act as a tool to inform the timing of thinning.