First thinning in sub-tropical eucalypt plantations grown for high-value solid-wood products: a review

SummaryThe decision to first thin a eucalypt plantation grown for high—value products must be based on the growth, wood quality and financial consequences of the decision. These factors are interrelated and at present some are difficult to quantify. This review summarises available knowledge on a range of factors influencing the decision to first thin in eucalypt plantations, with specific reference to sub-tropical eastern Australia.The most compelling reason for early thinning is to remove defective stems before they become dominant within the stand. In plantations of unimproved genetic stock, a high proportion of stems can be of inferior quality, and in these circumstances stand quality and value recovery can be greatly increased by early, possibly non-commercial, thinning. Stands of ‘intolerant’ species, such as Eucalyptus grandis, segregate into dominance classes earlier than do stands of more ‘tolerant’ species. Delaying thinning may allow defective stems to become dominant, suppressing some of the higher-quality stems which are then less able to respond to later thinning. In this regard, management options for eucalypts are generally much less flexible than for tolerant genera such as pines. This may be less important for stands of improved genetic material, which are more uniform, or for stands with a small proportion of defective stems.To maximise growth on final-crop trees, thinning should be undertaken as early as possible to take advantage of early rapid growth rates that cannot be attained later. Thinning too early, however, may reduce wood quality. The most common reason for downgrade in eucalypt plantation timber is knots. If pruning is not undertaken, delaying thinning until the lower branches die takes advantage of the excellent self-pruning behaviour of some sub-tropical species.Optimum thinning regimes are a trade-off between maximising growth on the smallest possible number of stems while fully utilising the site and minimising risk from biological and climatic factors. Maximising growth on the smallest number of stems allows critical threshold log sizes to be attained rapidly, thereby maximising the advantages from minimising time to financial returns. If thinning is too severe the site will not be fully occupied, resulting in lost production and problems such as weed growth, epicormic flushing and wind damage.The benefit or cost of early thinning depends on harvest costs, its effect on wood quality, product assortment, average piece sizes and available volumes per hectare. There are significant risks and opportunity costs associated with delaying thinning that may be greater than the perceived financial benefits—e.g. the immediate saving from not conducting a non-commercial thinning may be negated by a reduced total (net present) value recovery and lower internal rate of return over the rotation.