More trees or bigger trees?
Every plantation begins with a deceptively simple decision: how many trees should be allowed to grow? Plant many trees close together and the site is rapidly occupied, producing a large amount of wood per hectare. Give individual trees more space—or remove competitors through thinning—and something different happens: growth becomes concentrated in fewer stems, allowing them to become larger much faster. This trade-off lies at the heart of plantation forestry. In our study, Thinning regimes and initial spacing for Eucalyptus plantations in Brazil, we examined this question using 188 experimental plots in four clonal Eucalyptus trials in Bahia and Espírito Santo. The experiments compared different planting densities and a range of thinning strategies, from leaving stands untouched to removing most trees early in the rotation.
Before thinning, plantations established at 1,111 trees per hectare suffered considerably greater mortality than those planted at 667 trees per hectare, although early diameter growth was surprisingly similar. The major differences appeared once competition intensified. When stands were thinned early—at about 2.5 years—and only 150 trees per hectare were retained, growth became concentrated into those selected trees. By age 11.5 years, mean diameters of around 35–38 cm were achieved in these intensive regimes. The most intensively thinned treatment produced a mean tree diameter around 78% greater than the unthinned treatment. But there was a price: concentrating resources into fewer large trees reduced total stand basal area. In forestry, therefore, faster growth of individual trees does not necessarily mean greater production of wood at stand level.
There is no universally “best” thinning regime. The best strategy depends on what the forest is expected to produce. If the objective is pulp, energy or large quantities of relatively small-diameter wood, maintaining many trees can maximise stand stocking. If the objective is sawn timber or other higher-value solid wood products, early thinning can redirect the productive capacity of the site towards a much smaller number of large stems. Intermediate strategies can even produce several products during the same rotation, harvesting commercial wood in successive thinnings while keeping selected trees for a later final harvest. Plantation forestry is therefore not simply about maximising growth; it is about deciding where that growth should go. Understanding these trade-offs between biomass production, tree size, rotation length and product value remains an important part of research Biomass Production, (website).
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Ferraz Filho, A. C., Mola-Yudego, B., González-Olabarria, J. R., & Scolforo, J. R. S. (2018). Thinning regimes and initial spacing for Eucalyptus plantations in Brazil. Anais da Academia Brasileira de Ciências, 90(1), 255–265. https://doi.org/10.1590/0001-3765201720150453 [RG]
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