Silvicultural treatment effects on commercial timber volume and functional composition of a selectively logged Australian tropical forest over 48 years

Abstract

Post-logging silvicultural treatments involving thinning are commonly recommended to increase the commercial timber production under polycyclic harvesting regimes in tropical forests. However, thinning practices intentionally designed to improve commercial timber production may alter species composition and functional composition. Low, medium, and high intensity thinning following logging plus a logged only control was applied in an Australian tropical forest in 1969 to assess the long-term liberation thinning impacts on the commercial timber volume and the functional composition of the regenerating forest. Over 46 years following thinning, only medium- and high-intensity thinning promoted recruitment of commercial trees into the >10 cm diameter class. All three thinning treatments enhanced the standing volume of most desirable commercial species, mainly driven by the growth of remaining non-harvested trees in the 10–20 cm diameter class. The merchantable timber volume of other less desirable species was also promoted in the four treatments. Over the first 28 years after treatment (1969–1997), the average annual timber volume accumulation rate was highest in the medium-intensity thinning (1.97 m3 ha−1 year−1). The positive impacts of medium- and high-intensity thinning on commercial timber volume growth lasted longer than the low-intensity thinning. From 1997 to 2015, average annual volume increments increased in the medium- and high-intensity thinning, changed little in the low-intensity thinning, and decreased in the untreated control. Moreover, the medium- and high-intensity thinning led the community functional composition to shift towards early-successional species, which reduced community-weighted mean wood densities. The changes induced by intensive thinning treatments may affect the recovery of forest biomass and biodiversity. Hence, the trade-off between enhancing timber stocks and maintaining functional composition should be considered during planning and implementation of tropical forest management.