The influence of different restoration thinning treatments on tree growth in a depleted forest system

Land managers increasingly are interested in the means by which degraded or alienated forest ecosystems can be restored, typically for biodiversity gains. Indeed, there is some urgency to restore the integrity of dwindling intact forests across the globe, in part to halt an accelerating biodiversity crisis and also achieve sustainability goals. Restoration thinning can be an effective management tool where there is excessive stem density by utilising the selective removal of trees to return an ecologically more-desirable ecosystem structure and processes. A key objective of such thinning has typically been the hastened growth of retained stems and associated environmental attributes (e.g. tree hollows, coarse woody debris, understorey structure and other habitat) that provide benefits for forest flora and fauna. The Box-Ironbark forests of south-eastern Australia have been extensively exploited since European settlement and lost many of the ecological attributes considered beneficial for effective function and biodiversity conservation, especially large trees. Three thinning treatments, differing in their density and pattern of retained trees, were compared with controls to determine the most effective approach for restoring these systems and increasing the rate of recovery for biodiversity benefits. The thinning treatments applied different retention levels of stems, one which reflected conventional silvicultural practice and two designed to reflect a patchy forest structure. The response of tree diameter growth to thinning treatments, for multiple tree species, was examined approximately a decade after thinning. All three thinning treatments increased tree growth-rates similarly (0.32-0.57 cm/yr), compared with the controls (0.2-0.27 cm/yr). These data suggest that the choice of thinning treatment may not be critical for accelerating tree growth, and land managers can focus more strongly on the treatment that provides the best overall outcomes for biodiversity.