Thinning Eucalyptus delegatensis forest in Tasmania

Fuel reduction is essential to mitigate the increasing risk of wildfire resulting from changes in land management and climate change. In Australia, planned burning is the predominant method employed to reduce surface fuels. Mechanical thinning offers an additional approach to reducing elevated fuels, albeit there has been little research into its efficacy in eucalypt forests. Here, we report the effects of commercial thinning on tree demography, aboveground fuel loads, microclimate, and modelled fire behaviour at three sites in Tasmanian tall eucalypt forest. Compared to unthinned forest, thinned coupes contained on average 75% fewer live trees, 55% less live stand basal area, 60% less standing biomass and 14% less above-ground biomass. The biggest impact mechanical thinning had on fuel loads was the redistribution of biomass from standing biomass (average 206 Mg ha-1 more in unthinned) to surface fuels (152 Mg ha-1 more in thinned), with only 54 Mg ha-1 removed as harvested timber. Reduced canopy cover in thinned forest was associated with slightly elevated daytime temperatures and lower relative humidity compared to unthinned forest, indicating a possible small increase in fire weather conditions following thinning. Modelling under moderate fire weather suggests different fire behaviour between treatments, with thinned forests having more intense fires (fireline intensity 3.1 MW m-1 during moderate fire weather) and higher flames (4.3 m vs 3.4 m). In addition, large amounts of energy are embodied in localised debris piles with very high fuel loads. Application of the commercial thinning reported here does not reduce fire hazard. To do so demands drastically reducing debris through increased utilization of coarse woody debris and burning of fine residual fuels.