Impacts of commercial thinning on stand demography, fuel loads, microclimate and fire behaviour in Eucalyptus delegatensis forest in eastern Tasmania

Le résumé fourni par la source

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. Mechanical thinning redistributed 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. Thinning increased daytime temperatures and lowered relative humidity, but effects on microclimate were small compared to the temporal fluctuations. 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.

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