Thinning makes forests drier and more bushfire prone.

Commercial tree thinning in native forests is the practice of trying to improve the diameter and growth rates of selected trees by removing other trees. The aim of thinning is to improve the production of sawlogs from retained trees.

Thinning occurs naturally when young trees regrowth after bushfires or clearfell logging. Forests will naturally thinning out with only the strongest trees making it to maturity. Artificially cutting out trees is basically tree farming that speeds up the process. Small diameters trees removed are sold as woodchips to make the process economically viable.

Logging industry groups claim that biomass management through commercial tree thinning will reduce fuel loads and increase water yields from catchments.1 

However the science says thinning makes forests drier and more bushfire prone.

A 2006 review of commercial thinning conducted by University of Melbourne’s School of Forestry on behalf of VicForests demonstrates thinning in Victoria’s wet forests elevates bushfire risks.2

Thinnings operations will alter the climate on the forest floor with more open canopy, associated high wind speed and more direct sunlight giving rise higher temperatures, lower humidity and lower moisture content of the fuel itself.
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While it is expected that total fuel loads will be increased by thinning and fuels will be drier it is also expected that there will be less elevated fine fuels, which are critical to flame height development and forward rate of spread. However, it could be expected overall there should be an increased fire risk.3

The option of burning off the slash after thinning is not available as fire can easily destroy retained trees.  This is a planning constraint already acknowledged by forestry agencies in Tasmania:

One of the major planning constraints associated with thinning is the higher level of fuel present after the operation. It is not considered feasible in Tasmania to carry out fuel reduction burns in thinned coupes because of the high fuel loads and the sensitivity of the retained trees to fire. The location of thinned coupes amongst logging conventionally logged coupes is problematic, as it is not recommended that any regeneration burns take place within two kilometres of areas with high levels of flash fuel within two years of harvest (LaSala 2001).4

Logging industry groups often refer to thinning forests in the Wungong catchment Western Australian to increase water yields. However the preferred method to thinning in this case is undertaken with herbicides, with the dead trees left at the forest to decay.5  These forest ecosystems are also very different to those in Victoria.

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