A review of the impacts of sustainable harvesting, non-harvest management and wildfire on net carbon emissions from Australian native forests

Abstract

ABSTRACT A review of the science on net carbon dioxide emissions from continued harvesting or cessation of harvesting in Australian native forests was undertaken. Policy implications are discussed. Accurately estimating the long-term carbon (C) balance for forest harvesting regimes requires the use of a full life-cycle analysis (LCA) framework that includes changes in forest C stocks; the transport and processing of wood products; changes in C stocks in wood products in use and in landfill; and emissions avoided by using wood residues to replace fossil energy or by substituting wood products for more emissions-intensive construction materials. Assessing net changes in native forest C stocks requires consideration of patterns of C accumulation in biomass as forests age; forest C carrying capacity; C dynamics in soils; and the effects of wildfire on C stocks and dynamics. One Australian study has adopted a complete and accurate LCA approach. That study concluded that the harvesting of sustainably managed native forests and the subsequent use of forest biomass to produce harvested wood products or energy can make a positive contribution to mitigating national net C emissions. Other studies have overestimated the possible benefits of ceasing harvesting because they have either been incomplete, used inappropriate parameters to estimate components of the total C balance, or overestimated the rate of C gain in older forests and the ability of unharvested forests to store C for the long term. This has led to the incorrect conclusion that cessation of harvesting would provide lower long-term C emissions than sustainable management for wood production. A case study using C balance of Victorian 1939 regrowth mountain ash forest managed for sawlog and pulpwood production on a rotation of 75 years showed that Victorian Government statements that harvesting results in significantly increased C emissions are incorrect. Therefore, closing Victoria’s native forest timber industry will have negative outcomes in terms of C emissions and climate. The management of C in native forests needs to be integrated at the landscape scale with management for other forest values and attributes. Changes to C stocks in Australian native forests are driven much more by extensive wildfire than by harvesting. Harvesting affects only a small proportion of the forested landscape, and the C in annual log harvests equates to only about 0.6% of Australia’s total net anthropogenic greenhouse-gas (GHG) emissions. Adding emissions of C from the decomposition or combustion of slash produced during harvest increases this figure to 0.8%. These C removals from the forest are offset by sequestration of C in new regrowth and are supplemented by benefits derived from the use of harvested wood. In contrast, in very bad fire seasons such as the ‘Black Summer’ of 2019/20, C emissions were about twice Australia’s total annual anthropogenic GHG emissions and about 200 times greater than C removals in wood plus emissions from logging slash. When examined at the landscape scale, there is no evidence that harvesting leads to increased area burnt, fire severity or C emissions caused by wildfires. However, future wildfires in the large and contiguous areas of thick regrowth created after Black Summer poses a major threat to C stocks in all forests in the coming decades. Timber harvesting, providing it is well conducted in carefully selected parts of the landscape, can provide sustainable ongoing C benefits. A similar conclusion has been reached in numerous international studies.