A project undertaken at Western Sydney University, and supervised by Rachael Nolan
Over 7 million hectares were burnt by bushfires across eastern and south-eastern Australia during the unprecedented 2019-20 Black Summer fire season. The fires affected large swathes of eucalypt forests and woodlands, which are typically highly resilient to fire. The majority of eucalypt species can resprout new leaves, even after high intensity fire, facilitating rapid post-fire recovery. However, many of these forests were subject to the lowest rainfall and highest temperatures on record preceding the fires. Widespread reports of canopy death due to drought and heat-waves emerged in 2019-20, even in forests that remained unburnt (Figure 1). This raised important questions regarding the capacity of these usually resilient forests to recover from the combined impacts of fire, drought and heatwaves.
In this project, we surveyed 154 plots across a range of forest types, fire severities and soil types (Fig. 2). These plots spanned from northern NSW to East Gippsland in Victoria. We measured carbon stocks and tree mortality. This large and unique dataset that allows us to answer many questions about how our forests recover from fire.
In this study, our key findings are:
(1) Canopy fire was associated with greater declines in forest carbon stocks than understorey fire. The biggest effect of the fires on carbon stocks came from fire triggering topkill (i.e. death of above-ground stems). The conversion of live to dead biomass far exceeded the amount of carbon emitted during the fires. As these dead tree stems slowly decompose over time, they can emit large amounts of carbon. These trees are also much more likely to be consumed during subsequent fires, which may mean future fires produce more smoke and carbon emissions if fire recurs more frequently than historical norms.
(2) While many forests we surveyed were recovering well, with rates of tree mortality and topkill within historical ranges, one forest type we studied had much higher rates of tree mortality than expected. This was the driest forest type studied (Fig. 3).
(3) For some of the forests surveyed, we observed an atypical trend of increased mortality and topkill in the largest trees, particularly when they had structural damage from past fires. Trees in wet forests on more fertile soils had the highest levels of mortality.
Combined, our results highlight the importance of spatial variability in forest recovery following the Black Summer fires. These results have important implications for our understanding of how well our forests can store carbon, and how our forests may respond to shifting fire regimes under climate change.