Indirect effects of climate change on forest structure alters fuel availability in wet Eucalypt forests

Abstract

The direct effects of climate change are increasing the frequency of high-intensity fire events in many ecosystems across the globe, including wet Eucalypt forests of south-eastern (SE) Australia. Recurrent high-intensity fire can alter vegetation structure and composition, and the resultant alternative vegetation states may be more likely (positive feedback) or less likely (negative feedback) to burn again than the vegetation community replaced. These indirect effects of climate change have been reported for a range of different ecosystems across the globe. However, a common limitation to many empirical studies is the narrow temporal range of observations, often limited to a single fire season. In turn, this limits our understanding of the potential for vegetation-mediated indirect effects of climate change to generate positive or negative fire feedbacks across the range of climate conditions common to the region. In wet Eucalypt forests of SE Australia, dead fuel moisture content (FMC) is a key determinant of fire activity, and therefore a useful metric on which to quantify the potential for feedbacks across alternative forest states. To quantify potential for indirect effects of climate change to alter future fire activity, FMC was modelled in the open and at seven alternative forest states to wet Eucalypt forest using a process-based FMC model. The model was run using a long-term climate dataset spanning 1973 – 2020, which were transferred from macro- to microclimate values using forest structural properties derived from lidar. Hourly FMC outputs were summarised to fuel availability (FMC < 16% for one hour each day) to understand the potential for positive, negative of no feedbacks on potential fire activity. Mean annual FMC was significantly different between each alternative forest state across each of the 48 years of climate data – which act as a replicate for different climate conditions in our experimental design. By quantifying these differences using the metric of fuel availability, we have demonstrated that statistically significant differences in FMC translate into meaningful differences in the context of potential fire activity. Overall, the results show strong positive and negative feedbacks across the alternative forest states compared to the mature wet Eucalypt forest that they replaced, which were greater than age-related differences within the wet Eucalypt forest sites. Overall, our results support the hypothesis that indirect effects of climate change, acting through vegetation conversion to alternative forest states, have a substantial impact on the potential for future fire activity, with important implications for land and fire managers in this region.