OOS 26-4 - Assessing the impact of current and future fire regimes on old growth forest in south-eastern Australia

Old growth forest is an important ecological component of the fire prone temperate forest biome of south-eastern Australia. These forests provide many habitat features such as large hollow trees, complex vegetation structure and large woody debris, that are necessary for the persistence of some species of vertebrate fauna. Although fire plays a role in the development of some old growth structures (e.g., hollows, stags) in eucalypt forests, high severity wildfire is the primary cause of old growth decline. Climate change is increasing the frequency and severity of wildfires across south-eastern Australia, raising questions about the fate of old growth forest. Here, we (i) examine changes to the extent of old growth forest in south-eastern Australia under recent (2000 – 2020) and future fire regimes, and (ii) identify forest types and landscape locations where old growth forest has high resistance to degradation by wildfire. We focused on forests located in Victoria, Australia, owing to the prevalence of trees with contrasting fire response syndromes. Landsat derived fire severity mapping and fire regime simulation(s) were used to quantify the impact of recent and future fire regimes on old growth forest within fire sensitive (i.e., obligate seeders, basal resprouters) and fire resistant (i.e., epicormic resprouters) forest types. High severity wildfire decreased the extent of old growth forest dominated by fire sensitive tree species by ~40% between 2000 and 2020, with considerably lower rates of loss (~10%) being recorded for old growth dominated by fire tolerant epicormic resprouters. Fire regime simulations suggest that in fire sensitive forests the extent of fire regimes suitable for old growth development and retention will approximately halve under future climate compared to current climate (i.e., 43% vs 80% of the landscape). Conversely, the extent of fire tolerant forest experiencing fire regimes suitable for old growth will decrease slightly ( <5%) under future compared to current climate (i.e., 94% vs 98% of the landscape). Fire regimes suitable for old growth persistence were more likely to occur in sheltered topographic locations (e.g., moist gullies) than exposed environments (e.g., upper slopes). Our findings suggest that old growth eucalypt forests dominated by fire sensitive tree species are likely to contract under future fire regimes, whereas old growth dominated by fire resistant epicormic resprouters will remain somewhat static. Sheltered topographic locations have the potential to provide refugia for fire sensitive old growth under future climates, highlighting the need to protect these areas from timber harvesting.