Catastrophic wildfires are projected increase in frequency and severity over the coming century, particularly in areas with Mediterranean climates. Many ecosystems in these regions are considered fire-tolerant, including the extensive eucalypt forests, woodlands and heathlands of Australia. However, wildfires of unprecedented severity have recently occurred across southern Australia, providing unique opportunities to determine fire severity thresholds of historically fire tolerant ecosystems. This study investigated the post-fire regeneration of four vegetation types endemic to the southwest Australian Biodiversity Hotspot, following one of the most severe and extensive wildfires in national history. Focusing on four dominant vegetation types in the 98,000 ha fire scar (tall open forest, open forest/woodland, heathland and riparian), 38 old growth, previously long-unburnt sites across a gradient of fire severity were surveyed. At each site, plant species diversity and fungal fruiting body diversity was recorded, as well as tree mortality, canopy cover, overstorey seedling recruitment and epicormic resprouting.
Results/Conclusions
While almost no mature tree mortality occurred at low-moderate severity sites (<5%), high severity sites had over 50% tree mortality. As catastrophic wildfires become more frequent, this could lead to more even-aged stands of forest, changing future community dynamics and impacting fuel loads. Understorey plant community composition was significantly altered by fire, with these effects being most pronounced in high severity sites. Differences were largely driven by lack of recruitment of certain dominant understorey species and elevated abundance of fast-growing disturbance specialists. Fungal community composition was also significantly changed by fire severity, with no species in common between the low-moderate severity sites and the highest severity sites. Such changes in plant and fungal species dominance are likely linked, and could lead to long-term effects on ecosystem function. These findings indicate for the first time that southwest Australian vegetation types may have a fire tolerance threshold, correlating with similar studies following catastrophic wildfires in eastern Australia and western United States.