Evidence for state shift and generation of fire feedback loops in mesic forest driven by extreme fire severity and high fire frequency

Abstract The extent of severe fires is projected to increase with climate change. Furthermore, changes to the fire regime, including the frequency, severity or seasonality of fire, can reduce resilience and promote persistent changes in ecosystem state. Wet sclerophyll forests are found in potentially dynamic mosaics of rainforest and dry sclerophyll forests and contain species from both these contrasting community types. As such, they create an opportunity to study alternative state theory in which states are mediated by fire regimes. To assess the resilience of wet sclerophyll forests to extreme fire events we specifically asked; do mortality rates and recruitment after fire differ between sclerophyllous and non-sclerophyllous components of wet sclerophyll forests, how do these impacts differ along gradients of fire severity and frequency, and is there evidence of positive fire feedback loops, and if so what levels of fire severity and frequency thresholds influence state shifts towards dry sclerophyll forest? We surveyed all canopy (upper and mid canopy) and grass species, to represent three key plant groups; Eucalyptus trees, non-sclerophyllous trees and grasses. We found strong evidence that fire frequency and severity determined the initial trajectory of wet sclerophyll forest recovery. Key findings showed that extreme fire severity can have significant impacts on non-sclerophyllous tree mortality, with an average of 72% of trees killed, much greater than in Eucalyptus species (mean mortality = 9%). However, our findings also highlighted the importance of analysing past fire regime variables, with sites experiencing 4–5 fires in 60 years also experiencing mortality rates of above 75% for non-sclerophyllous trees. Our results support the conclusion that a long multi-decadal fire-free interval is essential for these recovering wet sclerophyll forests, both to rebuild the resilience of their non-sclerophyllous biota and to reduce the risk of recurrent high severity fires in future.