Increased Drought and Fire Intensity Regimes Reduce the Ecological Resilience of Mediterranean Forests in the South-West Australian Floristic Region

Sammanfattning: Future climate projections suggest an increase in average temperature as well as a decrease in average winter rainfall across the south-west Australian floristic region (SWAFR). These adverse future climatic conditions will amplify the intensity and frequency of disturbance events such as drought and fire. Mediterranean forests within the SWAFR are prone to drought and fire disturbance and have acquired resilience through the selection of drought and fire tolerable species. However, shifts in the magnitude of these disturbance events could increase the recovery period required for recruitment, causing a shift in forest structure and decreasing the resilience of these ecosystems to future disturbances. In this study, we investigated above-ground biomass (AGB) accumulation of understorey plants at sites within the Northern Jarrah Forest (NJF) that have experienced different degrees of drought and fire intensity. We found that within a disturbance event, sites experiencing either more severe drought and fire intensities on average accumulated substantially more understorey AGB than sites subjected to both low drought and moderate fire intensities. This suggests that understorey species within the SWAFR gain a competitive advantage in high drought and fire severity conditions and are highly tolerant to drought. However, the increase in understorey AGB accumulation also suggests a shift in overall forest structure to more dense, compact, low-ground small stems, which is known to increase fire probability. An increase in fire probability shortens the time period between fire intervals and can detrimentally affect forest recovery, especially in drought conditions. Therefore, these changes may shift ecosystems within the SWAFR to a state of non-equilibrium and reduce resilience to future disturbance events.