Recovery of south-eastern Australian temperate forest carbon is influenced by post-fire drought as well as fire severity

Abstract

During 2017–2019 south-eastern Australia experienced intense drought that threatened and damaged ecosystems and helped enable the Black Summer wildfires of 2019–2020. These wildfires burnt through about 7 M ha of mainly temperate Eucalyptus forests, releasing the equivalent of Australia's total net annual greenhouse gas emissions. For the state of Victoria, these fires burnt through about 1 M ha of highly productive forests, where 50 % of the area was burnt with high to extreme wildfire severity. This study reports the recovery of Eucalyptus sieberi dominated forests three years after the Black Summer wildfires, and five years after prescribed burn applied in May 2018, during the drought. Wildfire affected sites were burnt with high and low-to-moderate severity, while previously prescribed burnt sites were burnt at low-to-moderate severity and were not reburnt in the 2020 wildfires. For wildfire affected sites, 30 % of small diameter trees (10–20 cm) continued to die three years after fires, irrespective of fire severity. Three years after wildfire, live tree carbon reached 92 % of pre-fire level for low-to-moderate severity sites and 76 % for high severity sites, indicating longer recovery with more severe wildfire. For prescribed burnt sites, tree mortality significantly increased two years after the burn and the number of live trees thereafter remained stable to five years post fire. This trend was reflected in the non-recovery of live tree carbon reaching only 95 % of pre-fire level five years after the burn. Recruitment of new Eucalyptus seedlings was observed in sites burnt in wildfire regardless of fire severity (900 ± 316 trees ha⁻¹), but not in prescribed burnt sites. This was possibly driven by the differences in rainfall after fires, rather than fire type. For other forest carbon pools such as surface litter and CWD, recovery to pre-fire levels was observed within six months after fires. Moreover, for low-to-moderate severity burnt sites litter loads had doubled pre-fire mass three years after fire and continued to increase five years post fire.

Results from this study suggest that both fire severity and post-fire weather conditions play an important role in determining the rate of forest aboveground carbon recovery. Our findings indicate that as fire severity increases, the time required for recovery of live tree carbon to pre-fire level is longer. Even low impact prescribed burns during prolonged drought can lead to ongoing mortality that reduces the rate of carbon recovery. These observations indicate that increases in fire occurrence and severity associated with climate change can reduce the carbon carrying capacity of these highly productive temperate forests. The implication of this work is that the presumed recovery of aboveground carbon in forests following fires may be over-estimated or might not occur at all.