The relationship between fire severity and eucalypt health: implications for forest structure and carbon balance

The capacity of resprouting eucalypts to regenerate foliage determines the extent of fire induced structural change and carbon dynamics within Australian dry sclerophyll forests. Resprouting eucalypts are traditionally considered resilient to severe fire, yet records of post-fire mortality are highly varied, reflecting the limited sample size of previous studies and the complexity of factors that govern stem death. Fire regimes are predicted to become increasingly severe throughout Mediterranean ecosystems under anthropogenic climate change, increasing the risk of carbon loss within forest communities. This study sought to further the ecological understanding of the effects of fire severity on eucalypt mortality and coarse woody debris (CWD) dynamics within dry sclerophyll communities across southeast Australia. The extent of tissue death and the occurrence of resprouting were used to quantify the health response of eucalypts to fire disturbance. CWD was assessed using van Wagner’s line-intercept method. Relevant additional data was obtained from the NSW Bushfire Risk Management Research Hub. Fire severity and fire frequency values were derived from digital fire extent and severity maps based on satellite imagery. As predicted, eucalypt stem mortality was significantly influenced by stem diameter and fire severity, such that rates of stem death were greatest for small stems under extreme severity fire. Furthermore, stem mortality was significantly influenced by bark type, with smooth bark stems generally the most resilient to fire disturbance. CWD biomass was not significantly influenced by fire severity or frequency yet was significantly affected by fire type. CWD was reduced in plots burnt by prescribed fire and heightened in plots burnt by wildfire, relative to long unburnt forest. This suggests that lower intensity prescribed burns consume more CWD than they produce, whilst CWD production exceeds consumption under higher intensity wildfires. This study provides the largest and most reliable field-based estimates of stem death in dry sclerophyll forests to date. Under more severe fire regimes, disproportionate age class and bark type mortality will likely decrease forest diversity and structural complexity. Both CWD production and consumption will likely increased under future fire regimes, leading to a possible reduction in forest carbon if the consumption of dead wood exceeds the production of live biomass. Whilst gradual carbon loss and demographic shifts are 4 expected under more severe fire regimes, complete ecosystem transformation of resprouting eucalypt forests seems unlikely in the near future, given the persistence of the majority of large trees and the rapid development of lignotubers in small stems which often prevents whole tree mortality