The Effect of Antecedent Fire Severity on Reburn Severity and Fuel Structure in a Resprouting Eucalypt Forest in Victoria, Australia

Abstract

Research highlights—Feedbacks between fire severity, vegetation structure and ecosystem flammability are understudied in highly fire-tolerant forests that are dominated by epicormic resprouters. We examined the relationships between the severity of two overlapping fires in a resprouting eucalypt forest and the subsequent effect of fire severity on fuel structure. We found that the likelihood of a canopy fire was the highest in areas that had previously been exposed to a high level of canopy scorch or consumption. Fuel structure was sensitive to the time since the previous canopy fire, but not the number of canopy fires. Background and Objectives—Feedbacks between fire and vegetation may constrain or amplify the effect of climate change on future wildfire behaviour. Such feedbacks have been poorly studied in forests dominated by highly fire-tolerant epicormic resprouters. Here, we conducted a case study based on two overlapping fires within a eucalypt forest that was dominated by epicormic resprouters to examine (1) whether past wildfire severity affects future wildfire severity, and (2) how combinations of understorey fire and canopy fire within reburnt areas affect fuel properties. Materials and Methods—The study focused on ≈77,000 ha of forest in south-eastern Australia that was burnt by a wildfire in 2007 and reburnt in 2013. The study system was dominated by eucalyptus trees that can resprout epicormically following fires that substantially scorch or consume foliage in the canopy layer. We used satellite-derived mapping to assess whether the severity of the 2013 fire was affected by the severity of the 2007 fire. Five levels of fire severity were considered (lowest to highest): unburnt, low canopy scorch, moderate canopy scorch, high canopy scorch and canopy consumption. Field surveys were then used to assess whether combinations of understorey fire (<80% canopy scorch) and canopy fire (>90% canopy consumption) recorded over the 2007 and 2013 fires caused differences in fuel structure. Results—Reburn severity was influenced by antecedent fire severity under severe fire weather, with the likelihood of canopy-consuming fire increasing with increasing antecedent fire severity up to those classes causing a high degree of canopy disturbance (i.e., high canopy scorch or canopy consumption). The increased occurrence of canopy-consuming fire largely came at the expense of the moderate and high canopy scorch classes, suggesting that there was a shift from crown scorch to crown consumption. Antecedent fire severity had little effect on the severity patterns of the 2013 fire under nonsevere fire weather. Areas affected by canopy fire in 2007 and/or 2013 had greater vertical connectivity of fuels than sites that were reburnt by understorey fires, though we found no evidence that repeated canopy fires were having compounding effects on fuel structure. Conclusions—Our case study suggests that exposure to canopy-defoliating fires has the potential to increase the severity of subsequent fires in resprouting eucalypt forests in the short term. We propose that the increased vertical connectivity of fuels caused by resprouting and seedling recruitment were responsible for the elevated fire severity. The effect of antecedent fire severity on reburn severity will likely be constrained by a range of factors, such as fire weather.

Self-thinning forest understoreys reduce wildfire risk, even in a warming climate

“…High-severity fire has been found in some cases to promote denser regrowth in understoreys and consequent positive feedbacks to flammability (e.g. (Collins et al 2021b)). Severity data were unavailable for much of the period of our study, but the increased flammability that we measured may be broadly characterised as a response to prescribed burning, as that accounts for more than 80% of fire in the majority of our forested study area (Boer et al 2009).…”

The Carbon Cycle of Southeast Australia During 2019–2020: Drought, Fires, and Subsequent Recovery

“…Trends in climate variability will likely have a number of impacts on the carbon cycle of the region. A major risk for forest ecosystems is recurrent fires during recovery from the previous fires, with studies finding significant negative impacts on ecosystem function for both obligate seeder (D. M. J. S. Bowman et al., 2014) and resprouter‐dominated communities (Collins, Hunter, et al., 2021; Fairman et al., 2017). This risk may be compounded by longer recovery periods after fire due to frequent extreme heat and drought events.…”

High severity fire promotes a more flammable eucalypt forest structure

“…Both the increased understory cover and the reduced vertical separation in forests after high severity fire provide a mechanism to explain the results of Barker and Price (2018), who found that high severity fire in dry sclerophyll forests of eastern Australia is more likely after previous high severity fire over a range of times since fire from five to thirty years. This is supported by Collins et al (2021), who found that the difference in basal and epicormic resprouting patterns after different fire severities resulted in variations in canopy structure and vertical connectivity in Victorian Eucalypt forests 6-12 years post-fire. There was greater fuel connectivity after high severity fire, which was then associated with further high severity in a subsequent fire.…”

“…This is supported by Collins et al . (2021), who found that the difference in basal and epicormic resprouting patterns after different fire severities resulted in variations in canopy structure and vertical connectivity in Victorian Eucalypt forests 6–12 years post‐fire. There was greater fuel connectivity after high severity fire, which was then associated with further high severity in a subsequent fire.…”