Mortality and recruitment of fire-tolerant eucalypts as influenced by wildfire severity and recent prescribed fire

11 Pág.Mixed-species eucalypt forests of temperate Australia are assumed tolerant of most fire regimes based on the impressive capacity of the dominant eucalypts to resprout. However, empirical data to test this assumption are rare, limiting capacity to predict forest tolerance to emerging fire regimes including more frequent severe wildfires and extensive use of prescribed fire. We quantified tree mortality and regeneration in mixed-species eucalypt forests five years after an extensive wildfire that burnt under extreme fire weather. To examine combined site-level effects of wildfire and prescribed fire, our study included factorial replications of three wildfire severities, assessed as crown scorch and understorey consumption shortly after the wildfire (Unburnt, Low, High), and two times since last preceding fire (30 years since any fire). Our data indicate that while most trees survived low-severity wildfire through epicormic resprouting, this capacity was tested by high-severity wildfire. Five years after the wildfire, percentage mortalities of eucalypts in all size intervals from 10 to >70 cm diameter were significantly greater at High severity than Unburnt or Low severity sites, and included the near loss of the 10–20 cm cohort (93% mortality). Prolific seedling regeneration at High severity sites, and unreliable basal resprouting, indicated the importance of seedling recruitment to the resilience of these fire-tolerant forests. Recent prescribed fire had no clear effect on forest resistance (as tree survival) to wildfire, but decreased site-level resilience (as recruitment) by increasing mortalities of small stems. Our study indicates that high-severity wildfire has the potential to cause transitions to more open, simplified stand structures through increased tree mortality, including disproportionate losses in some size cohorts. Dependence on seedling recruitment could increase vulnerabilities to subsequent fires and future climates, potentially requiring direct management interventions to bolster forest resilience.This work was supported by the Australian Government’s Biodiversity Fund (grant number LSP-943972-876), and by the Victorian Department of Environment, Land, Water and Planning through the Integrated Forest Ecosystem Research program. We thank Richard Loyn (formally of ARI) for leading the Biodiversity Fund application, and members of the project’s Steering committee for their guidance (Jaymie Norris, Gordon Friend, Steve Leonard, Tim O’Brien, and Peter Wilcock). We also thank the following individuals for their diligent work in the field: David Bryant, Benjamin Castro, Garry Cheers, Phoebe Macak, Jessica Millett-Riley, Julio César Nájera-Umaña, Brendan Nugent, Steve Sinclair, Geoff Suter, Arn Tolsma, Liz Wemyss. Adrian Kitchingman and Matt White conducted desktop site selection, and David Duncan and Annette Muir provided advice on the study design.N