Exposure to canopy fire reduces the biomass and stability of carbon stored in fire tolerant eucalypt forests

Abstract

Short-interval high severity wildfires threaten the stability of carbon stocks across forest communities, particularly those dominated by ‘fire-sensitive’ trees. However, there is a dearth of research on the effect of these extreme fires on forests dominated by ‘fire-tolerant’ tree species that can resprout from their trunk and branches i.e. epicormic resprouters. We assessed the effect of the severity of two wildfires occurring in short succession (6-year interval) on live and dead above ground carbon stocks (≥2.5 cm diameter) and carbon stability (i.e. the proportion of total carbon stored in live trees) in a temperate eucalypt forest dominated by epicormic resprouters. Four factorial combinations of low severity understorey fire and high severity canopy fire were examined across two wildfires. Carbon stocks were estimated using two approaches, a standard approach that assigns trees to the live and dead pools based on the presence of live foliage, and a novel approach that accounts for partial mortality of branches and stems. When accounting for partial tree mortality, total carbon stocks were greatest following repeated understorey fire and lowest in areas that had recently experienced canopy fire irrespective of canopy fire frequency. Trends in total carbon were driven by substantial reductions in live carbon following canopy fire, which were partially offset by gains in the standing dead carbon pool. Exposure to canopy fire reduced the proportion of carbon stored in live trees, increasing the susceptibility of carbon stocks to future losses through decomposition and consumption by fire. Relative to sites recently exposed to a single canopy fire, exposure to repeated canopy fires did not result in further reductions to carbon stocks or their stability. The standard carbon estimates substantially overestimated live carbon (+59 %) and underestimated dead carbon (-52 %) in areas recently affected by canopy fire, thus underestimating the impact of high severity fires on carbon stability. Our results suggest that carbon stocks in forests dominated by epicormic resprouters are sensitive to losses following canopy fire but are resistant to compounding effects of short-interval high severity fires, owing to the survival of large live stems. Our results highlight the need to account for partial tree mortality and feedbacks between short-interval wildfires when estimating transitions between carbon pools in fire tolerant forests.