Carbon stocks and stability are diminished by short-interval wildfires in fire-tolerant eucalypt forests

Abstract

Forests play a key role in mitigating global climate change through carbon storage and sequestration. Wildfire affects forest carbon through combustion and by influencing forest mortality and regrowth, which are also influenced by a forest’s growth environment. Wildfires are becoming more severe and frequent in many temperate regions but the impacts of such changes–in association with projected warmer, drier climates–on forest carbon stocks, remains under-quantified in many temperate biomes. We compare the impacts of a single high-severity wildfire and two high-severity wildfires within a short interval (six years) on carbon stocks 2–3 years after the last wildfire in multiple carbon pools of fire-tolerant eucalypt forests in temperate, south-eastern Australia. These forests are assumed to contain relatively resistant carbon stores because the dominant eucalypts are rarely killed by fire, reliably resprouting after most fires. Mean carbon stocks in the principal aboveground pools were decreased by one, and particularly two, high-severity wildfires after controlling for site aridity, which was negatively related to aboveground carbon stocks. On average, the percentage of aboveground carbon stocks in live biomass decreased from 76% at Unburned sites to 62% at Single-burned and 47% at Double-burned sites, contributing to increases in standing deadwood carbon stocks from 11% at Unburned sites to 42% at Double-burned sites. As such, the proportion of carbon in large, living trees (an indicator of carbon stability and carbon-recovery capacity) was decreased after short-interval wildfires. Soil carbon stocks to 10-cm depth were also significantly reduced at all wildfire sites (likely due to erosion), which could also delay post-fire recovery of productivity. Overall, our findings indicate that both single and short-interval wildfires can significantly reduce the amount of carbon stored in fire-tolerant eucalypt forests, and that the potential for carbon-stock recovery could be compromised by predicted warmer and drier (i.e., more arid) future climates, and by soil feedbacks to productivity. Actively managing carbon stocks in fire-tolerant eucalypt forests could be required in the coming decades to mitigate these risks.