Estimating of Immediate Post-Fire Carbon Fluxes Using the Eddy-Covariance Technique

The increased frequency and intensity of extreme climate events appears to be leading to an intensification of wildfire regimes across the world. Wildfires typically affect multiple forest ecosystem services, with carbon sequestration being affected both directly, through the combustion of vegetation and soil organic matter, and indirectly, through perturbation of the energy balance, and photosynthetic and microbial activity.

The few existing eddy-covariance studies in burnt areas have suggested that forests can become carbon sources for months to decades after wildfire. However, these past studies have not looked into post-fire carbon fluxes over the initial phases of the fire-induced window-of-disturbance, except for a few days in one study. This knowledge gap is being addressed by the FIRE-C-BUDs project, which installed an eddy-covariance system in a young Maritime Pine stand just 40 days after a wildfire that occurred on 13 August 2017. The system has now been operating for almost two years.

The carbon fluxes recorded during the first post-fire year tended to be very low, but varied markedly with time-since-fire, as consequence of wildfire ash mobilization and vegetation/soil recovery, combined with seasonal weather conditions. Gap filling and footprint analysis therefore represented major challenges. These were addressed by categorizing the flux data based on soil moisture/temperature conditions.

No significant carbon emission was observed during the rainless period immediately after fire, but, surprisingly enough, minor uptakes did occur, apparently associated with early-morning dewfall. The first post-fire rain event did produce significant emissions but rain events during winter did not. The highest emissions during the first post-fire occurred in spring, while uptake prevailed during summer. The latter could be due to the summer of 2018 being relatively wet and fresh but does suggest that the study area was not a carbon source for a long period.