Estimating immediate post-fire carbon fluxes using the eddy-covariance technique

Wildfires typically affect multiple forest ecosystem services, with carbon sequestration being affected both directly, through the combustion of vegetation, litter and soil organic matter, and indirectly, through perturbation of the energy and matter balances. Post-fire carbon fluxes continue to be poorly studied at the ecosystem scale, especially during the initial window of disturbance when changes in environmental conditions can be very pronounced due to the deposition and subsequent mobilization of a wildfire ash layer and the recovery of the vegetation. Therefore, an eddy-covariance system was installed in a burnt area as soon as possible after a wildfire that had occurred on 13 August 2017 and has been operating from the 43rd post-fire day onwards. The study site was specifically selected in a Mediterranean woodland area dominated by maritime pine stands with a low stature that had burned at high severity. The carbon fluxes recorded during the first post-fire hydrological year tended to be very low so that a specific procedure for the analysis and, in particular, gap filling of the eddy-covariance data had to be developed. Still, the carbon fluxes varied noticeably during the first post-fire year, broadly revealing five consecutive periods. During the rainless period after the wildfire, fluxes were reduced but, somewhat surprisingly, indicated a net assimilation. With the onset of the autumn rainfall, fluxes increased and corresponded to a net emission, while they became insignificant with the start of the winter. From the midwinter onwards, net fluxes became negative, indicating a weak carbon update during spring followed by a strong uptake during summer. Over the first post-fire year as a whole, the cumulative net ecosystem exchange was -347 g C m(-2), revealing a relatively fast recovery of the carbon sink function of the ecosystem. This recovery was mainly due to understory species, both resprouter and seeder species, since pine recruitment was reduced. Specific periods during the first post-fire year were analyzed in detail to improve process understanding. Perhaps most surprisingly, dew formation and, more specifically, its subsequent evaporation were found to play a role in carbon emissions during the rainless period immediately after fire, involving a mechanism distinct from degassing the ash-soil pores by infiltrating water. The use of a special wavelet technique was fundamental for this inference.