Assessing the carbon dioxide balance of a degraded tropical peat swamp forest following multiple fire events of different intensities

Tropical peat swamp forest is a unique ecosystem rich in carbon and water, accumulating a huge amount of carbon as peat. However, the huge carbon pool has been threatened by oxidative peat decomposition and fire loss mainly because of deforestation and drainage. Fire causes acute carbon dioxide (CO2) emissions through the combustion of biomass and peat. Also, fire should change the CO2 balance of postfire ecosystems. Although it is crucial to quantify CO2 balance even after a fire event to understand the total fire impact, information based on field measurement is lacking. Thus, we had measured eddy CO2 flux above a repeatedly burned degraded peat forest for about 13 years since 2004. The site was a stable CO2 source of 147−290 g C m−2 yr−1 for five years after a stand-replacing fire in 2002. Unexpectedly, a moderate-severity fire in 2009 changed the site to a CO2 sink of about −600 g C m−2 yr−1. The drastic change would have been caused by a large decrease in the decomposition of plant debris, which had accumulated since the 2002 fire but was burned by the 2009 fire. In contrast, gross primary production (GPP) remained about the same even though vegetation was damaged, mainly because year-round wet conditions caused by a La Niña event promoted the regrowth of hygrophilous herbaceous plants and were favorable to their GPP. The site also had a low-severity fire and was drained in 2014 but did not return to a CO2 source. However, the net ecosystem CO2 uptake after the 2009 fire was insufficient to recover a large amount of fire CO2 emission. If CO2 emissions from four fires in 1999, 2002, 2009 and 2014 are counted, the site is expected to owe an outstanding CO2 debt of 25 kg C m−2. © 2021