Organic soil combustion in cypress swamps: Moisture effects and landscape implications for carbon release

Swamps, peatlands, and other wetland ecosystems can store vast amounts of carbon in organically-derived peat soils. Wildfires during severe droughts can produce smoldering combustion in these soils, releasing large quantities of carbon to the atmosphere and causing dramatic changes at the local scale due to plant mortality and hydrologic effects. I studied variation in moisture content and carbon loss from smoldering combustion in soils from pondcypress (Taxodium distichum var. imbricarium) swamps in Florida USA. In a lab study, soil moisture content near the surface (upper 10 cm) did not predict vertical depth of soil combustion. Mass loss of organic carbon from soil profiles was, however, negatively related (P < 0.01). I also studied spatial variation in soil moisture, as a predictor of potential soil combustion, at a range of distances from edges of cypress-swamp patches. A weak, but significant (P < 0.01), positive relationship exists between distance from edges and upper-layer soil moisture, indicating that some inhibitive effect on smoldering may be present proceeding toward the centers of larger swamp patches. Conservative estimates of SOC content in cypress peats (approximately 41% by mass, compared to a figure of 50% sometimes used in such studies) indicate substantial potential for soil carbon loss (over 4 kg m2) from wildfires in cypress swamps. This initial study on smoldering of cypress peats also makes recommendations for future efforts to study ground fires in these regionally important ecosystems.

► Smoldering combustion was investigated in organic soils from cypress swamps.
► Moisture predicted combustion loss of soil organic carbon, but not depth of burning.
► Sustained smoldering occurred in cypress peat soils at moisture levels above 200%.
► Ground fires in cypress swamps may release substantial carbon to the atmosphere.