SOIL-ATMOSPHERE FLUXES OF CARBON-MONOXIDE DURING EARLY STAGES OF POSTFIRE SUCCESSION IN UPLAND CANADIAN BOREAL FORESTS

Soil-atmosphere fluxes of carbon monoxide (GO) were investigated durin g BOREAS 1994 (June to September 1994) in forest sites near the northe rn study area (NSA) of the Boreal Ecosystem-Atmosphere Study (BOREAS). Fluxes and related ancillary data were measured for both upland black spruce (located on poorly drained clay-textured soils) and jack pine sites (well-drained sandy soils) that were in early stages of successi on following stand replacement fires that occurred within 7 years of B OREAS 1993. Nearby control stands that had not burned in the past 80 y ears were studied for comparison. Net fluxes measured by using transpa rent closed chambers were generally positive at the warmer, sunlit bur n sites but negative (sink activity) in the shaded, cooler control sit es. Carbon monoxide uptake in controls, which was first order with res pect to CO concentration, was little affected by covering the sampling chambers to exclude light. Median deposition velocities calculated fr om the uptake fluxes were 0.015 cm s(-1) at the black spruce control s ite and 0.0055 cm s(-1) at the jack pine control site, at the lower en d of the range of values observed by others in tropical and temperate ecosystems. Daytime CO fluxes at the burn sites were generally positiv e (10(11)-10(12) molecules cm(-2) s(-1)) and were lowered when solar i rradiance was excluded from the chambers by covering or when cloudines s or smoke reduced the light intensity. Net fluxes at the burn sites w ere controlled by competition between abiotic production, mainly at th e surface, and by oxidation deeper in the soil. Abiotic production, wh ich was attributable to photoproduction and thermal decomposition of t he surface organic layer and charcoal, strongly correlated with incide nt solar irradiance, and thus the greatest fluxes were observed during midday. Results of these studies indicate that the locally dependent changes in boreal fire return intervals that are linked to global clim ate change represent an important biospheric/physical feedback that is likely to alter the biosphere-atmosphere exchange of CO.