Micrometeorological changes to the boreal forest following wildfires: Airborne measurements during BOREAS

Fire is the dominant stand-replacing agent in the Canadian boreal forest, but few quantitative measurements are available on the micrometeorological effects of fire. Airborne measurements of surface temperature, albedo, energy exchange, and carbon dioxide fluxes were made during the BOREAS experiment along a 500-km transect through Saskatchewan and Manitoba, Canada. The flights included extensive burn areas, which were compared to areas that had not been burned recently. These data for 1-, 5-, and 7-year-old burns were supplemented with 15- and 30-year-old-burn data from the BOREAS northern study site near Thompson, Manitoba. Data were available for daytime only and included the June, July and September campaigns during 1994, and July of 1996. Surface radiometric temperature increased by up to 6°C and remained elevated even 15 years after fire. Net radiation was largely unaffected whereas albedo decreased in the first year post-fire but recovered by the fifth year. Sensible heat flux increased by 10 to 20% for the first few years after the fire and then decreased. Latent heat flux was largely unaffected or slightly decreased after the fire, causing the Bowen ratio to increase by about 50% for 7 years post-fire. Carbon dioxide flux was reduced for the 15-year period after fire with the greatest reduction to about 25% of control areas during the year following fire. Shorter-term carbon flux measurements from a tower on a one-year-old burn show the drastic reduction in both photosynthesis and respiration in the altered stand. Although the reduced carbon dioxide flux supports many model predictions of long-term reduction in carbon sequestration following fire, considerably more data are needed to determine the total impact of fire on the boreal-forest carbon balance.