The impact of harvesting and fire on energy and carbon fluxes in the boreal forest

Disturbances by fire and harvesting are thought to control the carbon balance of the Canadian boreal forest. However, there are few direct measurements of carbon fluxes following disturbances to provide data needed to refine mathematical models. The eddy covariance technique was used with paired towers to measure fluxes simultaneously at disturbed and undisturbed sites over periods of about one week during the growing season. The disturbances were: a one-year-old burned jackpine stand at the International Crown Fire Modelling Experiment site that experienced an intense crown fire near Fort Providence, Northwest Territories; a one-year-old clearcut aspen area at the EMEND project near Peace River, Alberta; and a ten-year-old burned, mixed forest near Prince Albert National Park, Saskatchewan. Nearby mature forest stands of the same types were also measured as controls. The harvested site had lower net radiation (Rn), sensible (H) and latent (LE) heat fluxes, and greater ground heat fluxes (G) than the mature forest. Daytime CO₂ fluxes were much reduced, although positive, but night-time CO₂ fluxes were identical to that of the mature aspen forest. It is hypothesised that the aspen roots remained living following harvesting, and dominated soil respiration. The overall effect was that the harvested site was a carbon source of about 2.4 g carbon m^-2 day^-1, while the mature site was a sink of about -4 g carbon m^-2 day^-1. The one-year-old burn had lower Rn, H and LE than the mature jackpine forest, and had a continuous CO₂ efflux of about 0.8 g carbon m^-2 day^-1 compared to the mature forest sink of -0.5 g carbon m^-2 day^-1. The carbon source was likely caused by decomposition of fire-killed vegetation. The 10-year-old burned site had similar H, LE, and G to the mature mixed forest site. Although the half-hour CO₂ fluxes were slightly less, there was no significant difference between the daily integration (-1.3 g carbon m^-2 day^-1 at mature site and -2.9 g carbon m^-2 day^-1 at 10-year-old burn site). It appears that most of the effect occurs within the first ten years following disturbance, but more data are needed on other forest and disturbance types for the first 20 years following the disturbance event.