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stage of the vegetation determines the radiative budget, energy balance partitioning, evapotranspiration and carbon dioxide flux. Here, we synthesize energy balance measurements from across the western boreal zone of North America as a function of stand age following fire. The data are from 22 sites in Alaska, Saskatchewan and Manitoba collected between 1998 and 2004 for a 150-year forest chronosequence. The summertime albedo immediately after a fire is about 0.05, increasing to about 0.12 for a period of about 30 years and then averaging about 0.08 for mature coniferous forests. A mature deciduous (aspen) forest has a higher summer albedo of about 0.16. Wintertime albedo decreases from a high of 0.7 for 5- to 30-year-old forests to about 0.2 for mature forests (deciduous and coniferous). Summer net radiation normalized to incoming solar radiation is lower in successional forests than in more mature forests by about 10%, except for the first 1–3 years after fire. This reduction in net radiative forcing is about 12–24 W m−2 as a daily average in summer (July). The summertime daily Bowen ratio exceeds 2 immediately after the fire, decreasing to about 0.5 for 15-year-old forests, with a wide range of 0.3–2 for mature forests depending on the forest type and soil water status. The magnitude of these changes is relatively large and may affect local, regional and perhaps global climates. Although fire has always determined stand renewal in these forests, increased future area burned could further alter the radiation balance and energy partitioning, causing a cooling feedback to counteract possible warming from carbon dioxide released by boreal fires.

Abstract: Fire and harvesting are major forest renewal processes in the Canadian boreal forest. The eddy covariance method was used to compare ecosystem fluxes of carbon dioxide between harvested and burned boreal forest sites in Saskatchewan, Canada. The harvest chronosequence had sites harvested in 2002 (HJP02), 1994 (HJP94) and 1975 (HJP75), whereas the fire chronosequence sites were burned in 1998 (F98), 1989 (F89), 1977 (F77) and 1929 (OJP). All sites were dominated by jack pine prior to the disturbance. During 2004 and 2005, net ecosystem production showed an average carbon gain (gCm−2 year−1) at F89=84, HJP75=80, HJP94=14 and OJP=20. The other sites lost carbon (gCm−2 year−1) at HJP02=−139, F98=−20, and F77=−58. Gross ecosystem production (GEP), ecosystem respiration (R e) and evapotranspiration tended to be greater at the burned sites than the harvested sites. The F89 and F77 sites had the strongest response of GEP to photosynthetically active radiation, and the strongest response of R e to soil temperature at the 2-cm depth. HJP02 had the weakest responses, followed by HJP94. This apparent greater ecosystem activity at the burned sites is likely caused by local differences in soil moisture and nutrients, differences in vegetation development, and differences in the decomposition of coarse woody debris. [Copyright &y& Elsevier]

Abstract: Fire and harvesting are important disturbances in the boreal forest, driving net biome production. Measurements of net ecosystem production (NEP) over mature forest stands have been made from flux towers for about a decade at the Boreal Ecosystem Research and Monitoring Sites (BERMS) in central Saskatchewan, Canada. Over the last few years, the network of towers has been expanded to include stands that were recently disturbed by fire or harvesting. These new towers are part of Fluxnet-Canada, a national network that ties into the international Fluxnet network. Data from 2001 and 2002 show that forests burned in 1998 (F98) or harvested in 1994 (HJP94) were net carbon sources to the atmosphere with annual NEP values in the range of −50 to −130gCm−2 y−1. A site burned in 1989 (F89) was a carbon sink of about 68gCm−2 y−1 in 2002. NEP at a mature jack pine site (SOJP) ranged from 41 to −23gCm−2 y−1 in this period, whereas, an old black spruce site (SOBS) ranged from 68 to 21gCm−2 y−1. A mature aspen site (SOA) had the greatest NEP with 361gCm−2 y−1 in 2001 and 139gCm−2 y−1 in 2002, the contrast between years attributed to differences in spring temperature and leaf emergence, as well as a long-term drought at the site. Evapotranspiration (ET) followed similar patterns although the F89 site had the highest ET. The deciduous tree components of SOA and F89 are likely responsible for lower summer Bowen ratios with greater energy going into ET, compared to coniferous sites. These data indicate that there is still a broad gap in our knowledge of carbon fluxes to forest stands between about 10 and 50 years of age. This is especially true in understanding the dynamics of the growth of successional vegetation along with heterotrophic respiration, highlighted by predictions of more boreal fire disturbance in the future. [Copyright &y& Elsevier]