Search

Abstract: A long-term flux measurement station has been established in a 74-year-old mixedwood forest ecosystem, located approximately 80km west of Timmins in northern Ontario, as part of the Fluxnet-Canada Research Network (FCRN). Measurements of energy, water vapour, and carbon dioxide fluxes have been made continuously since August 2003 using the eddy covariance technique, along with ancillary meteorological variables. The spatial structure of the site was evaluated using a variety of sources and techniques, including remote sensing, showing that this forest is mixed but relatively homogeneous. The canopy top height is remarkably constant at between 30 and 32m. The basal area varies from 18 to 27m2 ha−1, and the aboveground biomass ranges from 82 to 122Mgha−1. In this paper, we summarize the diurnal and seasonal patters of carbon dioxide exchange and water loss from September 1, 2003 to August 31, 2004. Net ecosystem productivity (NEP) is strongly related to temperature. Atmospheric vapour pressure deficit (VPD) in this ecosystem exerted strong biophysical control on the daily gross ecosystem productivity (GEP) and evapotranspiration. Seasonal change in shortwave albedo, as a result of the presence of mixed deciduous and coniferous species, was clearly evident. Albedo changes were comparable to the seasonal pattern of NEP. The dormant season lasts more than 6 months of the year at this station. This forest was a moderate sink of carbon over the measurement period. Annual values of GEP, ecosystem respiration (R), and NEP were 1075, 919, and 156±35gCm−2, respectively. [Copyright &y& Elsevier]

Abstract: Large-scale weather events such as the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and droughts are known to cause substantial interannual variation in the net ecosystem productivity (NEP) of tropical, temperate and boreal forests. Hypotheses for the impacts on NEP of changes in air temperature (T a) and precipitation associated with these events were tested at diurnal, seasonal and annual time scales using the terrestrial ecosystem model ecosys with measurements of CO2 and energy exchange from 1998 to 2006 at eddy covariance (EC) flux towers along a transcontinental transect of forest stands in the Fluxnet-Canada Research Network (FCRN). 1 [1] Now the Canada Carbon Project. These tests were supported at seasonal time scales by remotely-sensed vegetation indices, and at decadal time scales by wood growth increments from tree-ring and inventory studies. Collectively, results from this testing indicate that large-scale weather events during the study period caused spatially coherent changes in NEP, although these changes may vary with climate zone, species and topography. High T a episodes, such as occurred with greater frequency during ENSO/PDO events, adversely affected diurnal CO2 exchange of temperate and boreal conifers, but had little effect on that of a boreal deciduous forest. These contrasting responses of CO2 exchange to T a were attributed in the model to greater xylem resistance to water uptake in coniferous vs. deciduous trees. Sustained warming such as occurred during ENSO/PDO events extended the period of net C uptake and thus raised annual NEP at boreal coniferous and deciduous sites, but did not do so at a temperate coniferous site where annual NEP was reduced. However the rise in NEP of boreal conifers with warming was partially offset by the adverse effects of high T a on diurnal CO2 exchange, so that the rise in NEP with warming remained smaller than that at a boreal deciduous site. A 3-year drought during the study period adversely affected annual NEP of well-drained boreal deciduous forests but did not affect that of poorly-drained boreal conifers. This lack of effect was attributed in the model to low coniferous evapotranspiration rates and to subsurface water recharge. Drought effects on NEP were therefore largely determined by topography. These contrasting responses of different forest stands to warming and drought indicate divergent changes in forest growth with interannual changes in weather. Such divergent changes are consistent with the complex changes in forest NDVI and net C uptake observed over time in several large-scale remote-sensing studies. [Copyright &y& Elsevier]

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 in the boreal forest renews forest stands and changes the ecosystem properties. The successional 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 12–24Wm−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. [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]

Continuous measurements of carbon exchange using the eddy covariance (EC) technique were made at three boreal forest mature sites including Southern Old Aspen (SOA), Southern Old Black Spruce (SOBS) and Southern Old Jack Pine (SOJP) in 2000. Climatic conditions were slightly warmer than normal with precipitation exceeding evapotranspiration at each site. Annual ecosystem respiration (R) derived from daytime analyses of EC data was 1141, 815 and 521 g C m−2 per year and was consistently lower than nighttime EC estimates of 1193, 897 and 578 g C m−2 per year for SOA, SOBS and SOJP, respectively. The differences, however, were not statistically significant given the large uncertainty associated with each analytical technique. The uncertainty in annual net ecosystem productivity (NEP) was assessed by randomly simulating missing data and gap filling using simple biophysical algorithms. The uncertainty analysis supports the finding that each site was a net sink, and that differences in NEP were only significant between SOA and SOBS. The annual NEP and uncertainty for SOA, SOBS and SOJP was 122 (64–142), 35 (18–53) and 78 (61–91) g C m−2 per year, respectively. These relatively old growth forests represent a weak to moderate carbon sink. Despite having the shortest growing period, carbon sequestration was greatest at SOA because of its relatively large photosynthetic capacity (Amax). At the evergreen sites, Amax was marginally larger at SOBS; however, annual carbon sequestration was smaller as a result of greater R. The evergreen sites exhibited a pronounced mid-season reduction in NEP, which was attributed to a large increase in R while Amax had not reached its full capacity. Non-growing season R resulted in a carbon loss of 285, 120 and 64 g C m−2 and accounted for 70, 80 and 46% of the summertime NEP at SOA, SOBS and SOJP, respectively. Six years of EC data at SOA indicate that carbon sequestration at boreal aspen sites may benefit from warmer climatic conditions because R is relatively conservative and photosynthesis increases in response to a longer growing period. [Copyright &y& Elsevier]