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Abstract: British Columbia (BC), Canada is experiencing a severe mountain pine beetle (MPB) (Dendroctonus ponderosae) epidemic extending over an area of 135,000km2. The widespread mortality of lodgepole pine (Pinus contorta var. latifolia) caused by the beetle has significant implications for BC''s timber supply and Canada''s carbon (C) budget. This study used the eddy-covariance technique to examine how the beetle is impacting the net ecosystem production (NEP) of two attacked lodgepole pine-dominated stands in the central interior of BC. MPB-06 is an 83-year-old stand that was first attacked in 2006. At the start of 2007 roughly 60% of the canopy had been attacked and by October 2008 only 21% of the trees remained healthy. MPB-03, a 110-year-old stand, had >95% pine canopy mortality as a result of a 2003 beetle attack, and also differed from MPB-06 in that it had a more developed secondary structure (consisting of tree seedlings and saplings and sub-canopy and canopy trees) that survived the beetle attack. Eddy-covariance measurements were also conducted in two stands near MPB-06 harvested in 2005 and 1997 (CC-05 and CC-97) for approximately three-week periods each during the 2007 growing season. MPB-06 had an annual NEP of −82 and −33gCm−2, while MPB-03 had an NEP of −56 and 4gCm−2 in 2007 and 2008, respectively. In the 2007 and 2008 growing seasons (May–September), MPB-06 was a sink of 12 and 52gCm−2, while MPB-03 was a sink of 17 and 68gCm−2, respectively. The productivity at MPB-06 resulted from an increase in photosynthesis by the remaining healthy trees and understory vegetation, while at MPB-03 the secondary structure and understory vegetation showed a strong capacity to sequester C due to an opening up of the stand as a result of canopy mortality. Average daily values of NEP during the measurement periods at CC-97 and CC-05 were −0.37 and −0.87gCm−2, respectively, showing that even 10 years following harvesting, these stands are likely to remain growing season C sources. That MPB-06 and MPB-03 were growing season C sinks suggests that deferring the harvest of stands with significant levels of secondary structure could prevent MPB-attacked forested areas from becoming C sources over extended periods. [Copyright &y& Elsevier]

CO2 exchange was measured on the forest floor of a coastal temperate Douglas-fir forest located near Campbell River, British Columbia, Canada. Continuous measurements were obtained at six locations using an automated chamber system between April and December, 2000. Fluxes were measured every half hour by circulating chamber headspace air through a sampling manifold assembly and a closed-path infrared gas analyzer. Maximum CO2 fluxes measured varied by a factor of almost 3 between the chamber locations, while the highest daily average fluxes observed at two chamber locations occasionally reached values near 15 μmol C m−2 s−1. Generally, fluxes ranged between 2 and 10 μmol C m−2 s−1 during the measurement period. CO2 flux from the forest floor was strongly related to soil temperature with the highest correlation found with 5 cm depth temperature. A simple temperature dependent exponential model fit to the nighttime fluxes revealed Q10 values in the normal range of 2–3 during the warmer parts of the year, but values of 4–5 during cooler periods. Moss photosynthesis was negligible in four of the six chambers, while at the other locations, it reduced daytime half-hourly net CO2 flux by about 25%. Soil moisture had very little effect on forest floor CO2 flux. Hysteresis in the annual relationship between chamber fluxes and soil temperatures was observed. Net exchange from the six chambers was estimated to be 1920±530 g C m−2 per year, the higher estimates exceeding measurement of ecosystem respiration using year-round eddy correlation above the canopy at this site. This discrepancy is attributed to the inadequate number of chambers to obtain a reliable estimate of the spatial average soil CO2 flux at the site and uncertainty in the eddy covariance respiration measurements. [Copyright &y& Elsevier]

Stand-replacing disturbances, such as harvesting, change former forest net CO 2 sinks into net sources due to significantly reduced photosynthetic uptake and continued respiratory losses. To quantify these effects, this study used data from a Fluxnet-Canada Douglas-fir chronosequence on Vancouver Island, where the most mature site (62-year-old; DF49) was commercially harvested in 2011 creating a 77-ha clearcut (HDF11). Carbon (C) exchange was measured continuously using the eddy-covariance technique for more than a decade pre-harvest and for three years after harvesting. Automated non-steady-state chambers were used to measure soil respiration ( R s ) before and after harvesting. The mature stand transitioned from a moderate C sink (net ecosystem productivity (NEP) = 560 g C m −2 yr −1 ) before harvesting to a strong C source (NEP = −1000 g C m −2 yr −1 ) in the first year after harvesting. Gross ecosystem photosynthesis (GEP) decreased from 1890 g C m −2 yr −1 before harvesting to 130 g C m −2 yr −1 in the first year after harvesting, while ecosystem respiration ( R e ) decreased by just 15% from 1325 to 1130 g C m −2 yr −1 . This small decrease in R e suggests that heterotrophic respiration ( R h ) increased to partially compensate for a significant reduction in autotrophic respiration ( R a ) due to the loss of respiring roots, boles, branches and foliage. The post-harvest C balance results from HDF11 were also compared with those for a previously harvested stand (HDF00) in the same chronosequence. It was harvested in 2000 and was located 3 km away from HDF11. Considerable differences in NEP, GEP and R e were observed in the two clearcuts for the first two comparison years following harvesting, with HDF11 being a stronger source of C (NEP = −780 and −697 g C m −2 yr −1 ) than HDF00 (NEP = −620 and −520 g C m −2 yr −1 ). This was mostly due to greater R e at HDF11 likely due to a greater amount of decomposing organic matter and logging residue. These results show that caution is necessary when drawing conclusions about C fluxes from a single site in an ecozone. [ABSTRACT FROM AUTHOR]

Highlights: [•] Feather moss community dominated net forest-floor CO2 exchange (NFFE) in a boreal black spruce forest. [•] NFFE showed great inter-annual variability over an 8-year period. [•] Mean annual values of climate variables were poor predictors of inter-annual variability in NFFE. [•] Spring and summer conditions had a more dominant impact than fall conditions. [ABSTRACT FROM AUTHOR]

High prevalence of benzodiazepine use among citizens of Serbia has been described previously. The fourth year medical students are supposed to have sufficient knowledge on benzodiazepines, but it is also clear that medical students are not immune to substance use and abuse. This study was conducted to assess the perceptions of a Serbian medical students (fourth year), regarding benzodiazepines utilization. The study was conducted as cross-sectional, using structured questionnaire with 4 multiple-choice questions. The participation was voluntary and anonymous, 112 students participated and the response rate was 87%. Results showed that slightly above half of the students (n=64; 57.1%) classified benzodiazepines correctly as anxiolytics/hypnotics (sedatives), while the others (n=48; 42.9%) non-specifically classified benzodiazepines as drugs acting on central nervous system. The majority of students stated that they never use benzodiazepines (n=72; 64.3%). Among benzodiazepines users (n=40; 35.7%), 100% were occasional users (less than once a week). The main reason for benzodiazepines use was insomnia (n=16; 40.0%), anxiety (n=12; 30.0%) and other reasons, such as tremor, muscle spasms etc. (n=12; 30.0%). The majority of benzodiazepines users (n=28; 70.0%) stated that benzodiazepines did not impair their concentration and studying, while the rest (n=12; 30.0%) noticed mild impairment. Serbian fourth year medical students showed mildly disappointing knowledge on benzodiazepines regarding the classification. However, low rate of benzodiazepines use among students and the lack of recreational use indicate awareness of the risk of drugs misuse in this population. [Copyright &y& Elsevier]

Abstract: The eddy-covariance method has become a cornerstone in the study of the surface energy and water balances, yet the oft-reported lack of energy-balance closure adds uncertainty to its measurements of evapotranspiration. This study explores the issue of closure based on 10 years of energy and water balance measurements at seven flux-tower sites in the southern boreal forests of western Canada. The sites include mature aspen, black spruce and jack pine forests, three young jack pine forests following harvesting, and one fen. Careful attention was paid to the measurement of evapotranspiration E, surface available energy (net radiation minus storage), precipitation P, and soil water storage S including root-zone soil water and water table elevation. Stand-level outflow O at the flux-tower sites was estimated annually as a residual term in the vertical water balance (O = P − E −ΔS), with and without energy-closure adjustments to E. Six of the flux towers were located within or near the gauged White Gull Creek watershed (603km2). The comparison of measured streamflow from the White Gull Creek with estimated outflow, scaled from the flux towers to the watershed, provides an independent test of the need for energy-closure adjustments to flux-tower E. Measured streamflow from the White Gull Creek watershed had a long-term mean (S.D.) of 101 (54)mmy−1 but was highly variable among years, ranging from 30mmy−1 during an extreme dry year to 204mmy−1 during an extreme wet year. Stand-level outflow, estimated with an energy-closure adjustment to E, varied by land-cover type, from a mean (S.D.) of 30 (35)mmy−1 at the mature aspen forest and 52 (125)mmy−1 at the fen to 108 (85)mmy−1 at the mature black spruce forest and 187 (55) and 250 (64)mmy−1 at the mature and young (harvested) jack pine forests, respectively. The dominant land-cover type, mature black spruce forest, typified the hydrology of the boreal forest landscape, with outflow similar to the watershed mean. The warm-season, eddy-covariance sensible and latent heat fluxes were 15±4% (annual mean±S.D.) smaller than surface available energy, with reasonable similarity among sites and years. When scaled from stand to watershed, the tower-based estimates of outflow agreed with measured streamflow only when outflow was calculated with an energy-closure adjustment to E. The annual means for the 2002–2009 hydrologic years were: 131mmy−1 (measured streamflow); 182mmy−1 (tower-based outflow calculated without energy-closure adjustments to E); and 112 and 127mmy−1 (tower-based outflow calculated with energy-closure adjustments to E from two different schemes). This result corroborates the need for energy-closure adjustments to eddy-covariance measurements of E, but it does not provide guidance about how the adjustments should be implemented. [Copyright &y& Elsevier]

A simple aerodynamic-variance method is proposed to fill gaps in continuous CO2 flux measurements in rainy conditions, when open-path analysers do not function. The method requires turbulent conditions (friction velocity greater than 0.1ms−1), and uses measurements of mean wind speed, and standard deviations of temperature and CO2 concentration fluctuations to complement, and at times replace, eddy-covariance measurements of friction velocity, sensible heat flux and CO2 flux. Friction velocity is estimated from the mean wind speed with a flux-gradient relationship modified for the roughness sublayer. Since normalised standard deviations do not follow Monin-Obukhov similarity theory in the roughness sublayer, a simple classification scheme according to the scalar turbulence scale was used. This scheme is shown to produce sensible heat and CO2 flux estimates that are well correlated with the measured values. [ABSTRACT FROM AUTHOR]

A large area of boreal jack pine ( Pinus banksiana Lamb.) forest in Canada is recovering from clear-cut harvesting, and the carbon (C) balance of these regenerating forests remains uncertain. Net ecosystem CO2 exchange was measured using the eddy-covariance technique at four jack pine sites representing different stages of stand development: three postharvest sites (HJP02, HJP94, and HJP75) and one preharvest site (OJP). The four sites, located in the southern Canadian boreal forest, Saskatchewan, Canada, are typical of low productivity jack pine stands and were 2, 10, 29, and 90 years old in 2004, respectively. Mean annual net ecosystem production (NEP) for 2004 and 2005 was -137±11, 19±16, 73±28, and 22±30 g C m-2 yr-1 at HJP02, HJP94, HJP75 and OJP, respectively, showing the postharvest jack pine stands to be moderate C sources immediately after harvesting, weak sinks at 10 years, moderate C sinks at 30 years, then weak C sinks at 90 years. Mean annual gross ecosystem photosynthesis (GEP) for the 2 years was 96±10, 347±20, 576±34, and 583±35 g C m-2 yr-1 at HJP02, HJP94, HJP75, and OJP, respectively. The ratio of annual ecosystem respiration ( R) to annual GEP was 2.51±0.15, 0.95±0.04, 0.87±0.03, and 0.96±0.03. Seasonally, NEP peaked in May or June at all four sites but GEP and R were highest in July. R at a reference soil temperature of 10 °C, ecosystem quantum yield and photosynthetic capacity were lowest for the 2-year-old stand. R was most sensitive to soil temperature for the 90-year-old stand. The primary source of variability in NEP over the course of succession of the jack pine ecosystem following harvesting was stand age due to the changes in leaf area index. Intersite variability in GEP and R was an order of magnitude greater than interannual variability at OJP. For both young and old stands, GEP had greater interannual variability than R and played a more important role than R in interannual variation in NEP. Based on year-round flux measurements from 2000 to 2005, the 10-year stand had larger interannual variability in GEP and R than the 90-year stand. Interannual variability in NEP was driven primarily by early-growing-season temperature and growing-season length. Photosynthesis played a dominant role in the rapid rise in NEP early in stand development. Late in stand development, however, the subtle decrease in NEP resulted primarily from increasing respiration. [ABSTRACT FROM AUTHOR]

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]

In 2000–03, continuous eddy covariance measurements of carbon dioxide (CO2) flux were made above mature boreal aspen, black spruce, and jack pine forests in Saskatchewan, Canada, prior to and during a 3-year drought. During the 1st drought year, ecosystem respiration ( R) was reduced at the aspen site due to the drying of surface soil layers. Gross ecosystem photosynthesis (GEP) increased as a result of a warm spring and a slow decrease of deep soil moisture. These conditions resulted in the highest annual net ecosystem productivity (NEP) in the 9 years of flux measurements at this site. During 2002 and 2003, a reduction of 6% and 34% in NEP, respectively, compared to 2000 was observed as the result of reductions in both R and GEP, indicating a conservative response to the drought. Although the drought affected most of western Canada, there was considerable spatial variability in summer rainfall over the 100-km extent of the study area; summer rainfalls in 2001 and 2002 at the two conifer sites minimized the impact of the drought. In 2003, however, precipitation was similarly low at all three sites. Due to low topographic position and consequent poor drainage at the black spruce site and the coarse soil with low water-holding capacity at the jack pine site almost no reduction in R, GEP, and NEP was observed at these two sites. This study shows that the impact of drought on carbon sequestration by boreal forest ecosystems strongly depends on rainfall distribution, soil characteristics, topography, and the presence of vegetation that is well adapted to these conditions. [ABSTRACT FROM AUTHOR]

Life cycle analysis of climate and disturbance effects on forest net ecosystem productivity (NEP) is necessary to assess changes in forest carbon (C) stocks under current or future climates. Ecosystem models used in such assessments need to undergo well-constrained tests of their hypotheses for climate and disturbance effects on the processes that determine CO2 exchange between forests and the atmosphere. We tested the ability of the model ecosys to simulate diurnal changes in CO2 fluxes under changing air temperatures ( Ta) and soil water contents during forest regeneration with eddy covariance measurements over boreal jack pine ( Pinus banksiana) stands along a postclearcut chronosequence. Model hypotheses for hydraulic and nutrient constraints on CO2 fixation allowed ecosys to simulate the recovery of C cycling during the transition of boreal jack pine stands from C sources following clearcutting (NEP from −150 to −200 g C m−2 yr−1) to C sinks at maturity (NEP from 20 to 80 g C m−2 yr−1) with large interannual variability. Over a 126-year logging cycle, annualized NEP, C harvest, and net biome productivity (NBP =NEP–harvest removals) of boreal jack pine averaged 47, 33 and 14 g C m−2 yr−1. Under an IPCC SRES climate change scenario, rising Ta exacerbated hydraulic constraints that adversely affected NEP of boreal jack pine after 75 years. These adverse effects were avoided in the model by replacing the boreal jack pine ecotype with one adapted to warmer Ta. This replacement raised annualized NEP, C harvest, and NBP to 81, 56 and 25 g C m−2 yr−1 during a 126-year logging cycle under the same climate change scenario. [ABSTRACT FROM AUTHOR]

The carbon and water budgets of boreal and temperate broadleaf forests are sensitive to interannual climatic variability and are likely to respond to climate change. This study analyses 9 years of eddy-covariance data from the Boreal Ecosystem Research and Monitoring Sites (BERMS) Southern Old Aspen site in central Saskatchewan, Canada and characterizes the primary climatic controls on evapotranspiration, net ecosystem production ( FNEP), gross ecosystem photosynthesis ( P) and ecosystem respiration ( R). The study period was dominated by two climatic extremes: extreme warm and cool springs, which produced marked contrasts in the canopy duration, and a severe, 3-year drought. Annual FNEP varied among years from 55 to 367 g C m−2 (mean 172, SD 94). Interannual variability in FNEP was controlled primarily by factors that affected the R/ P ratio, which varied between 0.74 and 0.96 (mean 0.87, SD 0.06). Canopy duration enhanced P and FNEP with no apparent effect on R. The fraction of annual photosynthetically active radiation (PAR) that was absorbed by the canopy foliage varied from 38% in late leaf-emergence years to 51% in early leaf-emergence years. Photosynthetic light-use efficiency (mean 0.0275, SD 0.026 mol C mol−1 photons) was relatively constant during nondrought years but declined with drought intensity to a minimum of 0.0228 mol C mol−1 photons during the most severe drought year. The impact of drought on FNEP varied with drought intensity. Years of mild-to-moderate drought suppressed R while having little effect on P, so that FNEP was enhanced. Years of severe drought suppressed both R and P, causing either little change or a subtle reduction in FNEP. The analysis produced new insights into the dominance of canopy duration as the most important biophysical control on FNEP. The results suggested a simple conceptual model for annual FNEP in boreal deciduous forests. When water is not limiting, annual P is controlled by canopy duration via its influence on absorbed PAR at constant light-use efficiency. Water stress suppresses P, by reducing light-use efficiency, and R, by limiting growth and/or suppressing microbial respiration. The high photosynthetic light-use efficiency showed this site to be a highly productive boreal deciduous forest, with properties similar to many temperate deciduous forests. [ABSTRACT FROM AUTHOR]

Abstract: Closure of the surface energy balance provides an objective criterion for evaluating eddy-covariance (EC) flux measurements. This study analyses 5 years of EC carbon dioxide, water vapor, and sensible heat flux measurements from three mature boreal forest stands in central Saskatchewan, Canada. The EC sensible and latent heat fluxes, H and λE, underestimated the surface available energy by 11% (aspen), 15% (black spruce), and 14% (jack pine). At all sites, the energy-closure fraction CF responded similarly to the friction velocity u *, atmospheric stability, and time of day. At night, CF increased from ∼0.3 at very low-u * to an asymptotic maximum of ∼0.9 at u * above 0.35ms−1. During unstable-daytime periods, CF varied linearly from ∼0.7 at low-u * to ∼1.0 at high-u *. The energy imbalance pattern was similar among sites and may be characteristic of the continental, boreal forest. EC measurements of net ecosystem exchange F NEE have no objective, diagnostic parameter that is equivalent to CF. We therefore derived an analogous F NEE “closure fraction” CFNEE by normalizing measured F NEE against estimates from an empirical model that was tuned to the high-u * data. CF and CFNEE responded similarly to u *, atmospheric stability, and time of day. We discuss two implications for EC flux data post-processing. The results uphold the common practice of rejecting EC measurements during low-u * periods. They also lend support to the application of energy-closure adjustments to H, λE, and F NEE. [Copyright &y& Elsevier]

Abstract: The net ecosystem productivity (NEP) of boreal aspen is strongly affected by comparative rates of annual potential evapotranspiration (E a) and precipitation (P a). Changes in E a versus P a during future climate change will likely determine changes in aspen NEP and consequently the magnitude of the carbon sink/source of a significant part of the boreal forest. We hypothesize that the effects of E a versus P a on aspen NEP can be modelled with a soil–root–canopy hydraulic resistance scheme coupled to a canopy energy balance closure scheme that determines canopy water status and thereby CO2 uptake. As part of the ecosystem model ecosys, these schemes were used to model diurnal declines in CO2 and latent heat (LE) exchange during a 3-year drought (2001–2003) at the Fluxnet-Canada Research Network (FCRN) southern old aspen site (SOA). These declines were consistent with those measured by eddy covariance (EC) at SOA, except that ecosystem CO2 effluxes modelled during most nights were larger that those measured by EC or gap-filled from other EC measurements. Soil CO2 effluxes in the model were close to, but sometimes smaller than, those measured by automated surface chambers at SOA. Diurnal declines in CO2 exchange during the drought caused declines in annual NEP in the model, and in gap-filled EC measurements (model versus EC in gCm−2: 275 versus 367±110 in 2001, 82 versus 144±43 in 2002 and 23 versus 104±31 in 2003). Lower modelled NEP was attributed to the larger modelled CO2 effluxes. Ecosys was then used to predict changes in aspen net biome productivity (NBP=NEP − C lost from disturbance) caused by 6-year versus 3-year recurring droughts during 100-year fire cycles under current climate versus climate change projected under the IPCC SRES A1B scenario. Although NBP was adversely affected during recurring 6-year droughts under current climate, it recovered quickly during non-drought years so that long-term NBP was maintained at 4gCm−2 year−1. NBP rose by 10, 108 and 126gCm−2 year−1 during the first, second and third centuries under climate change with recurring 3-year droughts, indicating a gradual rise in sink activity by boreal aspen. However recurring 6-year droughts during climate change caused recurring negative NBP (C losses), gradually depleting aspen C reserves and eventually causing dieback of the aspen overstory during the third century of climate change. This dieback was followed by a large decline in NBP. We conclude that NBP of boreal aspen will rise gradually under current projections of climate change, except under prolonged (e.g. 6 years) recurring droughts, which would eventually cause aspen to die back and substantial amounts of C to be lost. [Copyright &y& Elsevier]

In 2001–03, continuous eddy covariance measurements of carbon dioxide (CO2) flux were made above mature boreal aspen, black spruce, and jack pine forests in Saskatchewan, Canada, prior to and during a 3−year drought. During the 1st drought year, ecosystem respiration ( R) was reduced at the aspen site due to the drying of surface soil layers. Gross ecosystem photosynthesis (GEP) increased as a result of a warm spring and a slow decrease of deep soil moisture. These conditions resulted in the highest annual net ecosystem productivity (NEP) in the 9 years of flux measurements at this site. During 2002 and 2003, a reduction of 6% and 34% in NEP, respectively, compared to 2000 was observed as the result of reductions in both R and GEP, indicating a conservative response to the drought. Although the drought affected most of western Canada, there was considerable spatial variability in summer rainfall over the 100−km extent of the study area; summer rainfalls in 2001 and 2002 at the two conifer sites minimized the impact of the drought. In 2003, however, precipitation was similarly low at all three sites. Due to low topographic position and consequent poor drainage at the black spruce site and the coarse soil with low water-holding capacity at the jack pine site almost no reduction in R, GEP, and NEP was observed at these two sites. This study shows that the impact of drought on carbon sequestration by boreal forest ecosystems strongly depends on rainfall distribution, soil characteristics, topography, and the presence of vegetation that is well adapted to these conditions. [ABSTRACT FROM AUTHOR]

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]

The seasonal phenology of the leaf area index (LAI) is a major determinant of net ecosystem production in deciduous forest ecosystems. This study describes seasonal and inter-annual differences in LAI in a boreal aspen-hazelnut forest in central Saskatchewan, Canada, between 1994 and 2003, and relates the differences in LAI to annual net ecosystem production (FNEP). A robust method is developed to fill gaps in the annual LAI cycle from systematic but sparse measurements using associated radiation and temperature indices. The ratio of the photosynthetically-active radiation reflectance to the shortwave reflectance is shown to have a particularly distinct LAI signature.Optical estimates of the fully-leafed LAI agreed well with measurements from autumn litterfall and showed moderate inter-annual variability for the trembling aspen overstory (mean ± S.D. of 2.44 ± 0.30) and the hazelnut understory (1.98 ± 0.44). Two features of the annual LAI cycle differed among years—the timing of leaf emergence in spring, which varied by up to four weeks, and the fully-leafed value for LAI, which varied between 3.66 and 5.22. The timing of leaf senescence in autumn was nearly constant among years. The seasonal cycles of FNEP and LAI were tightly coupled and the correspondence between their respective inter-annual differences was remarkable, particularly during leaf emergence in spring. Annual FNEP was positively correlated with the canopy duration and the annual maximum LAI, with increases in annual FNEP of 6.9gCm-2 for each additional day in full leaf and 83gCm-2 for each additional unit of LAI. [Copyright &y& Elsevier]

Continuous automatic chamber and eddy covariance (EC) measurements were made at an old aspen forest (SOA) located at southern boreal treeline in Saskatchewan, Canada to examine the temporal variability in soil (Rs), tree bole (Rb), and ecosystem respiration (RE) during 2001. Climatic conditions were significantly warmer and drier than the 30-year climate normal, resulting in lower RE and an unprecedented increase in net ecosystem productivity (NEP). In the 7-year record (1994, and 1996–2001) of CO2 exchange at SOA, the year 2001 showed the greatest carbon gain (300 g C m-2 year). Scaled chamber measurements (1315 g C m-2 per year) were 37% larger than the EC estimate of RE (961 g C m-2 per year). The difference between the scaled chambers and the EC estimate was reduced to 20% after correcting for lack of energy balance closure. Annual RE was approximately 170 g C m-2 per year lower than the average of the previous 6 years. Annual estimates of microbial-heterotrophic (Rh) (510 g C m-2 per year) and autotrophic respiration (Ra) (805 g C m-2 per year), based on chamber measurements, were used to help validate the EC estimate of RE. Ra represented 61% of the total chamber respiration. This fraction was used to partition RE into Ra and Rh to calculate net primary production (NPP). The values of NEP (300 g C m-2 per year) and NPP (675 g C m-2 per year) were more characteristic of temperate forests. The NPP/Pg ratio of 0.54 was within the range of recently published values using biometric techniques and supports that the annual ecosystem respiration budget and its partitioning was well constrained. We recognize, however, that this ratio will vary interannually depending on climatic conditions. In 7 years of annual EC CO2 flux measurements at SOA, this study provides the first evidence that drought can lead to a transient increase in CO2 sequestration resulting from a reduction of RE. [Copyright &y& Elsevier]

A process-based model of the transport of heat and water in the soil and surface energy balance components is extended to include production of CO2 from heterotrophic (microbial decomposition) and autotrophic (root) respiration, and transport of the resulting CO2 in both the gaseous and liquid phases. The production of CO2 is determined by the amount and type as well as distribution in the soil profile of organic matter and roots, and their respective first-order rate constants. Influences of soil water and temperature are considered through their effects on respiration, CO2 diffusivities and chemical equilibria among various C species in the soil–solution–air continuum. Also included is CO2 uptake by plant roots associated with root water uptake.Model calculations, using independently determined parameters, are compared with measurements of soil CO2 concentrations and chamber-measured forest floor fluxes in a second-growth 54-year-old Douglas-fir forest located on the east coast of Vancouver Island, Canada. Measured and modeled forest floor CO2 effluxes and soil CO2 concentration profiles, as well as soil water contents and temperatures showed good agreement. The efflux was most sensitive to soil temperature, and the influence of soil water content was relatively small. In this rapidly draining soil, the CO2 efflux, at time scales as small as half-hour, was very well approximated by the total production of CO2 in the soil profile, even during and after rainfall, which significantly increases soil water content. This is because CO2 diffusion in this forest soil is relatively rapid compared to changes in the rates of CO2 production. In this podzolic soil ecosystem with low soil pH, liquid-phase diffusion appears to play a minor role in CO2 transport. High soil CO2 concentrations, up to 10,000 μmol mol-1 in summer and 6000 μmol mol-1 in winter, and a positive downward gradient at the 1 m depth indicate some CO2 sources associated with very low CO2 diffusivity at deeper depths. [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]

Two years of continuous eddy covariance measurements were used to characterize the seasonal and annual variability of the latent and sensible heat fluxes above a 50-year-old, 33 m tall coastal Douglas-fir forest on the east coast of Vancouver Island, Canada. The total annual evaporation was found to be very conservative for this temperate coniferous rainforest despite variability in weather between the years (432 mm in 1998 and 435 mm in 1999). Winter evaporation was a significant component of the annual total, on average 27% of the mean 434 mm per year. Seasonal variations in the magnitude and direction of the sensible heat flux above the canopy were linked to changes in the surface conductance to water vapour transfer. The wet canopy tended to act as a sink for sensible heat, especially throughout the winter months, resulting in an average daily 24 h Bowen ratio of −1.7. This contrasted dramatically with summer daytime turbulent exchange, which was usually dominated by upward sensible heat flux during the summer months (April–September, inclusive). The total 24 h Bowen ratio for the summer was 1.1, with daily 24 h values reaching a maximum of 3.1 for dry-canopy conditions. The average rate of transpiration was 1.7 mm per day reaching a maximum of 3.7 mm day, while canopy conductance ranged from 1 to 30 mm s−1.Although the total winter and summer evaporation was similar between years, differences in the timing of the maximum evaporation rates and flux partitioning patterns resulted in considerable variability within the seasons. The cooler, cloudier and wetter weather of 1999 maintained relatively low evaporation rates and low Bowen ratios through the entire summer, while clear skies and hot and dry conditions in 1998 resulted in greater evaporation until mid-July. After this time, drought and a decrease in canopy conductance reduced evaporation and led to a daily daytime Bowen ratio as high as 3.8. [Copyright &y& Elsevier]

Compares the response of net ecosystem productivity (NEP) and evaporation in a boreal aspen forest and a black spruce forest in Canada. Relationship between carbon dioxide uptake and evaporation; Strength of carbon source of black spruce forest in warm condition; Basis of the carbon balance of both forests.

Turbulent flux measurements both above and beneath the canopy of a boreal aspen forest are described. Velocity skewness showed that, beneath the aspen canopy, turbulence was dominated by intermittent, downward penetrating gusts. Eulerian horizontal length scales calculated from integration of the autocorrelation function or spectral peaks were 9.0 and 1.4 times the mean aspen height of 21.5 m respectively. Above-canopy power spectral slopes for all velocity components followed the -2/3 power law, whereas beneath-canopy slopes were closer to -1 and showed a spectral short cut in the horizontal and vertical components. Cospectral patterns were similar both above and beneath the canopy. The Monin–Obukhov similarity function for the vertical wind velocity variance was a well-defined function of atmospheric stability, both above and beneath the canopy. Nocturnal flux underestimation and departures of this similarity function from that expected from Monin–Obukhov theory were a function of friction velocity. Energy balance closure greater than 80% was achieved at friction velocities greater than 0.30 and 0.10 m s-1, above and below the aspen canopy, respectively. Recalculating the latent heat flux using various averaging periods revealed a minimum of 15 min were required to capture 90% of the 30-min flux. Linear detrending reduced the flux at shorter averaging periods compared to block averaging. Lack of energy balance closure and erratic flux behaviour led to the recalculation of the latent and sensible heat fluxes using the ratio of net radiation to the sum of the energy balance terms. [ABSTRACT FROM AUTHOR]

AbstractTo evaluate the carbon budget of a boreal deciduous forest, we measured CO2 fluxes using the eddy covariance technique above an old aspen (OA) forest in Prince Albert National Park, Saskatchewan, Canada, in 1994 and 1996 as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). We found that the OA forest is a strong carbon sink sequestering 200 ± 30 and 130 ± 30 g C m–2 y–1 in 1994 and 1996, respectively. These measurements were 16–45% lower than an inventory result that the mean carbon increment was about 240 g C m–2 y–1 between 1919 and 1994, mainly due to the advanced age of the stand at the time of eddy covariance measurements. Assuming these rates to be representative of Canadian boreal deciduous forests (area ≈ 3 × 105 km2), it is likely they can sequester 40–60 Tg C y–1, which is 2–3% of the missing global carbon sink. The difference in carbon sequestration by the OA forest between 1994 and 1996 was mainly caused by the difference in leaf emergence date. The monthly mean air temperature during March–May 1994, was 4.8 °C higher than in 1996, resulting in leaf emergence being 18–24 days earlier in 1994 than 1996. The warm spring and early leaf emergence in 1994 enabled the aspen forest to exploit the long days and high solar irradiance of mid-to-late spring. In contrast, the 1996 OA growing season included only 32 days before the summer solstice. The earlier leaf emergence in 1994 resulted 16% more absorbed photosynthetically active radiation and a 90 g C m–2 y–1 increase in photosynthesis than 1996. The concomitant increase in respiration in the warmer year (1994) was only 20 g C m–2 y–1. These results show that an important control on carbon sequestration by boreal deciduous forests is spring temperature, via the influence of air temperature on the timing of leaf emergence. [ABSTRACT FROM AUTHOR]