AbstTactWater vapour and CO2 fluxes were measured using the eddy correlation method above and below the overstorey of a 21-m tall aspen stand in the boreal forest of central Saskatchewan as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Measurements were made at the 39.5-m and 4-m heights using 3-dimensional sonic anemometers (Kaijo-Denki and Solent, respectively) and closed-path gas analysers (LI-COR 6262) with 6-m and 4.7-m long heated sampling tubing, respectively. Continuous measurements were made from early October to mid-November 1993 and from early February to lateSeptember 1994. Soil CO2 flux (respiration) was measured using a LI-COR 6000-09 soil chamber and soil evaporation was measured using lysimetry.The leaf area index of the aspen and hazelnut understorey reached 1.8 and 3.3, respectively. The maximum daily evapotranspiration (£) rate was 5-6 mm d^^ Following leaf-out the hazelnut and soil accounted for 22% of the forest £. The estimated total £ was 403 mm for 1994. About 88% of the precipitation in 1994 was lost as evapotranspiration.During the growing season, the magnitude of half-hourly eddy fluxes of CO2 from the atmosphere into the forest reached 1.2 mg CO2 m'^ s^* (33 |imol C m~^ s"^) during the daytime. Downward eddy fluxes at the 4-m height were observed when the hazelnut was growing rapidly in June and July. Under well-ventilated night-time conditions, the eddy fluxes of CO2 above the aspen and hazelnut, corrected for canopy storage, increased exponentially with soil temperature at the 2-cm depth. Estimates of daytime respiration rates using these relationships agreed well with soil chamber measurements. During the 1994 growing season, the cumulative net ecosystem exchange (N££) was -3.5 t C ha"^ y"^ (a net gain by the system). For 1994, cumulative NEE, ecosystem respiration (K) and gross ecosystem photosynthesis (GEP = R-NEE) were estimated to be -1.3, 8.9 and 10.2 t C ha"^ y~^, respectively. Gross photosynthesis of the hazelnut was 32% of GEP.

Both species showed a decrease in canopy conductance as the saturation deficit increased and both showed an increase in canopy conductance as the photosynthetic active radiation increased. There was a linear relationship between forest leaf area index and forest canopy conductance. The timing, duration, and maximum leaf area of this deciduous boreal forest was found to be an important control on transpiration at both levels of the canopy. The full-leaf hazelnut daytime mean Priestley and Taylor [1972] a coefficient of 1.22 indicated transpiration was largely energy controlled and the quantity of energy received at the hazelnut surface was a function of aspen leaf area. The full-leaf aspen daytime mean c• of 0.91 indicated some stomatal control on transpiration, with a directly proportional relationship b6tWeen forest leaf area and forest canopy conductance, varying c• during much of the season through a range very sensitive to regional scale transpiration and surface-convective boundary layer feedbacks. IntroductionThe boreal forest represents one of the world's largest yet least understood ecosystems. Of the estimated 48.5 million km 2 total land area of the world's forests, 12.0 (25%) is covered by boreal forest, second only to the 17.0 (35%) covered by tropical rain forest. Boreal forest net primary productivity (ex---1 pressed as dry matter) accounts for an estimated 9.6 Gt yr (13%) of the world's forests 73.9, exceeding both temperate deciduous 8.4 (11%) and temperate coniferous 6.5 (9%) forests [Salisbury and Ross, 1978].•University of British Columbia, Vancouver, Canada. 2Atmospheric Environment Service, Downsview, Ontario, Canada.•Yale University, New Haven, Connecticut. Continuing with the work presented by Black et al. [1996], the objectives of this paper are (1) to describe the diurnal and seasonal patterns of the aspen overstory and hazelnut understory energy balance, (2) to describe the diurnal and seasonal patterns of canopy water vapor conductances for both the aspen and the hazelnut, and (3) to relate the canopy conductances to the ambient meteorological conditions at both the canopy and the regional levels. Site DescriptionThe study site (

Abstract.A boreal deciduous forest in Saskatchewan, Canada, sequestered 144i65, 80i60, 116-t-35 and 290-t-50 g C m -2 y-• in 1994, 1996, 1997 and 1998, respectively. The increased carbon sequestration was the result of a warmer spring and earlier leaf emergence, which significantly increased ecosystem photosynthesis, but had little effect on respiration. The high carbon sequestration in 1998 was coincident with one of the strongest E1 Nifio events of this century, and is considered a significant and unexpected benefit.

To 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.

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Water flux to the atmosphere was measured from a mature stand of aspen (Populus tremuloides Michx.) in Saskatchewan, Canada, as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). Diurnal and seasonal changes in transpiration were monitored using two sap flow techniques and were compared against the difference between eddy correlation measurements of water vapor flux made above and below the aspen canopy. The three methods showed similar diurnal and seasonal trends in water flux, although sap flow lagged the eddy correlation measurements by about 1 hour diurnally due to changes in water storage within the trees. During the growing season, all methods showed a linear increase in midday transpiration with above‐canopy vapor pressure deficit (VPD) up to ∼1 kPa, beyond which transpiration was relatively constant (VPD 1–2.5 kPa). A similar relationship was obtained when total daily transpiration was plotted against mean daytime VPD. The results are consistent with other observations that stomatal conductance of the aspen canopy decreases at high VPD. The complementary benefits of simultaneous monitoring of canopy transpiration by both eddy correlation and sap flow measurements are discussed.

Abstract. In conjunction with eddy covariance measurements of CO2 fluxes at the 39.5-m height over a 21.5-m-tall boreal aspen stand in northern Saskatchewan, CO2 concentration was measured at eight heights in order to calculate net ecosystem exchange. During both leafless and full-leaf periods, daytime vertical CO2 concentration gradients above 9 rn were weak (< 0.2 gmol mo1-1 m't), but were strong below this height. Little change in CO2 storage in the air column below 39.5 rn occurred during much of the daytime, while around sunrise and sunset CO2 storage changed mainly below 9 m. For the rest of the night, over 85% of the increase in CO2 storage occurred above 9 m. On some calm nights during the growing season, CO2 also accumulated below 9 rn resulting in a sudden upward CO2 flux at 39.5 rn following the resumption of mixing 2-3 hours after sunrise. A 1 O-day experiment was conducted to determine the spatial variability of CO2 flux in the trunk space. Two eddy covariance systems were mounted just above the understory about two tree heights apart.The correlation between CO2 fluxes were poor even under unstable (daytime) conditions, suggesting a relatively heterogeneous understory and soil. In contrast, the correlation between water vapor fluxes was high (r = 0.70) in unstable conditions. However, average daytime and nighttime CO2 fluxes over the 10 days agreed to within 5%. This suggests that partitioning net ecosystem exchange between overstory and understory on an hourly basis using a single-understory eddy covariance system is inadvisable; however, partitioning probably can be done quite reliably using 5-day average fluxes.