Z. Nesic

Energy balance and canopy conductance of a boreal aspen forest: Partitioning overstory and understory components

The energy balance components were measured throughout most of 1994 in and above a southern boreal aspen (Populus tremuloides Michx.) forest (53.629°N 106.200°W) with a hazelnut (Corylus cornuta Marsh.) understory as part of the Boreal Ecosystem-Atmosphere Study. The turbulent fluxes were measured at both levels using the eddy-covariance technique. After rejection of suspect data due to instationarity or inhomogeneity, occasional erratic behavior in turbulent fluxes and lack of energy balance closure led to a recalculation of the fluxes of sensible and latent heat using their ratio and the available energy. The seasonal development in leaf area was reflected in a strong seasonal pattern of the energy balance. Leaf growth began during the third week of May with a maximum forest leaf area index of 5.6 m 2 m -2 reached by mid-July. During the full-leaf period, aspen and hazelnut accounted for approximately 40 and 60% of the forest leaf area, respectively. Sensible heat was the dominant consumer of forest net radiation during the preleaf period, while latent heat accounted for the majority of forest net radiation during the leafed period. Hazelnut transpiration accounted for 25% of the forest transpiration during the summer months. During the full-leaf period (June 1 to September 7) daytime dry-canopy mean aspen and hazelnut canopy conductances were 330 mmol m -2 s -1 (8.4 mm s -1 ) (70% of the total forest conductance) and 113 mmol m -2 s -1 (2.9 mm s -1 ) (24% of the total forest conductance), respectively. Maximum aspen and hazelnut canopy conductances were 1200 mmol m -2 s -1 (30 mm s -1 ) and 910 mmol m -2 s -1 (23 mm s -1 ), respectively, and maximum stomatal conductances were 490 mmol m -2 s -1 (12.5 mm s -1 ) and 280 mmol m -2 s -1 (7 mm s -1 ), aspen and hazelnut, respectively. 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] α 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 α of 0.91 indicated some stomatal control on transpiration, with a directly proportional relationship between forest leaf area and forest canopy conductance, varying α during much of the season through a range very sensitive to regional scale transpiration and surface-convective boundary laver feedbacks.

Increased carbon sequestration by a boreal deciduous forest in years with a warm spring

A boreal deciduous forest in Saskatchewan, Canada, sequestered 144±65, 80±60, 116±35 and 290±50 g C m−2 y−1 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 El Nino events of this century, and is considered a significant and unexpected benefit.

Ecophysiological controls on the carbon balances of three southern boreal forests

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 52 1gCm −2 per year and was consistently lower than nighttime EC estimates of 1193, 897 and 57 8gCm −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 )gCm −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 6 4gCm −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.

Measuring forest floor CO2 fluxes in a Douglas-fir forest

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 mol Cm −2 s −1 . Generally, fluxes ranged between 2 and 10 mol Cm −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 CO 2 flux by about 25%. Soil moisture had very little effect on forest floor CO 2 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 CO 2 flux at the site and uncertainty in the eddy covariance respiration measurements.

Response of Net Ecosystem Productivity of Three Boreal Forest Stands to Drought

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.

Annual and seasonal variability of sensible and latent heat fluxes above a coastal Douglas-fir forest, British Columbia, Canada

Abstract 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, 33xa0m 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 (432xa0mm in 1998 and 435xa0mm in 1999). Winter evaporation was a significant component of the annual total, on average 27% of the mean 434xa0mm 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 24xa0h 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 24xa0h Bowen ratio for the summer was 1.1, with daily 24xa0h values reaching a maximum of 3.1 for dry-canopy conditions. The average rate of transpiration was 1.7xa0mm per day reaching a maximum of 3.7xa0mm day, while canopy conductance ranged from 1 to 30xa0mmxa0s −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.

The seasonal water and energy exchange above and within a boreal aspen forest

Abstract The seasonal water and energy exchange of a boreal aspen forest underlain by a hazelnut understory is described. Measurements of above-aspen latent and sensible heat, short-wave and net radiation, and photosynthetically active radiation are compared to those measured above the hazelnut understory. Understory radiation measurements were made using a tram system. Energy storage at each measurement height was determined, and measurements of the soil moisture, temperature, and heat flux were made using an array of probes. The mean annual air temperature and total precipitation during 1994 were 1.2°C and 488.4xa0mm, respectively, above the 1951–1980 average −0.2°C and total 462.6xa0mm. There was a pronounced seasonal development of leaves, with the maximum leaf area index of the hazelnut (3.3xa0m2xa0m−2) exceeding that of the aspen (2.3xa0m2xa0m−2). Beneath-aspen radiation decreased exponentially as the aspen leaf area increased, and the calculated effective extinction coefficients decreased as the plant area index increased. At full aspen leaf, 27, 23, and 20% of the above-aspen short-wave, net, and photosynthetically active radiation, respectively, reached the hazelnut. The diurnal energy balance at both heights showed pronounced seasonal trends. Sensible heat from the forest floor dominated during the leaf-free period, whereas latent heat from the overstory dominated during the leafed period. The fraction of the annual precipitation evaporated was 82–91%, with 67–68%, 26–28%, and 4–7% originating from the aspen, hazelnut, and soil, respectively. Over the leafed period, soil water was depleted from the root zone (0–60xa0cm depth) and accumulated between the 61–123xa0cm depth, overall resulting in a deficit of 34.7xa0mm between 0–123xa0cm depths. This soil water balance compared well with the daily integrated difference between precipitation and eddy-covariance determined measurements of evaporation.

Response of net ecosystem productivity of three boreal forest stands to drought (Ecosystems DOI: 10.1007/S10021-005-0082-X)

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.

Remote sensing of seasonal light use efficiency in temperate bog ecosystems

Despite storing approximately half of the atmosphere’s carbon, estimates of fluxes between wetlands and atmosphere under current and future climates are associated with large uncertainties, and it remains a challenge to determine human impacts on the net greenhouse gas balance of wetlands at the global scale. In this study we demonstrate that the relationship between photochemical reflectance index, derived from high spectral and temporal multi-angular observations, and vegetation light use efficiency was strong (r2u2009=u20090.64 and 0.58 at the hotspot and darkspot, respectively), and can be utilized to estimate carbon fluxes from remote at temperate bog ecosystems. These results improve our understanding of the interactions between vegetation physiology and spectral characteristics to understand seasonal magnitudes and variations in light use efficiency, opening new perspectives on the potential of this technique over extensive areas with different landcover.