Boreal Ecosystem-Atmosphere Study Research Articles

A soil‐vegetation‐atmosphere transfer scheme for the modeling of water and energy balance processes in high latitudes: 1. Model improvements

This paper describes the development of a process‐based water and energy balance model for use in high latitudes under both summertime and wintertime conditions. The model is developed as a part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). The model differs from its original version in the representation of hydrological processes specific to climatic and ecologic conditions in high latitudes: the impact of an organic layer on the water and energy balance of the boreal forest; the parameterization of frozen ground and snow accumulation and ablation; and the effect of open water bodies on the calculation of the radiative energy and water budget. The model can be run either in a fully distributed or an aggregated statistical mode. The resulting macroscale hydrological model can be used, off line, to study the energy and water balance of the boreal forest and potentially can be used as a land‐atmosphere parameterization in atmospheric models.

Water and carbon dioxide exchange at a boreal young jack pine forest in the BOREAS northern study area

Climatological measurements, including carbon dioxide flux density, were made from May to September 1994 and from May to November 1996 at a young jack pine (Pinus banksiana Lamb.) forest near Thompson, Manitoba, Canada, as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). The study periods were warmer and drier than the 24 year climate normals, and in general, 1996 was warmer but wetter than 1994. Volumetric soil moisture above a depth of 8 cm in 1994 was often less than 5%. Albedos for solar radiation and photosynthetic photon flux density (PPFD) were 0.13 and 0.05, respectively, in both years. The Bowen ratio for days of year 144–262 was 1.7 in 1994 and 1.5 in 1996, with maximum daily Bowen ratios from 4 to 6. The range in CO2 flux densities was the same in both years, with maximum uptake values of −0.55 mg m−2 s−1 in midsummer. Average diurnal patterns of CO2 exchange for 1994 were similar to those for 1996, characterized by maximum uptake in the morning and a gradual decrease through the afternoon. Rectangular hyperbolic response curves showing CO2 uptake in relation to incident PPFD show that changes between the morning and the afternoon, and because of environmental controls, were as great or greater than changes between designated 3 and 4 week measurement periods. Over a period of 119 days the young jack pine forest accumulated approximately −200 g C m−2 in 1994 and −267 g C m−2 in 1996. Reproduction of CO2 exchange trends will be important for modeling efforts involving northern ecosystems.

Point simulation of seasonal snow cover dynamics beneath boreal forest canopies

The accurate simulation of snowpack deposition and ablation beneath forested areas is complicated by the fact that the vegetation canopy strongly affects the snow surface energy balance. Data collected as part of the Boreal Ecosystem‐Atmosphere Study are used to derive a series of simple canopy adjustments and drive a two‐layer coupled energy‐ and mass‐balance snowmelt model to simulate the deposition and ablation of the seasonal snowpack at six sites within the boreal forest for the 1994–1995 snow season. Snow cover energy gain in the spring is strongly controlled by canopy cover and is dominated by net radiation fluxes which contribute from 66% to 92% of the snow cover energy balance. Turbulent fluxes comprise 11% of the net energy balance on average, with minor contributions from soil and advected energy fluxes. Simulated depths at the forested sites generally show good agreement with measured snow depths, indicated by model efficiencies ranging from 0.90 to 0.94, with root‐mean‐square differences less than 5 cm. Seasonal snow covers in the boreal environment may be more sensitive to land use transitions, rather than climate shifts, due to the strong control exerted by vegetation canopies on radiation transfer processes.

Seasonal gas exchange of black spruce using an automatic branch bag system

Four large cuvettes were used to measure whole-branch CO2 and H2O vapour exchange at the Boreal Ecosystem-Atmosphere Study, Southern Study Area, Old Black Spruce site. Measurements started before the spring thaw and continued until after the winter freeze-up. Daytime photosynthesis, nighttime respiration, transpiration rate, and branch conductances were zero at the start and the end of the measurement period. Maximum photosynthetic uptake rates were ca. 6 µmol·m-2·s-1 (0.02 µmol·g-1·s-1). Maximum nighttime respiration was ca. 0.8 µmol·g-1·s-1. Maximum transpiration rates were ca. 2 mmol·m-2·s-1 (7.1 µmol·g-1·s-1). Maximum branch conductance was ca. 35 mmol·m-2·s-1 (0.123 mmol·g-1·s-1). A simple model combining light and temperature responses accounted for 80-90% of the day-to-day variation in daytime carbon uptake. Total annual daytime carbon uptake ranged from 151 to 271 g·m-2 (530-868 mg·g-1). Total nighttime loss ranged from 12.7 to 15.5 g C·m-2 (22.4-27.3 mg C·g-1). Total transpiration ranged from 20.4 to 35.0 kg H2O·m-2 (70.2-112.2 g H2O·g-1).

Spatial Scaling of a Remotely Sensed Surface Parameter by Contexture

Various remote sensing sensors observe the Earth’s surface at different spatial resolutions. In deriving surface parameters using remotely sensed data, the transportability of algorithms from one resolution to another is often of great concern because of the surface heterogeneity. This article addresses this scaling issue through image degradation experiments using a Landsat TM image. It is shown theoretically that scaling problems in deriving surface parameters exist not only because of the nonlinearity in the relationships between remote sensing measurements such as NDVI (normalized difference vegetation index) and SR (simple ratio) and the parameters of interest, but also because of the discontinuity between contrasting cover types within a mixed pixel. To quantify the effects of the nonlinearity and discontinuity on scaling, it is found that contextural parameters are more effective than textural parameters. Contexture-based functions are derived for the estimation of the scaling effects on leaf area index (LAI) calculations using algorithms based on NDVI and SR separately. Based on NDVI–LAI and SR–LAI relationships that were derived at the Landsat TM scale (30 m) as part of the Boreal Ecosystem–Atmosphere Study (BOREAS), the effects of scaling on the retrieval of LAI were investigated using nine selected areas of the same size (990 m×990 m) but different water area fractions. The following conclusions are drawn from the investigation: 1) Negative biases in the estimation of LAI occur when either the NDVI or SR algorithm derived at a fine resolution (Landsat TM) is used for calculations at a coarse resolution (for example, AVHRR). 2) The amount of the biases depends on the surface heterogeneity. For a pure forest pixel, the bias caused by the nonlinearity of the NDVI algorithm was smaller than 2% and the linear SR algorithm induces no error in scaling. Therefore, the scaling problem for pure pixels may be ignored for many applications using either linear or nonlinear algorithms. 3) Large negative biases occur when a pixel contains interfaces between two or more contrasting surfaces. In the case of two contrasting surfaces between vegetation and open water, the biases can be up to 45% of the correct value depending of the water area fraction in the pixel. The biases in this case depend on contexture and little on texture. Simulations show that the most useful contextural parameter for quantifying the scaling effects in vegetation–water mixed pixels is the water area fraction within each degraded pixel. Algorithms for remote sensing applications can be transported from one scale to another, if the information on the water body size is available. This study shows the need for global water masks at high resolutions for the purpose of accurate derivation of surface parameters maps at various resolutions. In boreal regions, this is particularly important because of the large number of small water bodies. Crown

Measurement of Isoprene Emissions over a Black Spruce Stand Using a Tower-Based Relaxed Eddy-Accumulation System*

Abstract Daytime isoprene emissions were measured over a black spruce forest in Saskatchewan (Canada) during the Boreal Ecosystem Atmosphere Study of 1994. The relaxed eddy-accumulation (REA) technique was used to measure isoprene fluxes in parallel with gradient measurements, which are required for using the gradient transport (GT) theory. The average isoprene flux was 2.29 mg C m−2 h−1 in late July and decreased to 0.54 mg C m−2 h−1 in early September. The senescent needles and lower ambient air temperature were most likely the cause of the lower isoprene emissions measured in September. A relationship of isoprene flux with air temperature was derived at the canopy scale because canopy temperature is not readily available. High isoprene emissions were observed at temperatures above 25°C. These were most likely in relation to thermoprotection of photosynthesis. The diurnal trends measured by GT and REA were similar. Isoprene fluxes measured using GT were 63% lower than those using REA. The underestimatio...

Source Footprint Considerations in the Determination of Volatile Organic Compound Fluxes from Forest Canopies

Above-canopy sampling of trace gases to determine volatile organic compound (VOC) emissions should be interpreted in terms of footprint considerations. This can be accomplished by defining the upwind canopy areas effectively sampled under the given wind and stability conditions. Using solutions of the advection–diffusion equation adjusted through controlled tracer gas (SF6) release experiments in the Boreal Ecosystem Atmosphere Study of 1994, sampling of VOC concentrations and fluxes over a forest whose VOC sources are heterogeneous (Camp Borden) are studied. Analysis demonstrates that the variability observed in measured VOC fluxes could be accounted for by varying numbers of randomly distributed clumps of emitter species within a varying footprint. It suggests that heterogeneity of the forest canopy, in terms of source distribution of VOCs, has to be explicitly considered in estimates of source strength from above-canopy concentration/flux sampling.

Aircraft-based volatile organic compounds flux measurements with relaxed eddy accumulation

Regional estimates of fluxes of volatile organic compounds (VOCs) are required to improve our understanding of their role in the chemistry of the atmosphere. Flux measurements on such a scale can best be obtained using aircraft-based systems. These systems usually rely on the eddy covariance technique, which requires fast response gas sensors for flux measurement, but such sensors are not available for most organic compounds, therefore, the relaxed eddy-accumulation (REA) technique was selected. An aircraft-based REA sampling system was developed and used to measure isoprene emission over the boreal forest during the 1996 summer. Over a short period in July at the Boreal Ecosystem/Atmosphere Study (BOREAS) southern study area (SSA), the isoprene fluxes ranged from −0.06 to 1.79 μg m -2 s -1, with a mean of 0.59±0.34 μg m -2 s -1, while in August at the BOREAS northern study area (NSA) the isoprene fluxes ranged from 0.00 to 0.26 μg m -2 s -1, with a mean of 0.14±0.09 μg m -2 s -1. In the SSA, the isoprene fluxes over aspen ranged from 0.44 to 1.79 μg m -2 s -1, with a mean of 0.92±0.33 μg m -2 s -1, whereas over black spruce, isoprene fluxes ranged from −0.06 to 0.54 μg m -2 s -1, with a mean of 0.36±0.21 μg m -2 s -1. The isoprene fluxes were exponentially correlated with solar radiation and radiative surface temperature. High correlations between isoprene fluxes and the fluxes of CO 2 and latent heat were also observed. Carbon lost through isoprene emissions was about 0.7 and 0.8% of the CO 2 assimilation rate for aspen and black spruce, respectively. The results demonstrate that the aircraft-based relaxed eddy-accumulation technique is a promising approach for quantifying the atmosphere–surface exchange of VOCs on a regional scale.

Controls on Evaporation in a Boreal Spruce Forest

The surface energy balance over a boreal spruce forest is analyzed using 3 yr of 30-min-averaged data collected during the 1994–96 Boreal Ecosystem–Atmosphere Study experiment 40 km west of Thompson, Manitoba, to show the climatic controls on surface evapotranspiration. The seasonal variation of evaporation is shown: lowest in spring when the ground is frozen, highest in summer (although daytime evaporative fractions are only 0.4), and lower again in fall after frost. The surface sensible heat flux in contrast is high in spring, when evaporation is low. Evaporation is much higher when the surface, including the moss layer, is wet. At all temperatures (in summer), evaporative fraction falls with increasing light level, because of the high vegetative resistance of the forest system. Using a Monin–Obukhov formulation and a bulk vegetation model, the vegetative resistance for the boreal spruce forest system is calculated. This bulk vegetative resistance decreases with increasing photosynthetic radiation, decreases sharply with relative humidity, decreases with increasing surface water storage, and is lower on cloudy days than on sunny days with the same incoming photosynthetic radiation. Vegetative resistance at its midmorning minimum is lower by a factor of 4 when the moss surface is very wet. As over grassland sites, the lower surface resistance to evaporation directly influences the diurnal cycle of lifting condensation level and cloud-base height, which are much lower on days with a wet surface. The reduction of vegetative resistance under cloudy skies at the same incoming radiation level presumably reflects the more efficient use of diffuse radiation by the canopy for photosynthesis. Vegetative resistance is roughly doubled in spring, when the ground is frozen, and is higher in fall after frost. About 63% of the observed variance in vegetative resistance can be explained in terms of meteorological variables using multiple linear regression. Some measurement issues are addressed in an appendix. The residual in the energy balance falls with increasing wind speed, which may be due to a small (10%–15%) underestimation of the sensible and latent heat fluxes at low wind speeds. During spring melt, however, this residual has a high daytime value of 30% of net radiation. The residual is also much higher on sunny days than on cloudy days.

Investigation of directional reflectance in boreal forests with an improved four-scale model and airborne POLDER data

Airborne Polarization and Directional Earth Radiation (POLDER) data acquired during the boreal ecosystem-atmosphere study (BOREAS) and the four-scale model of Chen and Leblanc (1997) are used to investigate radiative transfer in boreal forest. The four-scale model is based on forest canopy architecture at different scales. New aspects are incorporated into the model to improve the physical representation of each canopy, as follows: 1) Elaborate branch architecture is added. 2) Different crown shapes are used for conifer and deciduous forests. 3) Bilayer version of the model is introduced for forest canopies with an important understory. 4) Natural repulsion effect is considered in the tree distribution statistics. Ground measurements from BOREAS sites are used as input parameters by the model to simulate measurements of bidirectional reflectance distribution function (BRDF) from four forest canopies (old black spruce, old aspen, and old and young jack pine) acquired by the POLDER instrument from May-July 1994. The model is able to reproduce with great accuracy the BRDF of the four forests. The importance of the branch architecture and the self-shadowing of the foliage is emphasized.

Characterization of the Reflectance Anisotropy of Three Boreal Forest Canopies in Spring–Summer

As a part of the Boreal Ecosystem-Atmosphere Study (BOREAS), measurements of the spectral reflectance anisotropy of three boreal forest canopies were studied for cloudless sky conditions at the phenological growth stages which were at or near maximum leaf area index at each site. The three sites were relatively homogeneous mature stands of black spruce, jack pine, and aspen located in the southern boreal zone of central Saskatchewan. Measurements of the spectal bidirectional reflectance factors with a 15° instrument field of view in three spectral bands centered at 662 nm, 826 nm, and 1658 nm were made with the PARABOLA instrument over a range of solar zenith angles typically varying from 35° (near solar noon) to 70°. The measured reflectance factors showed large anisotropy at all three sites and for all three wavelengths, with prominant backscatter peak reflectances, and strong retro solar view angle (hot spot) maximum reflectances in the visible (662 nm) and shortwave infrared (1658 nm) for the jack pine and black spruce sites, with a less pronounced hot spot at the aspen site. Pronounced effects of canopy and understory shadowing in the visible, as a function of solar zenith angle (SZA), were observed for the black spruce and jack pine sites, with resultant large linear increases in computed normalized difference and simple ratio vegetation indices as SZA increased for near-nadir view angles. Hemispheric spectral reflectances or spectral albedos were computed from angular integration of PARABOLA measured bidirectional reflectances. Visible (662 nm) hemispheric reflectances for the jack pine and black spruce canopies showed very little variation with solar zenith angle, while near-infrared hemispheric reflectances increased strongly with increasing SZA. Estimates were made of the total shortwave albedo for the aspen and jack pine sites from irradiance and reflectance weighting of the spectral hemispheric reflectances in the three measured wavelengths. Comparison of estimated to pyranometer measured total albedo showed all estimates to be biased high, but only by about 0.007–0.018, depending on which of two sets of pyranometer measured albedos were utilized for the comparison. The measured bidirectional reflectance factor (BRF) data sets reported in this study coupled with ancillary data of biophysical parameters collected at the same sites by BOREAS researchers provide a unique data set for the development and characterization of canopy bidirectional reflectance modeling and for the interpretation of remotely sensed data for boreal forest canopies.

Methane dynamics of a northern boreal beaver pond

The production, oxidation, and transport of methane (CH4) from a boreal beaver pond were measured during the ice-free period in order to explain the observed flux of CH4 between the pond and the atmosphere. The study pond was located in the northern study area (NSA) of the Boreal Ecosystem Atmosphere Study (BOREAS), near Thompson, Manitoba. Three pathways transported the CH4 produced in the rich, organic anaerobic sediments. In order of importance they were: bubble flux (ebullition), diffusion, and plant transport. The water-air flux of CH4 was related to windspeed and sediment temperature, whereas the bubble flux also showed a degree of synchronicity with changes in atmospheric pressure and water level. By mid-June the storage of CH4 in the sediments reached a quasi steady-state, and production minus oxidation approximated the water-air exchange. Since the dominant pathway of ebullition is little affected by oxidation, the mean unit area flux of CH4 for the vegetated and open portions of the beaver pond (155 and 320 mg CH4 m−2day−1) were larger than the flux observed in most wetlands of the NSA. CH4 in bubbles from vegetated sites was significantly enriched in 13C and significantly depleted in 2H relative to bubbles from open water sites, consistent with a shift in methane production mechanism from acetate fermentation to CO2 reduction for the vegetated and unvegetated sites, respectively. The radiocarbon content of methane was not different between the two types of sites (114.7 ± 4.2pMC, vegetated versus 108.1± 6.5pMC, open water).

Seasonal gas exchange of black spruce using an automatic branch bag system

Four large cuvettes were used to measure whole-branch CO2 and H2O vapour exchange at the Boreal Ecosystem-Atmosphere Study, Southern Study Area, Old Black Spruce site. Measurements started before the spring thaw and continued until after the winter freeze-up. Daytime photosynthesis, nighttime respiration, transpiration rate, and branch conductances were zero at the start and the end of the measurement period. Maximum photosynthetic uptake rates were ca. 6 µmol·m-2·s-1 (0.02 µmol·g-1·s-1). Maximum nighttime respiration was ca. 0.8 µmol·g-1·s-1. Maximum transpiration rates were ca. 2 mmol·m-2·s-1 (7.1 µmol·g-1·s-1). Maximum branch conductance was ca. 35 mmol·m-2·s-1 (0.123 mmol·g-1·s-1). A simple model combining light and temperature responses accounted for 80-90% of the day-to-day variation in daytime carbon uptake. Total annual daytime carbon uptake ranged from 151 to 271 g·m-2 (530-868 mg·g-1). Total nighttime loss ranged from 12.7 to 15.5 g C·m-2 (22.4-27.3 mg C·g-1). Total transpiration ranged from 20.4 to 35.0 kg H2O·m-2 (70.2-112.2 g H2O·g-1).

Effects of climatic variability on the annual carbon sequestration by a boreal aspen forest

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.

Seasonal patterns and controls on net ecosystem CO2 exchange in a boreal peatland complex

We measured seasonal patterns of net ecosystem exchange (NEE) of CO2 in a diverse peatland complex underlain by discontinuous permafrost in northern Manitoba, Canada, as part of the Boreal Ecosystems Atmosphere Study (BOREAS). Study sites spanned the full range of peatland trophic and moisture gradients found in boreal environments from bog (pH 3.9) to rich fen (pH 7.2). During midseason (July‐August, 1996), highest rates of NEE and respiration followed the trophic sequence of bog (5.4 to −3.9 μmol CO2 m−2 s−1) < poor fen (6.3 to −6.5 μmol CO2 m−2 s−1) < intermediate fen (10.5 to −7.8 μmol CO2 m−2 s−1) < rich fen (14.9 to −8.7 μmol CO2m−2 s−1). The sequence changed during spring (May‐June) and fall (September‐October) when ericaceous shrub (e.g., Chamaedaphne calyculata) bogs and sedge (Carex spp.) communities in poor to intermediate fens had higher maximum CO2 fixation rates than deciduous shrub‐dominated (Salix spp. and Betula spp.) rich fens. Timing of snowmelt and differential rates of peat surface thaw in microtopographic hummocks and hollows controlled the onset of carbon uptake in spring. Maximum photosynthesis and respiration were closely correlated throughout the growing season with a ratio of approximately 1/3 ecosystem respiration to maximum carbon uptake at all sites across the trophic gradient. Soil temperatures above the water table and timing of surface thaw and freeze‐up in the spring and fall were more important to net CO2 exchange than deep soil warming. This close coupling of maximum CO2 uptake and respiration to easily measurable variables, such as trophic status, peat temperature, and water table, will improve models of wetland carbon exchange. Although trophic status, aboveground net primary productivity, and surface temperatures were more important than water level in predicting respiration on a daily basis, the mean position of the water table was a good predictor (r2 = 0.63) of mean respiration rates across the range of plant community and moisture gradients. Q10 values ranged from 3.0 to 4.1 from bog to rich fen, but when normalized by above ground vascular plant biomass, the Q10 for all sites was 3.3.

Transport of carbon dioxide, water vapor, and ozone by turbulence and local circulations

Nocturnal land breezes and daytime lake breezes are studied using data collected by the Canadian Twin Otter aircraft and a deck boat which traversed Candle Lake during the Boreal Ecosystem‐Atmosphere Study (BOREAS). The nocturnal vertical transport of CO2, water vapor, and ozone over the lake consists of two parts: (1) mesoscale rising motion associated with land breeze convergence and (2) significant turbulence and vertical mixing driven by buoyancy in the lower part of the internal boundary layer and shear generation in the top part of the internal boundary layer. For comparison, the role of the lake in the daytime is examined in terms of formation of a stable internal boundary layer due to advection of warm air from land with small CO2 concentration over the cooler lake surface. Analysis of the aircraft and boat data indicates that the nocturnal land breeze plays an important role in the regional CO2 budget in the lake region. In the present study, CO2 is advected horizontally by a nocturnal land breeze circulation and vented vertically over Candle Lake (“chimney effect”). Such near‐surface horizontal transport implies that part of the respirated CO2 never reaches the tower observational level, particularly under light wind conditions. This study speculates that preferred locations of vertical venting of CO2 may also occur due to convergence of nocturnal drainage circulations or flow meandering, although probably weaker than that associated with the land breeze. These circulations partly explain recent findings that tower‐measured nocturnal turbulent fluxes of CO2 above the canopy and the subcanopy storage of CO2 frequently sum to less than the total respiration of CO2, leading to “missing CO2.” Unfortunately, the present study does not allow evaluation of all of the terms in the carbon dioxide budget.

Snow breezes in the boreal forest

This study examines thermally induced flows (or “snow breezes”) associated with snow cover in the boreal forests of Canada. Observations from a lake less than 4 km across were made as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS) winter field campaign. These are interpreted with the aid of idealized three‐dimensional mesoscale model simulations representing the forest‐lake contrast. Typically, strong forest‐lake temperature contrasts develop in the lowest 50 m of the atmosphere during the morning. The resulting pressure gradients induce low‐level onshore wind components across the lake. This snow breeze persists into the afternoon provided that large‐scale winds remain light. A characteristic snow breeze signature is clearly evident in wind observations averaged over 27 days of data, in agreement with model simulations. The study suggests that snow breezes will regularly develop over the many larger lakes and other unvegetated areas in the region.

Improving snow cover mapping in forests through the use of a canopy reflectance model

MODIS, the moderate resolution imaging spectroradiometer, will be launched in 1998 as part of the first earth observing system (EOS) platform. Global maps of land surface properties, including snow cover, will be created from MODIS imagery. The MODIS snow-cover mapping algorithm that will be used to produce daily maps of global snow cover extent at 500 m resolution is currently under development. With the exception of cloud cover, the largest limitation to producing a global daily snow cover product using MODIS is the presence of a forest canopy. A Landsat Thematic Mapper (TM) time-series of the southern Boreal Ecosystem-Atmosphere Study (BOREAS) study area in Prince Albert National Park, Saskatchewan, was used to evaluate the performance of the current MODIS snow-cover mapping algorithm in varying forest types. A snow reflectance model was used in conjunction with a canopy reflectance model (GeoSAIL) to model the reflectance of a snow-covered forest stand. Using these coupled models, the effects of varying forest type, canopy density, snow grain size and solar illumination geometry on the performance of the MODIS snow-cover mapping algorithm were investigated. Using both the TM images and the reflectance models, two changes to the current MODIS snow-cover mapping algorithm are proposed that will improve the algorithm's classification accuracy in forested areas. The improvements include using the normalized difference snow index and normalized difference vegetation index in combination to discriminate better between snow-covered and snow-free forests. A minimum albedo threshold of 10% in the visible wavelengths is also proposed. This will prevent dense forests with very low visible albedos from being classified incorrectly as snow. These two changes increase the amount of snow mapped in forests on snow-covered TM scenes, and decrease the area incorrectly identified as snow on non-snow-covered TM scenes.

Atmospheric Methane Consumption by Forest Soils and Extracted Bacteria at Different pH Values.

The effect of pH on atmospheric methane (CH4) consumption was studied with slurries of forest soils and with bacteria extracted from the same soils. Soil samples were collected from a mixed hardwood stand in New Hampshire, from jackpine and aspen stands at the BOREAS (Boreal Ecosystem Atmosphere Study) site near Thompson, northern Manitoba, from sites in southern Québec, including a beech stand and a meadow, and from a site in Ontario (cultivated humisol). Consumption of atmospheric CH4 (concentration, approximately 1.8 ppm) occurred at depths of >5 cm in both acidic (pH 4.5 to 5.2) and alkaline (pH 7.2 to 7.8) soils. In slurries of acidic soils, maximum activity occurred at different pH values (pH 4.0 to 6.5). Bacteria extracted from these soils by high-speed blending and density gradient centrifugation showed pH responses different from the pH responses of the slurries. In all cases, these bacteria had a methanotrophy pH optimum of 5.8 and exhibited no activity at pH 6.8 to 7.0, the pH optimum range for known methanotrophs. This difference in pH responses could be useful in modifying media currently used for isolation of these organisms. Methanotrophic activity was induced in previously non-CH4-consuming soils by preincubation with 5% (vol/vol) CH4 (50,000 µl of CH4 per liter) or by liquid enrichment with 20% CH4. The bacteria showed pH responses typical of known methanotrophs and not typical of preexisting consumers of ambient CH4. Furthermore, methanotrophs induced by high CH4 levels were more readily extracted from soil than preexisting ambient CH4 consumers were. In the alkaline soils, preexisting activity either was destroyed or resisted extraction by the procedure used. The results support the hypothesis that consumers of ambient CH4 in soils are physiologically distinct from the known methanotrophs.

Climatology of gravity waves in a forest

AbstractThe objectives of this study are to establish a climatology of gravity waves in a forest and to gain insights into the dynamics of this motion type. The site is part of the BOREAS (Boreal Ecosystem—Atmosphere Study) flux monitoring network. The analysis relies mainly on 5 Hz temperature time series observed with two arrays of fine‐wire thermocouples deployed in the vertical (up to 40 m above the ground) and horizontal (separation up to 150 m) from July to November 1996. Supporting measurements include mean wind speed and direction at two heights above the forest.Waves of various intensities are found over 40% of the night‐time observations, emphasizing the need to understand its role in transporting momentum and masses between the vegetation and the atmosphere. These waves are shear‐generated. They propagate in the direction of the mean wind at a speed that is most likely to fall between the mean wind speeds at z/h = 1.2 and 1.8, where z is the height above the ground and h is the mean tree height. The median values of wave speed, wavelength, wave vertical displacement, and wave frequency are 1.61 m s−1, 75 m, 10 m and 0.0214 Hz, respectively. The wave motion remains coherent in the horizontal over less than one wavelength but is in general persistent in time.