Boreal forest

John W. Pomeroy and Richard J. Harding

Relevance and characteristics

The boreal forest in winter is a complex mosaic of land surface types varying from closed coniferous canopies, mixed-wood deciduous forests, sparsely vegetated areas (clearings, wetlands, clear-cuts, burns) to ice and snow-covered lakes. At roughly 20% of the earth's land area, the boreal forest is the largest terrestrial type of land cover and extends in a circumpolar band across North America, Europe, and Asia. Canada, Russia, and the Scandinavian countries are dominated by boreal forest, although in most cases it lies to the north of major population centers. Economic activities in the boreal forest center on forest harvesting, tourism, and mining, yet many boreal forests retain an indigenous aboriginal population whose members conduct aspects of a traditional hunting, fishing, and gathering lifestyle. The large number of lakes and rivers (up to 40% of some boreal regions are covered by water) promote fishing and water transport that has been fundamental to the development of northern Canada and Siberia. Recent environmental concerns focus on the extensive clearcutting of boreal forest in Canada and Russia, episodic acidic precipitation (including snow) from anthropogenic pollution, and the apprehension that climate warming will result in a major northward shift in the boreal climate zone and loss of forest lands in the southern boreal forest.

One distinction of the boreal forest in comparison to more temperate forests is its long snow-covered period and cold winter temperatures (Harding and Pomeroy,

1996). The depth, density, and duration of snow cover is ecologically important to mammals and various microbial life-forms in this forest; in some cases the snow cover provides a thermally moderated habitat, in others a means of avoiding predators (Jones et al., 2001). Boreal forest productivity and carbon cycling are strongly influenced by the supply of available nitrogen and soil moisture. Snow influences productivity and carbon cycling by providing, upon melt and infiltration, a significant portion of the annual water and inorganic nitrogen input (Pomeroy et al., 1999a). The global boreal forest exerts a strong control on climate and because of its low winter albedo, its removal and the resulting higher albedo in spring might result in a cooling of the Northern Hemisphere (Thomas andRowntree, 1992). Snowmelt provides 40-60% of annual streamflow from boreal forests, with increases in snowmelt runoff of 24-75% when forest cover is removed (Hetherington, 1987).

Boreal forest snow covers are strongly influenced by the forest canopy, its interception of snow and radiation and dampening of wind speed and mixing above the snowpack's surface. Pomeroy et al. (1998a) observed in a mixed range of boreal forest cover types that 20-65% of cumulative snowfall was intercepted in early winter, and 10-45% of snowfall sublimated over the season. Leaf area strongly controls interception efficiency (Hedstrom and Pomeroy, 1998) and clearcutting or conversion of coniferous stands to deciduous species reduces interception to insignificant levels (Pomeroy and Granger, 1997). The energetics of intercepted snow in the boreal forest have been studied by Nakai et al. (1993, 1994, 1999), Lundberg and Halldin (1994), Harding and Pomeroy (1996), Pomeroy and Dion (1996), Pomeroy et al. (1998a) and Parviainen and Pomeroy (2000). These studies show that the albedo of snow-covered forest canopies is low (<0.2), that sublimation rates up to 3 kg m-2 d-1 are possible from snow-covered canopies and that the direction and magnitude of sensible and latent heat fluxes are influenced by the presence of snow in the canopy because it represents a "wetter," cooler surface than a snow-free canopy. Parviainen and Pomeroy (2000) suggest that sublimation is driven by local-scale advection of sensible heat from exposed branches heated in the sun to intercepted snow clumps and that the efficiency of this advec-tion is related to the fractal geometry of intercepted snow clumps (Pomeroy and Schmidt, 1993).

Studies of snow under the canopy have been directed towards snow accumulation and melt prediction. Boreal forest snow covers have relatively low coefficients of variation of snow water equivalent (0.04-0.14) and maximum densities near 200 kg m-3 (Pomeroy et al., 1998b). At small scales, the snow water equivalent generally decreases with distance from coniferous tree stems (Woo and Steer, 1986; Jones, 1987; Sturm, 1992) and at stand scales it decreases with increasing canopy density (Kuz'min, 1960; Pomeroy and Gray, 1995). The forest cover attenuates the magnitude of incoming shortwave radiation and large-scale advection of warm air, reducing the "connectivity" between subcanopy snow and the atmosphere. Ni et al. (1997) and Pomeroy and Dion (1996) have measured and modeled winter sub-canopy radiation and found its magnitude greatly reduced from the above canopy values and strongly dependent on solar zenith angle, leaf area, and needle orientation. Typically, subcanopy net radiation in a mature conifer stand is one-tenth that above the canopy at the time of snowmelt. Davis et al. (1997), Hardy et al. (1997a, b), and Metcalfe and Buttle (1995,1998) have measured and modeled snow ablation under boreal forest canopies and conclude that though the magnitude of net radiation is strongly reduced in forests and decreases with increasing canopy density, it still comprises the largest component of the energy balance because the subcanopy snow albedo drops substantially during melt as forest leaf litter and debris in the snowpack are exposed and because subcanopy turbulent fluxes are extremely small in magnitude and usually of the opposite direction. Faria et al. (2000) suggest that because subcanopy snowmelt energy and the pre-melt snow water equivalent have a spatial covariance, depletion of the snow-covered area is accelerated as the covari-ance increases. Pomeroy and Granger (1997) compared melt rates in various forest types and found that melt timing was accelerated three-fold in a clear-cut compared to under a mature boreal forest canopy because the net melt energy was up to four times greater in the clear-cut.

Site

Beartrap Creek (550 m a.s.l.) is a research basin, located at 54° N, 106° W, near the village of Waskesiu Lake, in Prince Albert National Park, Saskatchewan, Canada. The site has been the subject of intense investigations of boreal forest hydrology and climate under the Mackenzie global energy and water cycle experiment (MAGS), Prince Albert model forest hydrology study, and boreal ecosystem-atmosphere study (BOREAS). The region has a subhumid continental climate with six months of snow cover during a cold dry winter that experiences few melt events until April. Mean annual precipitation is 463 mm w.e. of which 33% occurs as winter snowfall. Topography is rolling with 700 m of local relief. Forest cover is typical of mature southern boreal forest: pine and mixed stands of aspen and white spruce on uplands, spruce, larch, and open muskeg in lowlands and about 15% covered by lakes.

The site studied is a mature, slightly open jack pine (Pinus banksiana) stand, 16-22 m tall with a winter leaf and stem area index of 2.2 m2 m-2 and canopy coverage of 82%. The fetch is level and uniform for about 100 m. Experiments were conducted in March, 1994 and 1996, using a canopy access tower (27 m). At the tower top two eddy correlation flux systems were installed, an Institute of Hydrology "Hydra" system for sensible and latent heat and a Gill Instruments "Solent" 3-axis sonic anemometer (Harding and Pomeroy, 1996). Above canopy net radiation and ground heat flux were measured using radiation and energy balance systems "REBS" net radiometers and heat flux plates and below canopy net radiation was measured using a Delta "T" tube net radiometer. Above canopy wind speed was measured using an RM Young propeller anemometer and temperature using a Vaisala HMP35CF hygro-thermometer. The intercepted snow load was measured using a suspended full size pine tree, which was weighed with an in-line force transducer (Hedstrom and Pomeroy, 1998). Weight of snow on the tree (kg) was converted to an areal mass (kg m-2) using an empirical conversion developed from comparing event-based snow interception on the single tree to areal interception determined from above canopy snowfall measurements and changes in snow accumulation along a line of 25 snow survey points in subfreezing conditions.

Energy balance

Two sets of energy balance and related surface conditions are shown. Figure 3.11 shows sensible and latent heat flux measurements made with a Hydra and checked against a Solent sonic anemometer along with net radiation above the canopy over a five day period in late March 1994. Fresh snowfall resulted in an initial intercepted load of about 4.5 kg m-2 on 27 March which then sublimated in temperatures ranging from -13 to 0 ° C until the end of 29 March, when above freezing temperatures (5 °C) resulted in melt and the unloading of any remaining snow.

Daily maximum temperatures then increased dramatically to 17 °Con31 March resulting in some early melt under the canopy. Four days had high net radiation inputs, i.e. negative peaks ranging from -450 to -500 W m-2, while 28 March was overcast with a peak net radiation of only -100 W m-2. When snow was in the canopy (27-29 March), daytime latent heat fluxes were directed away from the surface at approximately one-half the magnitude of net radiation. Sensible heat fluxes over the snow-covered canopy were similar in magnitude and direction to latent fluxes on the high-insolation days (27 and 29 March) but negligible on 28 March when low insolation resulted in minimal canopy heating. When the canopy snow load ablated (30 March), the daytime magnitude of sensible heat flux remained half that of net radiation but latent heat became negligible early in the day and directed downward later in the day. On the 31 March sensible heat behavior was unchanged but latent heat became directed upward at one-half the magnitude of sensible heat. This may reflect evaporation from melting snow beneath the canopy or, more likely given the magnitude, transpiration from the pine canopy induced by extraordinarily warm temperatures. The weighed tree did show some weight loss in this period reflecting desiccation due to evapotranspiration.

Figure 3.12 shows a consistently subfreezing sequence from the same site on 16-18 March 1996. A Solent sonic anemometer measured sensible heat fluxes (not latent) and the ablation rate of intercepted snow was measured using the weighed tree. The ablation rate was converted to equivalent energy units (flux) as if all the energy was consumed for phase change to vapor (a reasonable assumption given the -15 to -1 °C air temperatures).

0000 1200 0000 1200 0000 1200 0000 1200 0000 1200

Figure 3.11. Fluxes and climate measured above a jack pine stand in the southern boreal forest of Saskatchewan, Canada, March, 1994. (a) Latent heat, sensible heat, and net radiation fluxes measured five meters above an initially snow-covered canopy and (b) air temperature and wind speed measured five meters above the canopy.

0000 1200 0000 1200 0000 1200 0000 1200 0000 1200

Figure 3.11. Fluxes and climate measured above a jack pine stand in the southern boreal forest of Saskatchewan, Canada, March, 1994. (a) Latent heat, sensible heat, and net radiation fluxes measured five meters above an initially snow-covered canopy and (b) air temperature and wind speed measured five meters above the canopy.

Figure 3.12. Fluxes about a boreal jack pine canopy, Saskatchewan, Canada, March, 1996. (a) Sensible heat, net radiation, subcanopy net radiation, ground heat flux, and estimated latent heat flux from intercepted snow ablation. Above canopy fluxes were measured 5 m above the canopy, below canopy radiation 1 m above the snow cover and ground heat flux 5 cm into the soil. Intercepted snow ablation was measured using a weighed, suspended full-size jack pine tree. (b) Air temperature and wind speed measured 5 m above the pine canopy. Intercepted snow load measured using a weighed, suspended pine tree.

Figure 3.12. Fluxes about a boreal jack pine canopy, Saskatchewan, Canada, March, 1996. (a) Sensible heat, net radiation, subcanopy net radiation, ground heat flux, and estimated latent heat flux from intercepted snow ablation. Above canopy fluxes were measured 5 m above the canopy, below canopy radiation 1 m above the snow cover and ground heat flux 5 cm into the soil. Intercepted snow ablation was measured using a weighed, suspended full-size jack pine tree. (b) Air temperature and wind speed measured 5 m above the pine canopy. Intercepted snow load measured using a weighed, suspended pine tree.

Subcanopy net radiation and ground heat flux were also measured. An initial snow load of 4.2 kg m-2 ablated to 1.5 kg m-2 at the end of 16 March (strong winds, cold temperatures, and high insolation) and completely ablated by the end of 17 March. In this case sensible heat flux showed similar behavior to that in Fig. 3.11,at about one-half the magnitude of net radiation when the canopy is snow covered, increasing to three-quarters when snow free. Latent heat flux estimated from ablation equaled the sensible heat magnitude on the first (most snow-covered) day, then dropped to one-half the sensible magnitude on the second day and became negligible on the third day (snow-free canopy). Subcanopy net radiation was never more than one-tenth that of above canopy values, but remained slightly positive on 16 March when fresh snow covered the canopy and, along with a cool air mass, suppressed canopy temperatures and therefore downward longwave radiation. Ground heat fluxes were extremely small in magnitude and slowly fluctuated around zero.

Modeling aspects

The snow-covered canopy represents a separate snow layer that warrants its own mass and energy balance but is not represented by many land surface schemes (Essery, 1997). For instance, the ECMWF model recently used a routine that set the boreal forest canopy albedo to a high value (0.8) after a snowfall. When corrected to a much lower and appropriate value, air temperature predictions improved dramatically over the boreal region (Betts and Ball, 1997). CLASS and SiB are exceptions that do consider canopy snow, but calculate the snow interception process in a similar manner to rainfall and therefore underestimate intercepted load by an order of magnitude for large snowfalls (Pomeroy et al., 1998b). The interception models of Calder (1990) and Hedstrom and Pomeroy (1998) provide possible corrections to these schemes. Turbulent fluxes above a snow-covered canopy can be calculated using a resistance scheme with resistance set at 10 times the value of a rain-wetted canopy (Lundberg et al., 1998) or by varying the ratio of bulk transfer coefficients with snow load (Nakai et al., 1999). Parviainen and Pomeroy (2000) modeled the Beartrap Creek pine site using a nested control volume approach in which an energy and mass balance was conducted for an intercepted snow control volume, using the Reynolds number to calculate turbulent transfer between the intercepted snow and the atmosphere. The CLASS land surface scheme was then coupled to this small-scale calculation to calculate turbulent transfer between the canopy and atmosphere. Using a geometric radiative transfer model (GORT) to calculate subcanopy radiation (Ni et al., 1997) and SNTHERM to calculate snow-pack heat fluxes, snowmelt modeling was successful when snow surface albedo was reduced during melt and turbulent heat fluxes were given small values (Hardy etal, 1997b).

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Renewable Energy 101

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