The first boreal ecosystem we consider is that most analogous to water, namely, the wetland. During summer, P averaged 0.8 mm day-1 less than E for wetlands (Table 3). However, by definition, this comparison is incomplete for fens because water supply to this system exceeds rainfall by virtue of ground-water intrusion, although this is often difficult to quantify with certainty. In Table 3, the Saskatchewan, Zotino, and Schefferville sites were fens but it is not clear if ground water supplemented water supply in the other wetlands studied. Generally and on average, E = £eq at 2.6 mm day"1. Dry air inevitably entrains into the convective boundary layer (i.e., advection from aloft) on fine summer days in the boreal zone at a rate proportional to the surface sensible heat flux density. Consequently, the attainment of E = £eq suggests either that one of the aforementioned limits of the Penman- Monteith equation has been reached or that the surface was relatively dry, as can happen even for a wetland. Indeed, for the two cases where E < Eeq, conditions during the measurements were at least sometimes considered relatively dry. Although advection enhanced E at Southern James Bay, the similarity between P and E there is striking (Rouse et al, 1987). The topography and hydrology involved in the formation and maintenance of the wetlands studied may be such that these ecosystems are often not sufficiently extensive (i.e., the wetland landscape is patchy) to avoid the influence of horizontal advection.
For tundra sites at 60°-80°N, P has generally been observed to be nearly identical to E in summer, averaging 1.5 mm day-1. There is no clear trend of E or P changing with latitude for these sites.
TABLE 3 Average Rates of Precipitation (P) and Evaporation (E) and the Ratio of E and the Equilibrium Evaporation Rate (£/£eq) during Summer from Boreal /.one Vegetation
Southern James Bay Central Hudson Bay Kinosheo Lake Schefferville Thompson Zotino
Hardangervidda King Christian Island Axel Heiberg Island Imnavait Creek LI-pad
Happy Valley Deciduous broad-leaved forest (full leaf) Betula/Popnlus, Moscow Populus/Corylus, Prince Albert Deciduous needle-leaved forest (full leaf) Larix, Churchill Lttrix, Yakutsk Evergreen needle-leaved forest Pinns, ladraas Pinns, Lac du Bonnet Pinns, Norunda Pinns, Nipawin Pinns, Zotino Picea, Schefferville Picea, Candle Lake
58.7 N/94.1 W 51.6 N/81.8 W 54.9 N/66.7 W 55.9 N/98.4 W 61.0 N/89.0 E 54.0 N/105.0 W
60.8/16.5 E 50.2/95.9 W 60.3 N/17.3 E 53.0 N/i 04.0 W 61.0 N/89.0 E
2.9 1.0 Suyker and Verma, 1998
2.0 1.0 Skartveit et al., 1975 0.9 0.6 Addison and Bliss, 1980
1.3 n.d. Vourlitis and Oechel, 1997 1.5-2.7 0.6-0.9 McFadden et al., ¡998
4.7 1.1 Rauner, 1976
2.2 1.0 Black etal, 1996
2.1 0.8 Lafleur, 1992
3.4 0.9 Lindroth, 1985
1.4 0.8 Amiro and Wuschke, 1987 1.9 0.9 Grelleeffl/., 1997
1.5 0.6 Baldocchi et al., 1997
1.3 0.4 Kelliher etal., 1998
1.8 0.5 Fitzjarrald and Moore, 1994 2.0 0.7 Jarvis etal, 1997
'average for June through August from Muller (1982).
''from Baldocchi et al. (1997) but excludes a 110 mm storm, on a day halfway through the 117 day study and following a week of 23.1 mm rainfall, that was assumed to drain bevond the root zone of the sandv soil.
This suggests that local regimes can be influenced by other factors such as proximity to the sea and elevation/orography (see Vourlitis and Oechel, 1997). In one far-north study where only 9% of the site was covered by vascular plants, E « £C(] although E> P (Addison and Bliss, 1980). The tundra landscape is also patchy (see Skartveit et al., 1975). This leads to some variability in E although soil and sensible heat flux densities are much more spatially variable (McFadden et al, 1998). The relatively lower net available energy and consequently the sensible heat flux densities in the northern part of the boreal zone during summer probably limit entrainment there. The tight water balance and nutrient cycles and generally lower temperatures seem to have an overwhelming influence on the attained evaporation rate of tundra. Consequently, in general for tundra, we also find E = Ea].
Boreal zone forest stands are also part of a patchy landscape at least in terms of tree age, size, and density, which determine stand structure. More commonly, though, the stands of trees are interspersed with herbaceous and shrubby vegetation and wetlands. On average, P is found to be 0.5 mm day-1 less than E (= 2.3 mm day-1) for forests in summer. Rainfall interception has been included in this comparison because E comprised wet and dry canopy evaporation rates except for the Yakutsk study by Kelliher et al (1997). The difference between rainfall and evaporation rates is up to 2 mm day-1 for the two oldest studies. These studies also report by far the highest evaporation rates. These E measurements were made using the Bowen ratio technique, which is notoriously difficult to employ over forests because of the small gradients in temperature and humidity there. Except for Lalleur (1992), who also used this technique, the other measurements were made directly by eddy covariance.
For the two broad-leaved forests studied, E was found to be significantly greater than or equal to P but E ~ E on average, while £/£eq averaged only 0.7 for the needle-leaved forest studies. The two Larix forests had remarkably similar P and £, but £ was 0.6 mm day-1 higher at the eastern Siberian site mostly reflecting the effect of warmer temperatures there. Nevertheless, £/£cq < 1 for both Larix and Pinus forests. These data reflect the well-known degree of surface control of £ by needle-leaved forests that follows from examination of Eq. (4) as gs/g\ declines (McNaughton and
Jarvis, 1983). For nearly 30 years, beginning with the seminal Pinus sylvestris forest study of Stewart and Thom (1973), this has been interpreted to indicate or demonstrate the importance of stomatal control of forest evaporation. This is generally correct for relatively well-watered needle-leaved forests of the temperate zone (as noted by Kelliher et al (1998), precious few studies of forest £ have been done in stands subject to soil water deficit) that possess large leaf areas, but broad-leaved forests are not so straightforward (Baldocchi and Vogel, 1996). Moreover, extension of this temperate zone knowledge to the much drier and thus usually sparse forests of the boreal zone may be perilous. This is because forests of the boreal zone generally have two significant sources of £ namely, the tree canopy as implied above and the understorey including vegetation and soil (Table 4). Consequently, quantitative analysis of £ in terms of gs requires caution in terms of a basic tenet of Eq. (4) (i.e., commonality of height/location of the momentum sink and heat and water vapor sources). This is one of the many reasons we mostly examine boreal zone £ in terms of Eeq.
Looking more closely at the boreal Pinus forest data, while P is relatively similar for the five sites, £ from the three Pinus sylvestris stands varies by a factor of 2.6. This is mostly attributable to data from the two Swedish stands that are not far apart and yet E at Jadraas is nearly twice that at Norunda. However, £/£eq is the same and relatively large for both sites, suggesting that the difference in £ may be attributed to vagaries of the weather during the two studies. This may explain the large defiance of mass/water conservation during Lindroth's study, where E»P. In central Siberia P < E, reflecting a greater contribution of ground evaporation that was much more closely coupled to rainfall frequency limited to a major storm at the beginning of the 18-day study (Table 4). Tree transpiration does not vary so dynamically and, at first glance including Eqs. (2) and (4), its contribution to £ seems simply proportional to the overstorey leaf area index regardless of the plant lifeform, genus, or species. Although this may seem an intuitive conclusion, it only accounts for the physics involved in radiation interception and energy balance and the quantity of leaves. The recently published contrary data of Zimmermann et al (2000) are unique. They found that the stand-level transpiration rates of Pinus sylvestris trees in a central Siberian chronosequence (aged
TABLE 4 Measured Daily Values of the Percentage of Forest Evaporation (Average with Range in Parentheses) Emanating from the Understorey during Summer in Six Boreal Forests
Overstorey Leaf Area Index
% Evaporation from Understorey
Deciduous broad-leaved forest (full leaf)
Papains 5.1 Deciduous needle-leaved forest (full leaf)
Larix 1.5 Evergreen needle-leaved forest
Cory! us 22
Vaccinium, Arctostaphylos 50
Vaccinium, moss 17 Vaccinium, Arctostaphylos/lichen 25 (10-40)
lichen 54 (33-92)
Black et al., 1996
Lafleur and Schreader, 1994
Kelliher et al, 1997
Grelle et al, 1997 Baldocchi er ill., 1997 Kelliher et«/., 1998
TABLE 5 Maximum Hair-Hourly and Daily Forest Evaporation Rates (£„,„) Measured by the Eddy Covariance Technique above Each of Four Pinus sylvestris Stands during July 1996
Tree Age Half-Hourly £m ,x Daily £„„x Tree Leaf Nitrogen Content
(years) Tree Leaf Area Index1' (mm h"!) (mmd-1) Daily EnuJEeCi (mgg-1)
'from Rebmann ct aI.. 1999.
computed from leaf biomass and leaf area density data of Christian Wirth (personal communication). ' from Kelliher ct al., 1998.
The chronosequence of stands, located 40 km from the village of Zotino in central Siberia, includes an open woodland with a low density of relatively large trees of undetermined age. The values of were measured during fine weather when the tree canopy was dry. Also shown are the ratio of daily £I(W, to the equilibrium rate (£ ) and tree leaf nitrogen content.
28-430 years) during July 1995 were proportional to sapwood area and not leaf area index or tree age.
Forests are long-lived vegetation. Although fire probably always limits the lifespan of trees in the boreal zone, they can live for hundreds of years. We are aware of only one study of forest E in a chronosequence of stands. This was also conducted near Zotino, central Siberia, in July 1996; eddy covariance measurement systems were employed simultaneously at four sites within a ca. 20-km2 area (Rebmann et al., 1999; Table 5). The trees varied in age by over 200 years and all were growing on well-drained sand except for a 53-year-old stand located at a drainage collection area where the ground water was < 1 m beneath the surface throughout the measurements. This seemed to account for the relatively higher tree leaf area index there (Table 5). However, stand disturbance by fire and the time elapsed therafter are generally considered more influential in determining tree leaf area index in the region (Wirth et al., 2000). For comparison of the data, we begin with the capacity for evaporation by examining maximum half-hourly E during fine weather when the canopy was dry. It is essentially indistinguishable throughout the four stands. This suggests no effect of tree aging, in agreement with Zimmermann et al. (2000), although the dominance of ground evaporation in the well-drained stands when E is at a maximum after rainfall was also important. This leads to examination of the maximum daily E during fine weather and the corresponding _Eeq. The daily data from the 7- and 215-year-old stands and the open woodland were from the first fine day after a significant 12 mm of rain. Nevertheless, the daily data vary across the chronosequence, as noted by Schulze et al. (1999) for average values. The highest values of daily maximum E and £/£cq were obtained in stands with the highest tree leaf area index. These stands had the highest tree canopy transpiration rates. The 53-year-old stand had a relatively high tree leaf nitrogen content and the highest leaf area index and E, emphasizing the contribution of tree transpiration. By contrast, owing to the relatively short time since disturbance, the 7-year-old stand had the highest tree leaf nitrogen content but the lowest leaf area index and significantly lower E and E/Eiq. Soil evaporation composed half of E in the 215-year-old stand (Kelliher et al,
1998) and the fraction was probably greater in the youngest stand and open woodland. Even on the day after rainfall, atmospheric demand evidently can exceed water supply in these younger stands so that soil evaporation is limited by a drying surface layer.
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