Evaporation from an eastern Siberian larch forest

AGRICULTURAL AND FOREST METEOROLOGY

Ei

ELSEVIER

Agricultural and Forest Meteorology 85 (1997) 135 - 147

Evaporation from an eastern Siberian larch forest F.M. Kelliher a,., D.Y. Hollinger a,1, E.-D. Schulze b, N.N. Vygodskaya c, J.N. Byers a, J.E. Hunt a, T.M. McSeveny a, I. Milukova c, A. Sogatchev c, A. Varlargin c, W. Ziegler d, A. Arneth b.2, G. Bauer b a Manaaki Whenua-Landcare Research, P.O. Box 69, Lincoln, New Zealand b Unwersitiit Bayreuth, Lehrstuhlfiir Pflazeni~kologie, Box 101251, D-95440 Bayreuth, Germany c Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Leninsky Prospect 33, 117071, Moscow, Russia d Comenius University, Department of Biophysical and Chemical Physics, Mlynska Dolina F1, CS-842 15, Bratislava, Slovak Republic Received 8 July 1996; accepted 19 November 1996

Abstract Total forest evaporation (AE), understorey evaporation, and environmental variables were measured on nine summer days under different weather conditions in a 130-year-old stand of Lar/x gmelinii (Rupr.) Rupr. trees located 160 km south of Yakutsk in eastern Siberia, Russia (61°N, 128°E, 300m above sea-level (a.s.l.)). Tree and broad-leaved understorey vegetation one-sided leaf area indices were 1.5 and 1.0, respectively. Agreement of AE and sensible heat flux (H), both measured by eddy covariance, and the available energy (R a) was generally good: (H + A E ) = 0.83R a + 9 W m -2 with r 2 = 0.92 for 364 half-bour periods and the mean + 95% confidence limit was 129 + 17 for (H + AE) and 144 _ 19 for R a. Dally E was 1.6-2.2mm, less than half of the potential evaporation rate and accounting for 31-50% of Ra, with the lowest percentage on clear days. A perusal of the sparse literature revealed that average daily E of boreal coniferous forest during the tree growing season (1.9 mm day-1 for this study) is relatively conservative, suggesting that low evaporation rates are a feature of this biome's energy balance. Using the Penman-Monteith equation, the maximum bulk-surface conductance (Gsmax) was 10mms -1. E and Gs were regulated by irradiance, air saturation deficit, and surface soil water content during a week-long dry period following 20mm rainfall. From lysimeter measurements, 50% of E emanated from the understorey at a rate proportional to R a. Based on the measurements and published climatological data, including average annual precipitation equal to 213mm, water balance calculations indicated growing season forest E equal to 169mm, the occurrence of a late summer-autumn soil water deficit, and annual runoff of 44 mm by snowmelt. © 1997 Elsevier Science B.V. Keywords: Larch; Larix gmelinii; Total forest evaporation; Understorey evaporation

1. Introduction * Corresponding author z Present address: US Forest Service, P.O. Box 640, Durham, NIl 03824, USA. 2 Present address: Lincoln University, Department of Soil Science, P.O. Box 84, Lincoln, New Zealand.

The eastern Siberian taiga is an immense forest, dominated by the deciduous conifer tree Lar/x gmelinii (Rupr.) Rupr. that grows throughout a 1 × 10 6 km 2 area (Shvidenko and Nilsson, 1994). De-

0168-1923/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S 0 1 6 8 - 1 9 2 3 ( 9 6 ) 0 2 4 2 4 - 0

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F.M. Kelliher et al. / Agricultural and Forest Meteorology 85 (1997) 135-147

spite the potential importance of this vast Asian boreal region's surface-atmosphere energy exchange for regulating northern hemisphere climate, there is scant micrometeorological information about Siberian larch forest (HoUinger et al., 1995; Lloyd et al., 1996) although habitat and tree growth information, and analyses of relevant tree physiological data, have recently been published (Schulze et al., 1995; Arneth et al., 1996; Vygodskaya et al., 1997). We know that the atmosphere both controls and responds to the partitioning of solar energy into sensible and latent heat (evaporation) fluxes at vegetated surfaces, largely as a result of stomatal regulation. Evaporation is also a major component in the water balance, influencing water availability, water quality, and nuwient cycling. These in turn influence ecosystem dynamics including carbon sequestration and storage. In this paper, we examine a set of 30m in measurements of available energy, total forest sensible heat and latent heat fluxes, understorey evaporation and environmental variables made on nine summer days with different weather conditions in an eastern Siberian larch forest. Total evaporation rates are scrutinised using independent energy balance measurements. Bulk surface (vegetation and soil) conductances ( Q ) are derived from the measurements using the Penman-Monteith equation, and Gs is related to irradiance, air saturation deficit, time since rainfall and soil water content. Understorey evaporation rate, measured using small weighing lysimeters, is related to forest evaporation rate and the available energy. Based on the measurements and published climatological data, a simple calculation of growing season forest evaporation rate is compared with corresponding potential evaporation and rainfall rates for eastern Siberia. These analyses are used to identify and quantify the features of this biome's energy balance.

2. Study site and methods Measurements were made in a 130-year-old stand of Lar/x gmelinii (Rupr.) Rupr. located 160km south of Yakutsk city in eastern Siberia, Russia (61°N, 128°E, 300m above sea-level (a.s.1.)). For the trees, average height was 12m, although some canopy emergents nearly reached 20 m, average stem diame-

ter was l l 0 m m , density was 1750ha -1 and onesided leaf area index was 1.5 based on tree harvesting. There was an evergreen, broad-leaved Vaccinium and Arctostaphylos spp. understorey vegetation of approximately 0.05 m height with a one-sided leaf area index of 1.0 determined from plot harvesting. A detailed study of the vegetation growth and biomass was done by Schulze et al. (1995). Topography was very gently undulating with broad, shallow valleys dissecting the region that was entirely forested. According to the USDA taxonomic classification, the soil was an inceptisol known as Pergelic Cryochrept (L. Basher, personal communication, 1993). There was a leaf litter horizon of 30 mm depth and the top 50 mm depth of the mineral soil horizon was enriched with humus (carbon content 10%). The texture of all soil horizons was silt loam. Soil bulk density increased with depth from 0.41 t m -3 for the surface (0-100mm depth) layer to 0.77tm -3 for 100-200mm depth, to a maximum of 1.46tm -3 at depth 600-700 mm. Gravimetric forest floor and soil samples were collected at 1-2day intervals during the study to a depth of 600ram. The measured temperature at 700ram depth was only 1-2°C throughout the study because of the underlying permafrost. Soil water release measurements were later made in the laboratory on undisturbed cores. Roots were observed throughout the soil pits, and over the 700 mm depth of unfrozen soil, there was an available water storage capacity of l l 3 m m based on applied tensions of 10 and 1500kPa. Anticipating the results of our field measurements, water contents for the surface soil (0-100 mm depth) and deeper layers at 10kPa, 100kPa and 1500kPa tensions were 0.25m3m -3, 0.22m3m -3 and 0.15m3m -3 and 0.35 -t- 0.02 m 3 m - 3 (95% confidence limit), 0.29 5: 0.02m 3 m -3 and 0.18 -t- 0.02m 3 m -3, respectively. On 14, 15, and 21-27 July 1993, total forest latent heat flux density (AE, where A and E are the latent heat of vaporisation and evaporation rate, respectively) was determined by the eddy covariance technique using 30 min covariances of high-frequency (10Hz) measurements of vertical wind speed (w; three-dimensional sonic anemometer, Model SWS- 11 with 3KE probe, Applied Technologies, Boulder, CO, USA) and atmospheric water vapour density (Pv; open-path UV absorption hygrometer, Model

F.M. Kelliher et al. /Agricultural and Forest Meteorology 85 (1997) 135-147

KH20, Campbell Scientific, Logan, UT, USA). The basic equation was E = ( ~ - ~ , where the primes indicate deviation from mean quantities and the overbar signifies time., average. The sensible heat flux ( H ) was also measured by eddy covariance using the sonic anemometer with the scalar being air temperature (Ta) ( H = pcp(-wr~,where p is air density and c o is the specific heal: of air at constant pressure). All measurements were made at a height of 22 m, about 5 - 7 m above adjacent trees, using a prefabricated aluminium tower erected next to a tree after removal of its branches. Eddy covariance calculations included two-angle coordinate rotation, ,employment of a digital recursive filter, and correction for sensible heat flux effects on Pv following Kelliher et al. (1992a). We also corrected the hygrometer (Serial Number 1044) measurements for the; effects of oxygen density fluctuations owing to pressure and temperature changes following Tanner et al. (1993). Briefly, and after measuring the sensitivity to water vapour (k w) by placing a chamber across the gap of the hygrometer and flowing air through a precision water vapour generator (Model WG-600, Analytical Development Company, Hoddesdon, UK)(k w = 0.1190, being the slope of the relation between the natural log of the hygrometer output voltage (mV) and the air water vapour density (gm-3)), the laboratory calibration air supply was sequentially diluted with five flow rates of nitrogen gas to create different oxygen concentrations while temperature and pressure were kept constant. Analogous to k w, the oxygen absorption coefficient (k o) was thus determined to be 0.0066. In terms of the field-measured fluxes, h E m and H m, the oxygen correction was written (Tanner et al., 1993) as h E = h E m + K o h E m, where K o = 0.23(ko/kw)[h(Hm/hEm)/T~]. Consequently, for Ta = 20°C, K o = O.107(Hm/hem). The sonic anemometer and hygrometer were also used to measure 30 inin averages of wind speed and air saturation deficit (D), respectively. Air temperature was measured at a height of 19 m with a shielded, artificially ventilated thermistor. Rainfall was measured with a tipping bucket gauge of resolution 0.2 mm mounted on ~tlaetower near the top of the tree canopy. During the study, rainfall occurred only on 16-20 July. Examination of E begins with the forest energy

137

balance (R a = H + hE), where Ra is the available energy, being the difference between net radiation (measured above the forest with a Middleton-type net radiometer), tree biomass and air energy storage rate, and the forest floor heat flux (G). The determination of R a included a combination of measurements and calculations. The tree biomass measurements were described by Schulze et al. (1995). Above-ground tree biomass was 8.1 kgm -2 with a water content of 0.7 kg kg-1 so that an equivalent water depth was 5.6mm. Following Stewart (1988) and using the heat capacity of water, the tree biomass energy storage rate was estimated from 30rain changes in T~ (dTa) as 15dTa with a 30min phase shift to approximate biomass temperature changes. Similarly calculated energy storage rates in the 22 m tall column of air were about the same. Calibrated heat-flux plates (Weaver and Campbell, 1985) buffed at a depth of 20 mm and accompanying thermocoupies at depths 5 and 15 mm were placed in nine locations within a 200m 2 circular area beneath the net radiometer for measurement of G. Bulk surface (vegetation and soil) conductances (Gs) were derived from the micrometeorological measurements using the Penman-Monteith equation as

I/Q =pO/E+ (1/G,)[eRa/hE- ( e +

1)]

(1)

The aerodynamic conductance (G~) was estimated from the sonic anemometer measurements as described by Kelliher et al. (1992a). The other new term E is the rate of change of latent heat content of saturated air with a change of sensible heat content ( e = l , 2, 3 and 4 at about 6°C, 18°C, 24°C and 32°C, respectively). We scrutinised the calculated values of Gs carefully according to the stringent editing criteria of Hollinger et al. (1995). In addition, a small number of arguably spurious high values of G~ were not included because (1) the wind was from the direction of a small lake and marshy ground, suggesting that a higher evaporation rate surface was part of the eddy flux 'footprint' (see Fig. 1 of Hollinger et al. (1995)) or (2) the surface was suspected to be partially wet following rain or dew. Determination of this latter condition was supported by observations and notes made during the measurements, but it also involved some judgement. The partially wet surface condition may be of specialist

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F.M. KeUiheret al. /Agricultural and Forest Meteorology 85 (1997) 135-147

interest with respect to evaporation by epiphytic and ground plants such as lichens and mosses that do not have stomata. However, employment of the Penman-Monteith equation for a partially wet surface with strict adherence to the single-source tenet requires the assumption of a single 'effective' surface temperature for the wet and dry portions (Appendix 1 of Kelliher et al. (1986) and an implicit assumption in Monteith's analysis (Monteith, 1977)). Restricting the analysis to Gs from a 'dry' surface was thus concluded to have strengthened our data set and allowed a direct connection with the micrometeorological literature (e.g. Kelliher et al., 1995a). We examined variation in Gs by boundary-line analysis and a series of least-squares regressions to quantify the constraints G~ imposed by quantum irradiance (Q) and air saturation deficit (D), both measured above the forest, and soil water content (0). First, a comparison of the maximum value of Gs (asmax) and vegetation leaf nitrogen content was done to consider the influence of plant nutrition following Schulze et al. (1994). A hyperbolic saturating function was then set on the upper boundary of a plot of Gs and Q as G~ -- Gsmax[a/(o + Qs0)], where Qs0 was the value of Q when Gs = Gsmax/2. This saturating function was physiologically based on the anticipated vegetation stomatal response to Q (Leuning, 1995) as well as the simplicity involved in having only one new parameter, Qs0- We next examined the relationship between Gs and D under nonlight limiting conditions (i.e. for Q much greater than the light saturating value) as G~ = Gsmax(1 + D / D s o ) - l , where Ds0 was the value of D when Gs = Gsmax/2. It can be shown that this form of the Gs:D relation, known as the Lohammer function, leads to a linear relationship between evaporation and conductance consistent with current understanding of the connection between these two variables in terms of leaf stomatal physiology (Leuning, 1995). Finally, and anticipating our results, we compared the time courses of 0 and G~ over a week-long dry period following 20 m m rainfall, under non-light limiting conditions and for different levels of D. This final analysis demonstrated an interaction between two environmental variables, D and 0, on Gs. This precluded our employment of the widely used multiplicative constraint function model of Gs (Jarvis, 1976) because the effects of some environmental

variables on G~ were not independent and simply multiplicative. Understorey evaporation was measured on a 2 h basis from the weight loss of five lysimeters, thinwalled plastic (370 mm by 370 mm by 150 mm depth) sleeves encasing undisturbed understorey vegetation-forest floor-soil cores. Fresh cores were obtained for the lysimeters every 2 days. Quantum irradiance was measured above the forest and at the forest floor with a network of 12 calibrated gallium arsenide photo-diodes (Model G l l 1 8 , Hamamatsu, Solid State Division, Hamamatsu City, Japan) to estimate radiation attentuation by the tree canopy. Using the eddy covariance measurements, average rainfall measured at Yakutsk (Miiller, 1982) and an assumed rainfall interception rate, we calculated a simple growing season (June-August) estimate of forest evaporation rate. First, and anticipating our results and discussion, we set the evaporation rate when the canopy was dry ( E d) to 1.9mmday -1, being the average value for the five complete days of measurements. On days with rain (about 10days per month; MUller, 1982), we reduced E d by 50%, reflecting a reduction of transpiration when the canopy was wet. Rainfall interception and thus wet canopy evaporation rate ( E w) were estimated separately from E d as 20% of rainfall during July and August assuming full development of the Lar/x canopy. This was the percentage measured in a New Zealand Pinus radiata D. Don forest of similar tree leaf area index (1.7; Kelliher et al., 1992b). Our Calculation of E w thus also assumed a similarity of rainfall regimes in New Zealand and Eastern Siberia. In June, the percentage of rainfall equal to E w was reduced to 15%, reflecting tree canopy development during that month.

3. Results and discussion Employing the energy balance, agreement between the sum of the turbulent fluxes H and AE and the available energy ( R a) was generally good such that ( H + AE) = 0.83R a + 9 W m -2 (r z = 0.92) for 364 half-hour periods. There was a tendency for ( H + AE) to be less than R a (see also Fig. 1 and Table 1), but the means ___95% confidence limit were not significantly different at 129 _+ 17 for ( H +

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F.M. Kelliher et al. /Agricultural and Forest Meteorology 85 (1997) 135-147

AE) and 144 _ 19 for R a. Using the same instrumentation and techniques, a similar tendency, but smaller disparity of about 5-10%, was observed earlier for energy balance measurements made in a New Zealand broad-leaved forest (Kelliher et al., 1992a). At least part of this energy balance disparity may be attributable to differences in sampling areas for the eddy flux and available energy measurements. Footprint analysis, after Schuepp et al. (1990), suggested that 90% of the eddy flux source in Siberia was typically located in the f'trst 650m upwind of the tower. By contrast, for a 90% view factor, the above-forest net radiometer sampled an areal diameter (centred at the tower) of only 30 m in Siberia and the nine forest floo:r heat flux plates were located next to the tower within a circular area of diameter 16m. This may also have involved advection during periods of partial and variable cloud cover that were common during the afternoon in the continental Siberian climate (Fig. 1). We concluded that forest evaporation was measured by eddy covariance in eastern Siberia with an accuracy of order 10-15%. A comparison of data from two representative days, 21 and 23 July 1993, illustrates diurnal partitioning of R a into H and AE (Fig. 2). These measurements were made following 20 mm rainfall over 16-20 July. The first day of comparison was clear and components of R a ranged from - 5 5 to 5 8 8 W m -2 for net :radiation, - 3 8 to 6 4 W m -2 for tree biomass and air storage rate and - 1 5 to 7 4 W m -2 for soil heat flux, respectively. About 50% of the 6.0mm (water depth equivalent) of R a was dissipated as H (Table 1). Daily forest evaporation was 2.2 mm, and the daily average Bowen ratio ( H / E ) was thus equal to 1.5. From late morning until early evening, AE generally reached a broad plateau of about 1 4 0 W m -2 (0.2mmh-~). Measurements made 2 days later, under partly cloudy, warmer and higher air saturation deficit conditions (Fig. 2), resulted in a similar daily forest evaporation of 2.0 mm. However, the daily average Bowen ratio on 23 July was about 30% lower at 1.1. There was good correspondence of timing and percentage changes in R a and H on this day, but the covariance of changes in R a and AE was variable. The average daily forest evaporation rate was 1 . 9 _ 0 . 3 m m d a y -1 for the five complete days of measurement (Table 1). This value was in good

21 July1993

IIA [', Ra

A

3OO 2OO ,°

0

4

8

12 16 Localtime (bourn)

20

24

Fig. 1. Half-hourlyaveragesof the available energy (Ra, dashed line) measured by micmmeteorologicalmethods, and the sum of sensible and latent heat fluxes (H + AE, continuous line) measured by the eddy covariancetechnique above an eastern Siberian larch forest on 21 July 1993 (A) and 23 July 1993 (I3).

agreement with data from the literature, and average daily E of boreal coniferous forests during the tree growing season appears to be relatively conservative (see Table 2, see also Fig. 15 of Sellers et al. (1995)). The same conclusion was reached earlier for European coniferous forests on the basis of annual E and growing season monthly mean daytime AE ( W m -2 ), excluding evaporation of intercepted rainfall, by Roberts (1983) and Lindroth (1985), respectively. An estimate of average daily E for European coniferous forests during May-October was 1.8 mm day -1, calculated using the mean data and footnote information from Table 1 of Roberts (1983)(i.e. (333 mm year- 1 _ 50 mm year- 1)/153 days year- 1). Coniferous forest E was much less than potential E in Europe and in the boreal zone. For example, climate data from Yakutsk indicated average daily potential E was 3.9 mm d a y - 1, 4.5 mm d a y - 1 and 3.5 m l T l day--1 during June, July and August, respectively (Miiller, 1982).

140

F.M. Kelliher et al. / Agricultural and Forest Meteorology 85 (1997) 135-147 4

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Local time (hours), 23 July 1993

Fig. 2. Half-hourly averages of eastern Siberian larch forest meteorological and energy balance components on 21 July 1993 (A and B) and 23 July 1993 (C and D); air saturation deficit (D, bold line) and wind speed (u, fine line) above the forest, available energy (R a, dashed line), sensible heat (H, fine line) and latent heat (AE, bold line) flux densities.

The lysimeters indicated that 50% of forest evaporation on 21 and 23 July emanated from the understorey (Table 3). This percentage may be compared with an average daily quantum radiation transmittance for the 2days of 0.34 + 0.21 and 0.34 _ 0.15. The understorey radiation regime was thus spatially

Table 1 Daily available energy (Re) measured by micrometeorological methods, and sensible heat ( H ) and latent heat (AE, evaporation) flux densities measured by the eddy covariance technique above an eastern Siberian larch forest July 1993

Ra

H

AE

H/AE

(H + AE)/R a

4.4 3.8 5.3 4.9 4.8

1.9 0.8 1.5 1.0 1.1

0.88 0.88 0.87 0.90 0.98

M J m - 2 day- l 14 15 21 22 23

14.4 7.6 15.0 10.9 10.3

8.2 2.9 7.7 4.9 5.3

Also shown is the daily average Bowen ratio ( H / A E ) and an energy balance ratio ( H + A E ) / R a.

variable. The relatively large ratio of percentage evaporation from understorey to percentage radiation transmittance to the understorey suggests that there was a significant contribution to evaporation of turbulent energy exchange beneath the sparse tree canopy (Kelliher et al., 1990, Kelliher et al., 1992a). As a simple model, there was a linear relation between understorey latent heat flux (AE u) and R a with a slope equal to [0.2E/(e + 1)] ( r 2 = 0.55 for 26 2 h periods; Fig. 3). Using the Penman-Monteith equation with our micrometeorological measurements and editing criteria, the maximum bulk-surface conductance (Gsmax) was 10mms -1. This is half the average value of G ~ x = 20 mm s-~ found for natural vegetation in a recent global review (Kelliher et al., 1995a). However, our Gsmax was within the range of data from Canadian boreal forest containing Larix laricina (Du Roi) K. Koch) as 48% (Qm~ = 20mm s- 1; Lafleur, 1992) and 13% of the tree population (Gsmax= 8 r a m s - l ; Fitzjarrald and Moore, 1994). For the Larix gmelinii trees in our forest, maximum stomatal

F.M. Kelliher et al. /Agricultural and Forest Meteorology 85 (1997) 135-147

141

Table 2 Average daily evaporation rates (E, mmday -I ) of boreal coniferous forests measured by micrometeorological methods during the tree growing season Tree species

Site

Latitude, longitude

Average daily E

Dates of measurements

Source

(mmday-l ) Larix gmelinii Larix laricina Larix laricinaPicea mariana Pinus banksiana Pinus banksiana Pinus sylvestris

Yakutsk Schefferville Churchill

61°N, 128°E 54°52'N, 66°40'W 58°45'N, 94°04'W

1.9 1.8 2.1

14-27 July 1993 8 July-5 August 1 9 9 0 17 June-1 September 1989

This study Fitzjarrald and Moore (1994) Lafleur (1992)

Lac da Bonnet Nipawin Jadraas

50°15'N, 95°53'W 53°55'N, 104°41'W 60°49'N, 16°30'E

1.4 1.9 3.4

1 May-6 October 1985 1 June-31 August 1 9 9 4 May-September 1976, 1978

Amiro and Wusehke (1987) Baldocchi et al. (1997) Lindroth (1985)

conductance (gsmax) derived from sap flow, air saturation and leaf area measurements was comparable with gsmax of other Lar/x species in the literature (Vygodskaya et al., 1997) and within 11% of the average gsmax for woody plants from the comprehensive review of KiSrner (1994). However, Qmax, unlike gsmax, includes understorey evaporation and we think that the low Gsmaxand forest evaporation rates were associated with the low leaf nitrogen concentration and low carbon assimilation capacity of the vegetation (Schulze et al., 1994). From above-ground nitrogen concentration and biomass measurements, tree and understorey leaf nitrogen concentration were only 16mgg -1 and 9 m g g -1, respectively, and nearly 90% of the above-ground nitrogen was tied up in tree wood and bark (Schulze et al., 1995). The limitation of available nitrogen may also be attributed to low atrnospheric inputs in the pristine Table 3 Daily total forest evaporation rate (E) measured by the eddy covariance technique above an eastern Siberian larch forest, average daily understorey evaporation rate (E u + 1 S.D.) measured by five small weighing lysimeters, and the fraction of E emanating from the understorey July 1993

e

Eu

Eu / E

0.6±0.1 1.1±0.1 1.0±0.1 1.0±0.2 1.0±0.3 1.0±0.2

0.38 0.50 0.50 0.50 -

nunday- l 15 21 22 23 24 25

1.6 2.2 2.0 2.0 -

boreal environment according to data from neighbouring northern Alaska (Schulze et al., 1995). For other Lar/x species growing in North America and Europe, published leaf nitrogen concentrations varied from 20 to 26mgg -1 (Schulze et al., 1994). The set light-saturating boundary-line relationship between Gs and Q gave a value of Qs0 = 100p~molm -2 s - l (Fig. 4(A)). In our forest, treescale values of Q50 derived from sap flow and humidity measurements are generally larger, with a range of 97-304 p,molm -2 s -1 and average of 164 + 7 1 p , molm -2 s - l (Arneth et al., 1996). Our Qso value is within the range of 25-340 p,molm -2 s -1 obtained in three pine forest studies reviewed by Kelliher et al. (1993). Non-light-saturated Gs data (i.e. when Q < 600 Ixmol m -2 s - l ; Fig. 4(A)) were then eliminated, but the relationship between the remaining Gs and D data was increasingly variable as Gs increased and D decreased below about 2 kPa (Fig. 4(B)). This limited conventional statistical interpretation of these data and accurate predictions from the Lohammer function model. Consequently, the definition of Ds0 = 0.5kPa by non-linear regression accounted for only 16% of the remaining variation in Gs (Gs = 10/[1 + ( D / 0 . 5 0 ) ] , the continuous curve in Fig. 4(B)). There was a much tighter relation between G~ and D during the afternoon, even when D < 2 kPa, but the Lohammer function model fit to these data significantly underestimated G~ for D < 2kPa (Gs = 10/[1 + (D/0.67)], r 2 = 0.46, curve not shown in Fig. 4(B)). Regression analysis accounted for 63% of remaining variation in the afternoon data when the

F.M. Kelliher et al. / Agricultural and Forest Meteorology 85 (1997) 135-147

142

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Fig. 3. The relationship between 26 2 h averages of understorey latent heat flux density (AE,) measured by lysimetry and the above-forest available energy flux density (R a) multiplied by ~ / ( e + 1), where E is the rate of change of latent heat content of saturated air with a change of sensible heat content. The line is a regression through the origin of slope 0.20 ( r 2 = 0.55).

was attributable to variability in the maximum value of stomatal conductance obtained by different short shoots because the relative diurnal courses of stomatal conductance were very similar amongst the measured shoots. Variability in tree stomatal conductance may have contributed to variability in the larger-scale surface conductance because, during the morning, a relatively de-coupled Gs could reflect the evaporation rate from a different portion of the tree canopy (i.e. upper and outer sun crown) than later in the day when the sun was higher in the sky and wind speed increased. Sun elevation and wind speed may

10

E

E

model included a second floating parameter ( Q = 1 0 / [ - 1 . 2 3 + (D/0.37)], the dashed curve in Fig. 4(B)). Scatter in the Gs-D relation did not change significantly when G~ was replaced by an estimate of the tree canopy conductance calculated by replacing forest E with ( E - E u) in the Penman-Monteith equation (data not shown). Increasing variability of G~ with decreasing D has been observed elsewhere in coniferous forests based on leaf-scale ( Whitehead et al., 1996; Vygodskaya et al., 1997), individual-treescale data derived from sap flow measurements (Granier and Loustau, 1994; Arneth et al., 1996; Whitehead et al., 1996) and tree-canopy-scale data from eddy covariance measurements (Baldocchi et al., 1997). Most of the scatter in our Q-D relation occurred during the morning, when D < 2 kPa. There was a tendency for G~ to be inversely related to wind speed at a given D in the morning, and wind speeds generally increased significantly during the afternoon, when Gs tended to be more closely coupled to D (Fig. 2(A) and Fig. 2(C)). The coupling of evaporation rate, and hence G~, to D depends on turbulence generated by tall, rough forest canopies in proportion to wind speed (Hollinger et al., 1994). An aerodynamic conductance is included in the Penman-Monteith equation (McNaughton and Jarvis, 1983), but our observations may suggest an additional influence of wind speed on G~. Stomatal conductance in the tree canopy was also most variable during the morning (Vygodskaya et al., 1997). This

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Fig. 4. (A) The relationship between 247 30min values of bulksurface conductance (Gs), derived from micrometoorological measurements using the Penman-Monteith equation, and measured above-forest quantum irradiance (Q). A hyperbolic-saturating curve was set by inspection to the upper boundary of the data as described in the text. (B) The relationship between 162 30min values of Gs and measured above-forest air saturation deficit (D) for Q > 600p, m o l m -2 s -I . The filled symbols are for data collected in the morning, and open symbols are for the afternoon data. The continuous curve known as the Lohammer function, 10/[1 + ( D / 0 . 5 0 ) ] , was fitted to all of the data by non-linear regression (r 2 = 0.16). The dashed curve, 1 0 / [ - 1.2 + ( D / 0 . 3 7 ) ] , was fitted to the afternoon data (r 2 = 0.63, n = 97).

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F.M. Kelliher et al. /Agricultural and Forest Meteorology 85 (1997) 135-147

also have been important for the illumination and coupling of leaves of the virtually ground-level understorey vegetation,, which contributed half the forest's evaporation, to D measured above the forest at a height of 22 m. Understorey evaporation rate was proportional to R a, which tended to peak 4 - 8 h before D, which, in turn, was the principal environmental variable driving tree transpiration during most of the day (Ameth et al., 1996). Our variable conductance data also suggest that rates of stomatal opening and closing, and transpiration, in parts of the tree crowns may be regulated at different times through the day by physiological factors resulting from partial shading. Stomatal opening appears to be .'in inherently slower process in coniferous trees than stomatal closing (Whitehead and Teskey, 1995) that occurred during the afternoon in our forest. Imposing sudden, but transient, changes in the fraction of illuminated foliage area of a wellwatered, field-grown Pinus radiata tree led to significant changes in leaf- and tree-scale conductances because of a perturbation of the hydraulic pathway in the xylem (Whitehead et al., 1996). There were generally higher values of Gs at given values of Q and D in the morning than in the afternoon (data not shown, but see Fig. 4(B)). This diurnal hysteresis was observed at the leaf- and tree-scales in this forest (Arneth et al., 1996; Vygodskaya et al., 1997) and for leaf-scale data of many other plant species (L~Ssch and Schulze, 1994). The lack of Gs recovery in the afternoon when D decreased may be explained by plant water stress (Schulze, 1994; Whitehead et al., 1996). Nevertheless, diurnal hysteresis in conductance data remains an enigma. The surface conductances were mostly from the week-long rainfree period of 21-27 July. It was postulated that there', could be a drying time effect on Gs in addition to the effects of Q and D. However, the relevance of this hypothesis is predicated on week-long dry periods being a feature of the eastern Siberian growing season climate. Following a rainy day during the growing season, there was a 10% chance that a week-long dry period would occur, according to a simple exponential probability distribution model based on a Poisson process (KeUiher et al., 1995b; Berninger et al., 1996). This included the growing season occurrence of ten rainy days per

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month (Miiller, 1982) in Yakutsk when calculating the model's power coefficient (Fig. 5). Combining this 10% chance with the ten rainy days per month suggested the climatological average occurrence of one week-long dry period per month during the growing season. The model prediction was in close agreement with data from a 12year rainfall record at Yakutsk (Fig. 5) (34 dry periods (following a rainy day) of 7days or longer occurred during the 12 growing seasons of 1975-1986, so that (34 per 12 years) there were 2.8 dry periods in each 3month growing season (Razuvaev et al. (1993)). The gravimetric soil water measurements during the week-long rainfree period suggested that drying was confined to the upper 0.1 m of soil, which was enriched in humus and thus of a relatively low bulk density of 0.41 t m -3 (Fig. 6(A)). Although exact quantitative comparison of these field measurements with laboratory soil core water release data can be problematical, the surface soil was nevertheless fairly dry (i.e. comparable surface soil core data were 0.15 m 3 m - 3 at 1500 kPa tension) whereas the deeper layers were generally not (i.e. comparable soil core data were 0.29 ___0.02m 3 m -3 at 100kPa tension). There was a statistically significant linear decrease in surface volumetric water content of 0.005 d a y - 1 ( r 2 = 0.35) during the week (Fig. 6(B)). Downward trends in G s, when Q was greater than

144

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6001xmolm -2 s -1 and within D classes, were also evident during the week (Fig. 7). Linear regressions of G~ and time (days) since rainfall showed that, with increasing D, there was a significantly decreased sensitivity of G~ to surface soil drying during the week (regression slopes were - 1 . 1 0 + 0.20, - 0 . 5 1 _ 0.13 and - 0 . 0 9 _ 0.06 for D of 0-1 kPa, 1-2 kPa and 2 - 4 kPa, respectively). Somewhat as an artefact of the least-squares method, and seemingly independent of the absolute scatter in Q , these decreasing slopes corresponded to the regressions having smaller coefficients of determination; r 2 = 0.64, r e = 0.22 and r e = 0.04, respectively. The decreasing slopes were caused mainly by decreasing values of the maximum G~ evident in the regression offsets, which were 7.4 mm s- l, 4.9 mm s- 1 and 2.2 mm s -1, respectively. The only comparable field data in the literature come from Canada where similar interactions between stomatal conductance, D and soil water potential for conifer forest overstorey and broad-leaved understorey species were quantified by Black and Kelliher (1989). As in this study,

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F.M. KeUiher et al. / Agricultural and Forest Meteorology 85 (1997) 135-147

was estimated, as described above, to be 22mm. Based on the eddy flux measurements and supporting data from the literature (Table 3), the average growing season dry canopy evaporation (calculated using a constant daily value of E d, including tree canopy and understorey, of 1.9mmday -1) was 147 mm. Consequently, total forest evaporation ( E d + E w) was 169mm, or 79% of the average annual precipitation. A significantly higher percentage of 88% was obtained from a year of eddy flux and precipitation measurements made above a Canadian boreal broadleaf forest (Black et al., 1996). Nevertheless, our estimate of total forest evaporation was 57 mm in excess of the corresponding average growing season rainfall. Consequently, there was a depletion of the available soil water storage capacity. This water store was equal to l l 3 m m over the 700mm unfrozen depth of soil observed during the warmest month of the growing season. Utilising this soil water for E d leads to the occurrence of a soil water deficit of 57 mm at the end of the growing season. We next consider the 101 mm of precipitation that occurs, mostly as snow, during September-May. In the eastern Siberian climate, this water remains frozen and unavailable, and evaporation and sublimation are assumed here to be negligible, until snowmelt in the spring. Upon refilling the 57 mm soil water deficit in spring, there thus remains 44mm of water for drainage and runoff. In eastern Siberia, snowmelt is an important generator of runoff. To consider this and complete the discussion requires a switch from the forest water balance to a brief synopsis of the regional-scale hydrology. About half of annual flow in the middle courses of major Siberian rivers such as the Lena, located 80 km north of the study site, occurs during the springtime months of April and May (Beckinsale, 1'969; Haines et al., 1988). This generally leads to tremendous flooding because the fiver mouths, located further north, remain frozen until early June and there is little topographic relief in eastern Siberia (Beckinsale, 1969). This sudden springtime flood and declining summer river flows close the annual water balance calculations. Although necessarily approximate in nature, this brief water balance and regional hydrology discussion supports our conclusion that there was substantial environmental conslxaint of growing season forest evaporation in eastern Siberia.

145

4. Conclusions During the growing season, forest evaporation was half the potential rate and relatively conservative at about 2 mm day- 1 on rainfree days. Half of forest evaporation emanated from the understorey because the low tree leaf area intercepted only about twothirds of the irradiance and there appeared to be significant advection of sensible heat from the tree canopy to the ground. The vertically separate, dualsource nature of evaporation from this boreal forest, and partial shading of plant canopies during the morning, made analysis of surface conductance and environmental variables problematic. Nevertheless, surface conductance was limited by irradiance only at very low levels of less than 10% of full sunlight. Furthermore, high air saturation deficits and low surface soil water contents appeared to be features of the growing season climate in eastern Siberia and these interacted to significantly reduce surface conductance below the maximum value obtained under optimal conditions. Finally, we believe the low forest evaporation rates in eastern Siberia were connected to the low leaf nitrogen content and carbon assimilation capacity of the vegetation. Evaporation from the Siberian forest may thus be sensitive to disturbance of the nitrogen cycle by agents such as fire, logging, or warming as has been suggested by some for the boreal latitudes in future.

Acknowledgements The Manaaki Whenua-Landcare Research team was funded by a long-term grant for atmospheric research from the New Zealand Foundation for Research, Science and Technology. For presentation of a draft of this paper, F.M. Kelliher is grateful for travel funds to attend the International Symposium on Forest Hydrology 1994 provided by the organising committee and the Department of Environmental Science and Natural Resources, Tokyo University of Agriculture and Technology. The encouragement of Dr. JunPei Kubota and Professor Takehiko Ohta for presenting our work at the Symposium was very much appreciated. E.-D. Schulze acknowledges the support of the German Bundesminister f'fir Ern~ihrung

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F.M. KeUiher et al. /Agricultural and Forest Meteorology 85 (1997) 135-147

Landwirtschaft und Forsten and the Humboldt Foundation (Max-Planck Forschungspreis). A travel grant for the journey from New Zealand to Germany was awarded to D.Y. Hollinger under the New ZealandFederal Republic of Germany Agreement for Scientific and Technological Co-operation. The soil particle size and water release measurements were done by John Claydon. The senior author thanks colleagues Rick Jackson and Barry Fabey, and two anonymous referees, for their valuable critiques of this paper. Finally, we express our sincere thanks to Inge Schulze for keeping us supplied with good food during our month living in the Siberian forest.

References Amiro, B.D. and Wuschke, E.E., 1987. Evapotranspiration from a boreal forest drainage basin using an energy balance/eddy correlation technique. Boundary-Layer Meteorol., 38: 125139. Arneth, A., Kelliher, F.M., Bauer, G., Hollinger, D.Y., Byers, J.N., Hunt, J.E., McSeveny, T.M., Ziegler, W., Vygodskaya, N.N., Milukova, I., Sogachov, A., Varlargin, A. and Schulze, E.-D., 1996. Environmental regulation of xylem sap flow and total conductance of Lar/x gmelinii trees in eastern Siberia. Tree Physiol., 16: 247-255. Baldocchi, D.D., Vogel, C.A. and Hall, B., 1997. Seasonal variation of energy and water vapour exchange rates above and below a boreal jack pine forest canopy. J. Geophys. Res., in press. Beckinsale, R.P., 1969. River regimes. In: R.J. Chorley (Editor), Water, Earth and Man. Methuen, London, pp. 455-471. Berninger, F., MSkel~i, A. and Hari, P., 1996. Optimal control of gas exchange during drought: empirical evidence. Ann. Bot., 77: 469-476. Black, T.A. and Kelliher, F.M., 1989. Processes controlling understorey evapotranspiration. Philos. Trans. R. Soc. London, Ser. B, 324: 207-231. Black, T.A., den Hartog, G., Neumann, H.H., Blanken, P.D., Yang, P.C., Russell, C., Nesic, Z., Lee, X., Chen, S.G., Staebler, R. and Novak, M.D., 1996. Annual cycles of water vapour and carbon dioxide fluxes in and above a boreal aspen forest. J. Geophys. Res., 2: 219-229. Fitzjarrald, D.R. and Moore, K.E., 1994. Growing season boundary layer climate and surface exchanges in a subarctic lichen woodland. J. Geophys. Res.--Atmos., 99(D1): 1899-1917. Granier, A. and Loustau, D., 1994. Measuring and modelling the transpiration of a maritime pine canopy from sap-flow data. Agric. For. Meteorol., 71: 61-81. Haines, A.T., Finlayson, B.L. and McMahon, T.A., 1988. A global classification of river regimes. Appl. Geogr., 8: 255272.

Hollinger, D.Y., Kelliher, F.M., Schulze, E.-D. and K~stner, B.M,M., 1994. Coupling of tree transpiration to atmospheric turbulence. Nature, 371: 60-62. Hollinger, D.Y., Kelliher, F.M., Schulze, E.-D., Vygodskaya, N.N., Varlargin, A., Milukova, I., Byers, J.N., Sogachov, A., Hunt, J.E., McSeveny, T.M., Kobak, K.I., Bauer, G. and Ameth, A., 1995. Initial assessment of multi-scale measures of CO 2 and H20 flux in Siberian taiga. J. Biogeogr., 22: 425431. Jarvis, P.G., 1976. The interpretation of variation in leaf water potential and stomatal conductance found in canopies in the field. Philos. Trans. R. Soc. London, Ser. B, 273: 593-610. Kelliher, F.M., Black, T.A. and Price, D.T., 1986. Estimating effects of understory removal from a Douglas-ftr forest using a two-layer canopy evapotranspiration model. Water Resour. Res., 22: 1891-1899. Kelliher, F.M., Whitehead, D., McAneney, K.J. and Judd, M.J., 1990. Partitioning evapotranspiration into tree and understorey components in two young Pinus radiata D. Don stands. Agric. For. Meteorol., 50:211-227. Kelliher, F.M., K~Sstner,B.M., Hollinger, D.Y., Byers, J.N., Hunt, J.E., McSeveny, T.M., Meserth, R., Weir, P.L. and Schulze, E.-D., 1992a. Evaporation, xylem sap flow, and tree transpiration in a New Zealand broad-leaved forest. Agric. For. Meteorol., 62: 53-73. Kelliher, F.M., Whitehead, D. and Pollock, D.S., 1992b. Rainfall interception by trees and slash in a young Pinus radiata D. Don stand. J. Hydrol., 131: 187-204. Kelliher, F.M., Leuning, R. and Schulze, E.-D., 1993. Evaporation and canopy characteristics of coniferous forests and grassland. Oecologia, 95: 153-163. Kelliher, F.M., Leuning, R., Raupach, M.R. and Schulze, E.-D., 1995a. Maximum conductances for evaporation from global vegetation types. Agric. For. Meteorol., 73: 1-16. Kelliher, F.M., Marden, M., Watson, A.J. and Arulchelvam, I.M., 1995b. Estimating the risk of landsliding using historical extreme river flood data. J. Hydrol. (New Zealand), 33: 123129. K~imer, Ch., 1994. Leaf diffusive conductances in the major vegetation types of the globe. In: E.-D. Schulze and M.M. Caldwell (Editors), Ecophysiology of Photosynthesis. Ecological Studies, Vol. 100. Springer, Heidelberg, pp. 463-490. Lafleur, P.M., 1992. Energy balance and evapotranspiration from a subarctic forest. Agric. For. Meteorol., 58: 163-175. Leuning, R., 1995. A critical appraisal of a combined stomatalphotosynthesis model for C 3 plants. Plant Cell Environ., 18: 339-355. Lindroth, A., 1985. Seasonal and diurnal variation of energy budget components in coniferous forests. J. Hydrol., 82: 1-15. Lloyd, J,, Krnijt, B., Hollinger, D.Y., Grace, J., Francey, R.J., Wong, S.-C., Kelliher, F.M., Miranda, A.C., Farquhar, G.D., Gash, J.H.C., Vygodskaya, N.N., Wright, I.R., Miranda, H.S. and Schulze, E.-D., 1996. Vegetation effects on the isotopic composition of CO 2 at local and regional scales: theoretical aspects and a comparison between rain forest in Amazonia and a boreal forest in Siberia. Aust. J. Plant Physiol., 23: 371-392. L~sch, R. and Schulze, E.-D., 1994. Internal coordination of plant responses to drought and evaporative demand. In: E.-D.

F.M. Kelliher et al.// Agricultural and Forest Meteorology 85 (1997) 135-147 Schulze and M.M. Caldwell (Editors), Ecophysiology of Photosynthesis, Ecological Studies, Vol. 100. Springer, Heidelberg, pp. 185-204. McNaughton, K.G. and Jarvis, P.G., 1983. Predicting the effects of vegetation changes on transpiration and evaporation. In: T.T. Kozlowski (Editor), Water Deficits and Plant Growth, Vol. VII. Academic Press, New York, pp. 1-47. Monteith, J.L., 1977. Resistance of a partially wet canopy: whose equation fails? Boundary-Layer Meteorol., 12: 379-383. Miiller, M., 1982. Selected Climatic Data for a Global Set of Standard Stations for Vegetation Science. Dr. W. Junk, The Hague, 306 pp. Razuvaev, V.N., Apasova, E.G, Martuganov, R.A., Vose, R.S. and Steurer, P.M., 1993. Daily temperature and precipitation data for 223 USSR stations. ORNL/CDIAC-56, NDP-040, Environmental Sciences Division Publication 4194, Carbon Dioxide Information Analysis Center, Oak Ridge, TN, 47 pp. Roberts, J., 1983. Forest transpiration: a conservative hydrological process? J. Hydrol., 66: 133-141. Schuepp, P.H., LeClerc, M.Y., MacPherson, J.L. and Desjardin, R.L., 1990. Footprint predictions of scalar fluxes from analytical solutions of the diffusion equation. Boundary-Layer Meteorol., 50: 355-373. Schulze, E.-D., 1994. The regulation of plant transpiration: Interactions of feedforward, feedback, and futile cycles. In: E.-D. Schulze (Editor), Flux Control in Biological Systems--from Enzymes to Populations and Ecosystems. Academic Press, New York, pp. 203-235. Schulze, E.-D., Kelliher, F.M., KiSrner, Ch., Lloyd, J. and Leuning, R., 1994. Relationships between maximum stomatal conductance, carbon assimilation rate and plant nitrogen nutrition: a global scaling exercise. Annu. Rev. Ecol. Syst., 25: 629-660. Schulze, E.-D., Schulze, W., Kelliher, F.M., Vygodskaya, N.N., Ziegler, W., Kobak, K.I., Arneth, A., Kustesova, W.A., Sogatchev, A., Issajev, A., Bauer, G. and Hollinger, D.Y., 1995.

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Above-ground biomass and nitrogen nutrition in a chronosequence of pristine Dahurian Larix stands in eastern Siberia. Can. J. For. Res., 25: 943-960. Sellers, P., Hall, F., Margolis, H., Kelly, B., Baldocchi, D., den Hartog, G., Cihlar, J., Ryan, M.G., Goodison, B., Crill, P., Ranson, K.J., Lettenmaier, D. and Wickland, D., 1995. The boreal ecosystem-atmosphere study (BOREAS): an overview and early results from the 1994 field year. Bull. Am. Meteorol. Soc., 76: 1549-1577. Shvidenko, A. and Nilssou, S., 1994. What do we know about the Siberian forests? Ambio, 23: 396-404. Stewart, J.B., 1988. Modelling surface conductance of pine forest. Agric. For. Meteorol., 43: 19-35. Tanner, B.D., Swiatek, E. and Greene, J.P., 1993. Density fluctuations and use of the krypton hygrometer in surface flux measurements. In: Management of Irrigation and Drainage Systems. Proc. American Society of Civil Engineers Meeting, Park City, UT, 21-23 July 1993. ASCE, New York, pp. 945-952. Vygodskaya, N.N., Milukova, I., Varlargin, A., Tatarinov, F., Sorgachov, A., Kobak, K.I., Bauer, G., Hollinger, D.Y., Kelliher, F.M. and Schulze, E.-D., 1997. Leaf conductance and CO 2 assimilation of Larix gmelinii under natural conditions of eastern Siberian boreal forest. Tree Physiol., in press. Weaver, H.L. and Campbell, G.S., 1985. Use of Peltier coolers as soil heat flux transducers. Soil Sci. Soc. Am. J., 49: 10651067. Whitehead, D. and Teskey, R.O., 1995. Dynamic response of stomata to changing irradiance in loblolly pine (Pinus taeda L.). Tree Physiol., 15: 245-251. Whitehead, D., Livingston, N.J., Kelliher, F.M., Hogan, K.P., Pepin, S., McSeveny, T.M. and Byers, J.N., 1996. Response of transpiration and photosynthesis to a transient change in illuminated foliage area for a Pinus radiata D. Don tree. Plant Cell Environ., 19: 949-957.