Evaporation, xylem sap flow, and tree transpiration in a New Zealand broad-leaved forest

Agricultural and Forest Meteorology, 62 (1992) 53-73

53

Elsevier Science Publishers B.V., A m s t e r d a m

Evaporation, xylem sap flow, and tree transpiration in a New Zealand broad-leaved forest F.M. Kelliher a, B.M.M. K6stner b, D.Y. Hollinger a, J.N. Byers a, J.E. Hunt ~, T.M. McSeveny a, R. Meserth b, P.L. Weir" a n d E.-D. Schulze b "Forest Research Institute. PO Box 31-011, Christchurch, New Zealand bLehrstuhl Pflanzen6kologie der Universitat Bayreuth and Bayreuther Institut fur Terrestrische Okosystemforschung, Box 10 12 51, 8580 Bayreuth, Germany (Received 10 January 1992; revision accepted 18 June 1992)

ABSTRACT Kelliher, F.M., K6stner, B.M.M., Hollinger, D.Y., Byers, J.N., Hunt, J.E., McSeveny, T.M., Meserth, R., Weir, P.L. and Schulze, E.-D., 1992. Evaporation, xylem sap flow, and tree transpiration in a New Zealand broad-leaved forest. Agric. For. Meteorol., 62:53 73. Total evaporation (E), forest floor evaporation (El), tree xylem sap flow (F), and environmental parameters were measured on 6 consecutive late-summer days under different weather conditions in a well-watered, temperate broad-leaved forest. Two tree species, Nothofagusfusca (Hook. f.) Oerst. (red beech) and N. menziesii (Hook. f.) Oerst. (silver beech), formed a vertically structured, complex canopy with a one-sided leaf area index of 7. The forest comprises 30-40 trees ha ~ of emergent red beech up to 36 m tall and 1.7 m diameter, above a mixed species canopy of 200 trees ha ~ about 20-30 m tall and 0.4 m average diameter, and approximately 900 trees ha ~ of sub-canopy, mostly silver beech < 20 m tall and 0.1 m average diameter. Agreement of E (determined by eddy covariance) and the difference between available energy and sensible heat flux densities was generally within 10% on half-hourly and daily bases. On clear days, the Bowen ratio obtained a broad plateau of about 1-2 for most of the time with much lower and even negative values around sunrise and sunset. Variable cloudiness caused substantial variation in available energy and the Bowen ratio. After rain, daytime Bowen ratios were somewhat lower and relatively constant at about 0.8 when the tree canopy was partially wet. Lysimeter measurements indicated that Ef was a significant evaporation component and accounted for 10-20% of E, with rates up to 0.5 m m day ~. Agreement between F(measured by a xylem sap flow method in a representative 337 m 2 plot of 14 trees) and tree canopy transpiration (E - El) was reasonable, with an average disparity of order 10-20% or 0.3 _+ 0.1 m m day i (standard deviation) when the tree canopy was dry. Within the plot, F typically varied by more than an order of magnitude. Tree social position, assessed by emergence of crown from the general canopy level, strongly affected an individual's contribution to plot sap flux density. About 50% of daily plot F emanated from only three emergent trees. Diurnal variation in coupling of the tree canopy to its aerial environment reflected changes in humidity and wind speed corresponding with changes in stomatal and aerodynamic conductances. Consequently,

Correspondence to: F.M. Kelliher, Forest Research Institute, PO Box 31-011, C h r i s t c h u r c h , New Zealand.

0168-1923/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

54

F.M. KELLIHER ET AL,

varying proportions of radiative and advective energy were involved in determining tree transpiration rate. Wet canopy evaporation rate was also examined because it significantly influences when transpiration takes place as rain falls on about 200 days of the year in the forest studied. Effects of leaf size and plant nutrition on tree transpiration and the partitioning of available energy are also discussed.

INTRODUCTION

Tree transpiration in a well-watered forest depends on biologically-mediated responses to an aerial environment that can, in turn through feedback effects of the tree canopy on radiation, energy balance, turbulence, and humidity regimes, determine the state of the environment itself. The structure of undisturbed, mature Nothofagus (beech) forest in New Zealand is complex, and the mosaic of trees of greatly differing size has a variable canopy architecture (Stewart and Rose, 1990). These natural stands are therefore ideal for studying the general forest micrometeorology question of canopy (surface) function and its regulation by structural form and stomatal physiology. Tree transpiration in such a natural forest is expected to be variable, and different spatial scales of measurement will be needed to assess tree transpiration processes and to obtain stand level integration. In this paper, we examine a set of half-hourly measurements of total evaporation, forest floor evaporation, tree xylem sap flow, and environmental parameters made from sunrise to sunset on 6 consecutive, late summer days with different weather conditions in a mature beech forest. Two methods of estimating tree canopy transpiration are compared. The first method is a 'top-down' approach based on the difference between eddy convariance measurements of total evaporation and forest floor evaporation determined by lysimetry. A useful feature of this method is that total evaporation rates can be scrutinized independently by using energy balance measurements. The second method is a 'bottom-up' approach summing tree xylem sap flow measurements made in a small representative plot. After comparison of the two methods, we explore environmental control of transpiration at tree and canopy scales. METHODS

Site description The measurement site was an undisturbed broad-leaved, evergreen forest with two tree species Nothofagus fusca (Hook. f.) Oerst. (red beech) and N. menziesii (Hook. f.) Oerst. (silver beech), located near Maruia in the South Island of New Zealand (42o13 ' S, 172o15 , E, 400 m elevation). The forest was on a glacial terrace in the centre of a valley about 5 km wide, with a fetch of about 1 km in the prevailing daytime upwind directions of southwest and

T R E E T R A N S P I R A T I O N IN A BROAD-LEAVED FOREST

55

northwest. Beyond the entirely forested fetch is a mosaic of forest and agricultural land uses in the valley b o t t o m though the adjoining hills and region are largely forested by undisturbed Nothofagus stands. F r o m measurements made in a representative 60 m x 60 m forest plot, the closed-canopy forest contained about 1100 trees ha -I (770 trees ha -L and 330 trees ha -I for silver and red beech, respectively) of diameter > 25mm. Basal area was 62 m 2 h a - i (17 m 2 ha-~ and 43 m 2 ha-~ for silver and red beech, respectively.) The age of silver and red beech trees was up to 400 years (Stewart and Rose, 1990). The tree canopy varied in height from 20 to 30 m with a crown depth of 10-15 m. By using rectangular hyperbola relationships between diameter and height derived from the harvested tree measurements of Stewart and Rose (1990), 90% of silver and 72% of red beech were estimated as < 20m tall (sub-canopy trees) and 35 trees ha ~ of red beech as > 30 m tall (emergent trees). Leaves of silver and red beech are hypostomatous (stomata on the abaxial surface). Red beech leaves are ellipsoidal with a major-to-minor axes ratio of 1.6 and a typical length of about 30 mm. Silver beech leaves are smaller and are comparable in shape with one-half ellipses (i.e. fans) with a representative length = basal width = 10mm. One-sided tree canopy leaf area index for the stand in late summer was estimated as 7 from litter collection and phenological measurements. Litter collection over 2 years indicated that 80% of the total leaf area was from red beech.

Evaporation measurements On 6 consecutive late-summer days, 12-17 March 1991, total forest evaporation, forest floor evaporation, and environmental parameters were measured from sunrise until sunset. Total forest evaporation (E) was determined by the eddy correlation technique using 30 rain covariances (Baldocchi et al., 1988) of high-frequency (10Hz) measurements of vertical wind speed (w) and atmospheric water vapour density (Pv) with a basic equation of E = p(w' Pv') where p is air density, the primes indicate deviation from mean quantities, and the overbar signifies time average. Measurements were made at a height of 36m, 5 - 1 0 m above the adjacent trees. Instruments were attached to a horizontal boom atop a remotely-operated antenna rotator on a vertical pole mounted above a 32 m tall scaffolding tower erected without removing any trees. Vertical wind speed was measured with a three-dimensional sonic anemometer (model SWS-11 with a model 3KE 3-axis probe, Applied Technologies, Boulder, CO). An open-path ultraviolet absorption hygrometer (model KH20, Campbell Scientific, Logan, UT) was used to determine water vapour density. Evaporation calculations were done using the algorithm of McMillen (1988), with two-angle coordinate rotation, a digital-recursive filter time constant of 200s (Baldocchi et al., 1985), and

56

F.M. KELL1HER ET AL.

correction for sensible heat flux effects on water vapour density, after Webb et al. (1980). Footprint analysis, after Schuepp et al. (1990) with a horizontal wind speed of 3 m s ~, suggested that 50, 75, and 90% of the eddy flux source was typically located in the first 130, 315, and 860 m upwind of the tower, respectively. Examination of Ebegins with the forest energy balance (Rn-J-G = H + 2E) where R, is net radiation flux density, J is tree-canopy biomass and air energy storage rate, G is forest floor heat flux density, H is sensible heat flux density, and )~ is the latent heat of vaporisation. Net radiation was measured with a model CN1 net radiometer (Carter-Scott Design, Fairfield, Vict.) at a height of 32.5 m on a horizontal boom extended 2 m from the tower. An expression for tree-canopy biomass energy storage rate as a function of half-hourly air temperature changes (dT~, with T~ also measured at height 32.5 m using a thermistor in a ventilated insulated shield) was derived from estimates of biomass, measurements of biomass water content, and conversion of the tree canopy to a thermally equivalent sheet of water. Tree-canopy biomass was estimated as 10.45 kg m -2 (25% of total above-ground biomass) from the tree stocking and diameter measurements (Beets, 1980), biomass water content was l kgkg ~, equivalent water depth was 10.4ram, and (using the heat capacity of water) biomass energy storage rate (W m -2) was equal to 24 dT,~ (°K). A 30min phase shift in determining dTd was used to approximate tree-canopy biomass temperature changes after Stewart (1988). Energy storage rates in the 36m tall column of air were of a similar magnitude, determined by the centred-difference method of Stewart (1988). Concurrent half-hourly measurements of G were made at six locations using calibrated heat-flux plates (Weaver and Campbell, 1985) and thermocouples buried to a depth of 20 mm, after Kelliher et al. (1990). Eddy correlation measurements of H were made using the sonic anemometer, with the scalar being air temperature and H pep (w' T~) where Cp is the specific heat of air at constant pressure. Auxiliary meteorological measurements included wind direction, wind speed, and relative humidity (measured with a model 2011 capacitative humidity sensor (Skye Instruments, Powys, UK) next to the T,d thermistor). Wet-bulb temperatures were later determined, using the interative algorithm of Abbott and Tabony (1985). Visible irradiance was measured above the forest and at the forest floor with a network or 32 calibrated photo-resistive sensors over 3 days to estimate radiation attenuation by the tree canopy. Forest floor R. was then estimated from the above forest R n measurements and this approximation of tree-canopy radiation attentuation. Interpretation of tree canopy/environment interactions included measurement of momentum flux with the sonic anemometer and scaling by a windflow parameter, the Reynold's stress, which after coordinate rotation is proportional by p to (w'u') where u is horizontal wind speed (Stull, 1988). The =

TREE TRANSPIRATION JN A BROAD-LEAVED FOREST

57

conventional tangential rotation velocity scale u , is then (~7~71°5) and the aerodynamic conductance for m o m e n t u m transfer (g,m) is (U, 2/fi), with the overbars signifying time averages. This conductance is a component of total aerodynamic conductance for water-vapour transfer (gAv) that has been estimated as {1/[1/g,m + (1/atgbv)]} where a t is tree leaf area index and gbv is a tree-canopy average of leaf-boundary layer conductance for water-vapour transfer (Kelliher et al., 1990). Forest-floor evaporation (E0 was estimated on a daily basis from the weight loss of six lysimeters, thin-walled plastic 150 m m diameter by 120 mm deep sleeves encasing undisturbed forest floor/soil cores. An estimate of tree-canopy transpiration rate (Et) results from combining eddy correlation and lysimeter measurements to give E t = E - El. Gravimetric forest floor and soil samples indicated that the trees were well watered on the 6 measurement days. An additional set of 25 hourly lysimeter measurements made on three other well watered days during December 1988-February 1989 was used to examine the relationship between Ef and forest floor available energy. The measurement period was generally dry, but there was 2.4 m m of rain between 21:00 h on 13 March and 01:00 h on 14 March. Tree-canopy throughfall, collected in six 12m long x 0.1 m wide troughs at the forest floor and measured every 6 rain with a tipping-bucket gauge system, was 1.2 m m for the storm, of which 1.0 m m occurred during rainfall and 0.2 m m was tree-canopy drip afterwards. Throughfall was not recorded until after the initial 0.8 m m of rainfall over the first 24 rain of the storm. Stemflow gauges on eight representative trees were empty at the end of the storm. A network of six cylindrical impedence wetness sensors indicated the tree canopy was not completely dry until 12:00h on 14 March.

Xylem sap .flow measurements Xylem sap flow was measured at a height of 1.4m in 14 trees on a representative 337 m 2 plot (Table 1) located 75 m northwest of the tower. The plot was a polygon determined by connecting bisects of outer plot trees and their nearest neighbouring trees outside the plot (K6stner et al., 1992). An additional 12 small trees (eight silver beech and four red beech) were present in the plot, with heights of 3 - 8 m and diameters 30-120mm. The plot was usually in a portion of the estimated forest footprint contributing significantly to the eddy covariance measurements of E, except on the afternoon of 15 March when the wind emanated from the opposite direction. Mass flow rate of water through the xylem of trees (Q~) was estimated from half-hourly steady-state energy balances o f undisturbed sapwood (to a depth of 50mm) heated by electrodes to a constant 3 ° (Th) above the temperature of unheated sapwood (Tu) (Steinwald Electronics, Marktredwitz, Germany), located at the same

58

F.M. KELLIHER ET AL.

TABLE 1 Some average characteristics (and standard errors) of the 14 trees in a xylem sap flow measurement plot (337m 2) at Maruia by species and social position Social position

Trees

Diameter (m)

Height (m)

Crown depth (m)

Crown area (m 3)

3 4 1

0.6 (0.1) 0.4 (0.01) 0.3

34 (0.9) 30 (0.6) 23

18 (1) 14 (0.6) 10

73 (10) 38 (6) 14

1 5

0.6 0.3 (0.01)

30 24 (0.8)

18 12 (0.7)

54 20 (4)

Red beech

emergent canopy sub-canopy Silver beech

canopy sub-canopy

height but about 0.2 m away, as Qi =

ki

Pi Cw(th-

Tu)

(1)

where P~ is power input to tree i (in units of W, a 30 min average), Cw is the specific heat of water and k i = C i / [ d ( e - 1)] with Ci, d, and e being stem circumference of tree i, distance between electrodes, and number of electrodes, respectively (Pearcy et al., 1989; K6stner et al., 1992). Power input is the difference between power supplied to the stem and losses caused by convection by sap flow and also by conduction and radial-heat loss to the surrounding air. Conduction and radial-heat losses were constant because of the constant heated-unheated sapwood temperature difference, and were equal to the minimum power input, obtained at pre-dawn on 14 March when fog and darkness assured that xylem sap flow approached zero. For the 14 measurement trees on 17 March, averages of sapwood water content and density were 0.80kgkg l ( + 0 . 1 3 k g k g - I ) and 5 4 0 k g m - 3 ( + 2 0 k g m 3), respectively. Bark thickness varied from 3 to 14mm (average = 8mm). For a 1.2m vertical band centred about the height of instrumentation, the stems of measurement trees were surrounded by 150mm thick fibre-glass insulation (manufacturer's stated thermal conductivity = 0 . 0 4 4 W m - t oK - t ) and covered with plastic sheeting. Estimation of stand sap-flux density requires areal extrapolation of Q~, which has been done using sapwood area (Granier et al., 1990; Diawara et al., 1991), leaf area (Cermak, 1989), and estimates of occupied ground area for measurement trees (Hatton and Vertessy, 1990). Histochemical sapwood depth of wood cores taken from the 14 plot trees varied from 35 to 82 mm with an average of 61 m m (standard deviation = 18mm). However, temperature measurements at depths 5, 15, 25, and 35 mm below bark of thickness 10 mm

T R E E T R A N S P I R A T I O N IN A BROAD-LEAVED FOREST

4 I

I

I

59

I /.~.~- s . J i

I

I

I

I

I

7

16

\~,

--h

t

° /

-4

16

20

0

4

8

~'~,.

,'/

~

- J

, Ref

_ 5

_15

25

,

,

T

7

; J'o

12

16

20

0

4

35

8

[-

Hour (NZST)

Fig. 1. Half-hourly averages of differential sapwood temperature at depths 5 ( - - ) , 15 ( - - - - ) , 25 (. . . . ), and 35 (. . . . ) mm, with respect to a reference temperature adjacent to but outside the stem ( - - ) . Sapwood temperature measurements were made just above electrodes that heated the stem to a depth of 50mm from 10:00h on the second of 2 fine late summer days at Maruia. The tree was a 32 m tall emergent red beech.

and above heated sapwood indicated that sap flow was confined to the outer 20 m m of xylem (physiologically active sapwood; Fig. 1). Consequently, our 50 m m deep xylem mass flow rate measurements included all depths of sap flow. Leaf area of individual trees was not measured in our stand. Sap-flux density (F) was thus determined on a plot basis by summing Q~ of the 14 trees and dividing by 337m 2 (ground area). An alternative calculation for individual trees determined F by dividing Q~ by an estimate of occupied ground area calculated from a vertical projection of the basal tree crown perimeter, based on anascope measurements. RESULTS AND DISCUSSION

The Jorest energy balance The March measurement period provided a variety of environmental conditions, with clear (12, 15, and 17 March), partly cloudy (13 and 16 March), and wet (14 March) days. The forest energy balance requires estimation of available energy. The Maruia stand contained substantial tree canopy biomass, but some energy still passed through the canopy to the forest floor. At sunrise and sunset, the energy storage term (] + G) was similar in magnitude but opposite in sign to R,, so that there was little available energy. Maximum ] (up to 90 W m - 2 ) w a s obtained 1-2 h after sunrise, and minimum

60

F.M. KELLIHER ET AL.

values (as low as - 9 3 W m -2) occurred at sunset. The range of G was 15-41 W m -2, with maxima and minima 1-2 h after midday and at sunset, respectively. Generally though, (J + G) accounted for about 10% of the available energy. Available energy partitioning into sensible and latent heat is indicated by the Bowen ratio ( = H/2E). On the 2 complete clear measurement days (12 and 17 March), the Bowen ratio tended to obtain a broad plateau of about 1-2 between about 09:00-17:00h (Figs. 2(a), 2(f)). There was a similar range of Bowen ratios during the afternoon of 15 March. Lower, and even negative Bowen ratios were generally obtained around sunrise and sunset. Variable cloudiness on 13 and 16 March caused substantial half-hourly fluctuations in available energy, which generally corresponded with similar changes in the Bowen ratio (Fig. 2(b)). For instance, the Bowen ratio decreased from 2 to 0 between 14:30 and 17:30h on 13 March, in association with a decrease in available energy from 602 to 117Win 2. After rain on 14 March, the Bowen ratio was generally about 0.8 when the tree canopy was wet (Fig. 2(c)). Later in the afternoon when the tree canopy was dry, the Bowen ratio was variable between about 0.5 and 1.3, in contrast to the generally higher values observed for the three completely dry days. These lower Bowen ratios may suggest that tree-water stress was less during a warm dry afternoon following a wet morning, compared with behaviour on a completely dry day (see also later discussion of hysteresis in the relationship of xylem sap flow and air saturation deficit). Generally, much lower Bowen ratios, in the range 0.3-0.8, have been observed above dry canopies in North American and European temperate broad-leaved forests (Droppo and Hamilton, 1973; Verma et al., 1986; Bernhofer and Gay, 1989). However, Balddocchi et al. (1985) observed that H exceeded 2E throughout a fine spring day in a broad-leaved forest, with Bowen ratios up to 3. In contrast, in a tropical broad-leaved forest, average daily Bowen ratios were only 0.3 on fine days (Shuttleworth et al., 1984), with substantial decreases and even a change in sign when the canopy was wet (Fitzjarrald et al., 1988). Weather conditions (particularly, 7~, D, and A) were similar to this study in the other temperate forest studies except that 7~ and D were generally much greater in the Verma et al. (1986) study (up to 30°C and 2.5 kPa, respectively), and the forests had similar heights of about 2025m. Lower D obtained per unit available energy, as would occur in a maritime climate, would be expected to yield relatively higher Bowen ratios (Monteith, 1965). This is evident in the substantial differences in the Bowen ratio obtained by Baldocchi et al. (1985) and Verma et al. (1986) at the same site on days about 15 months apart, when there was adequate soil water. The relatively high Bowen ratios observed in this study are caused by the forest itself and tree structure. Red and silver beech leaves are five to ten times smaller than leaves in the North American broad-leaved forests cited above. -

0.4

0.8

0.4

0.8

1,2

1,6

200

400

o

_

d)

I

8

I

I

10

I

14

I

q

Hour (NZST)

I

Hour (NZST)

12

[

I

16

I

4-

18

I

_

I

8

L

1()

I

10

I

,/

14

I

k~

14

I

Hour (NZST)

12

I

Hour (NZST)

12

I

q

16

I

I

18

I

18

".~,..'~.'1~ 16

I

i

~

-I

8

.

8

.

10

I

10

I

.

14

I

.

14

I

.

Hour (NZST)

12

I

.

Hour (NZST)

12

I

Q

16

k

16

I

18

I

18

I

'u)

1

2E

3

4

0

1

2g

3

Fig. 2. Half-hourly averages of beech forest meteorological and energy balance components at Maruia on 12-17 March 1991 ((a) (f), respectively): air saturation deficit (D, ~ 4 ~ ) and wind speed (u, o- -O) above the forest, available energy (R,, J G, 6__A), and sensible heat (H, ~ - - e ) and latent heat ().E, O- 43) flux densities. On 14 March, the tree canopy was wet until 12:00h.

:z-

-~ 200

~

~" 0 ~E 600

(:3

-?

400

0 E 600

123

1.2

1.6

u

©

~J

© >

7

Z

5

Z

70

62

F.M. KELLIHER ET AL.

Leaf boundary-layer conductance is inversely proportional to leaf size and is directly related to total forest aerodynamic conductance. In combination with leaf area index and tree-canopy architecture, leaf size can particularly affect H, but also 2E, and thus the Bowen ratio. The high Bowen ratios in our forest are related to comparatively large boundary-layer conductances for small leaves, leading to greater sensible heat transfer per unit available energy. At the same time, stomatal characteristics are important to 2E* Canopy conductances of Nothofagus (considered later in the discussion) are generally lower than those in the North American forests, causing low 2E and a concomitant increase in the Bowen ratio. The precision of estimates of total evaporation rate can also be scrutinised by forest energy balance measurements. Agreement between half-hourly values of (H + 2E) and (Rn - J - - G) was generally good ((H + 2E) = 0.97 x ( R n - J - G ) - 14(Wm 2), R 2 = 0.90 for 126 30-minute averages during 12-17 March). However, (H + 2E) tended to be less than (Rn - J - G) by 5 0 - 1 5 0 W m 2 during periods of intermittent cloudiness that may be attributable to significant differences in sampling area between (H + 2E) and (R n - J - G) in a variable radiation regime. On a daily basis, (H + 2E) was within 3-16% of(Rn - J - G) (0.2-0.8 m m d a y ~, expressed as water-depth equivalents, Table 2).

Forest-floor evaporation, tree-canopy transpiration, and xylem sap flow Forest-floor evaporation (E0 was a significant component of E (Table 2) and was related to the available energy (the difference between forest floor Rn (R,f, 5% of above-forest Ro) and G) (Fig. 3). We estimated 2El (expressed in energy flux density units of W m -2) as { ~a(Rnf - G)/(a + 1)} where ~ is the change in latent heat relative to the change of sensible heat of saturated air TABLE 2 Daytime available energy (R n -- J -- G) and sensible heat flux (H), expressed as water-depth equivalents, and total evaporation (E) measured by micrometeorological methods, forest-floor evaporation (El) measured with micro-lysimeters, and an estimate of tree transpiration (E - El) on 6 days in March 1991 at Maruia. Comparative daily (24h) sap fluxes of 14 trees in a 337m 2 plot were converted to flux densities (F) by division by plot area. On 14 March, the tree canopy was wet until 12:00h March

Hours

R n -- J

G

H

E

Ef

E -

Ef

F

( l m 2day i) ( m m d a y i) 12 13 14 15 16 17

06:30-19:30 06:30-19:00 08:30-19:30 12:00-19:30 10:30-19:30 07:30-19:00

6.0 5.2 3.6 3.7 4.3 6.4

3.4 2.4 1.3 2.3 2.1 3.6

2.4 2.0 1.9 2,0 1,7 2,3

0.5 0.4 0.2 0.3 0.2 0.3

1.9 1.6 1.7 1.7 1.5 2.0

1.5 1.4 0.8 1.5 1.3 1.5

63

TREE T R A N S P I R A T I O N IN A BROAD-LEAVED FOREST

60|

I

i

I

I

E 40

0

0 0

°

t

i

0 Q ./ O d. 0 .-

0

o/ ; oI. Q) I "

0

10 20 Equilibrium LEf (W m -2)

30

Fig. 3. The relationship between hourly averages of forest floor latent heat flux density (2E0 measured by lysimetry and equilibrium 2El, estimated as described in the text, on 3 fine s u m m e r days in the beech forest at Maruia. The dotted line is a regression t h r o u g h the origin of slope 1.75 (R 2 = 0.6) and a solid 1 to 1 line is also shown.

(1.27 at 10°C), and { e R n f - - G)/(e + 1)} is the equilibrium 2E r. The coefficient e was estimated as 1.75 under well-watered conditions from linear regression through the origin of the 25 hourly data pairs of daytime measurements (R 2 = 0.60), A value of e > 1.0 suggests that the forest floor was well coupled with air from above the ground, which imposed an air saturation deficit greater than the forest-floor equilibrium air-saturation deficit (Black and Kelliher, 1989). Although we base our calculations on the estimate of forest floor Rn, we are confident that ~ > 1.0, that is, air reached the forest floor as intermittent gusts probably originating above the tree canopy (e.g., Kelliher et al., 1990). Theoretical calculations suggest that quiescent periods between such gusts inside forests are too short for the air-saturation deficit at the forest floor to reach the equilibrium value (Finnegan and Raupach, 1987). Gust penetration through the tree canopy to the forest floor has potential implications for atmospheric mixing and the state of the environment within the tree canopy. Even so, D measured at the forest floor (D0 was significantly less than that above the tree canopy (D)(D(kPa) = 2.22Dr(kPa) + 0.03, R: = 0.98 for 81 30-rain averages, during 11-13 September 1990). Tree-canopy transpiration, derived from half-hourly E measurements and estimates of E r on 2 representative days, was generally greater than plot sap-flux density F, although differences were often < 0.05 m m h ~ (Fig. 4). Measured values of F were much less variable than estimated E - Er, particularly during the intermittently cloudy periods on 13 March. Initiation of F measured at the base of the tree stems appeared to lag E - Ef by 1-2 h in the early morning. At least two possible sources of evaporation might have inflated our early-morning estimates of tree transpiration as E - Ef and perhaps contributed to the later initiation of F at the base of stems. First,

64

F.M. K E L L I H E R ET AL.

0.5

a)

xz 0.4 E E 0.3

0.2

~ o.1 u-

0 8

10

12

14

Hour (NZST)

16

18

8

10

12

14

16

18

Hour (NZST)

Fig. 4. Half-hourly measurements to total beech-forest evaporation (E) minus forest-floor evaporation (Er) (E - E~. = tree canopy transpiration, ~--O) and tree xylem sap-flux density on a 337 m 2 plot (F, o - - ~ ) at Maruia on 12 (a) and 13 (b) March 1991. Sap-flux densities were determined by division of sap fluxes by plot ground area.

many epiphytic mosses and lichens were present in the tree canopy, and evaporation from these plants (without stomata) may have begun before tree stomata opened in the early morning. Second, dew fell overnight and its subsequent evaporation probably also preceded tree transpiration. Beech stomata are nearly closed in darkness and air saturation deficit is very low at night so that tree transpiration takes place during the daylight hours (K6stner et al., 1992). Although daytime measured totals of E - Ef were up to 0.9 m m d a y -~ greater than corresponding values of F, the average difference between the two estimates of tree-canopy transpiration was 0.3 _+ 0 . 1 m m d a y -j (standard deviation) (Table 2). Differences between the two methods were greatest on 14 March, when wet-canopy evaporation contributed to E - Ef during the morning, and on the following afternoon (when F was 0.8 mm), which was the only time tower measurements of E were made upwind of the xylem sap-flow plot. Undoubtedly, some evaporation (particularly on 14 March) emanated from the epiphytic lichens and moss in the tree canopy. The energy balance analysis suggests the possibility of an error of some tenths of a millimeter in daily E, and standard deviations for the lysimeter measurements of Ef were 0.02-0.05 mm day-1. Confining F measurements to 14 trees could also have contributed some error in the comparison. A comparative sap flow/energy balance study in Pinus pinaster (Ait.) plantation over 5 fine autumn days indicated that agreement between daily sap-flux density of ten trees and estimated tree transpiration rate was within 15%, although stand and understorey evaporation were not measured directly (Diawara et al., 1991). In another P. pinaster plantation studied over 19 summer days, there was essentially no disparity between sap-flux density of eight trees and eddy covariance estimates of tree transpiration rate when the probable (unmeasured) understorey evaporation was taken into account (Granier et al., 1990). Conversely, sap-flux density of six Pinus radiata D. Don

1 1~131=. I i X / ~ l ~ l , ~ r ' l i'~/~. 1 IK.tl'4 11 ~1 / ~ D I ~ t J / ~ , L t - L I Z / 4 ~

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shaded ground area of a representative plot at Maruia on 13 March 1991. For each tree, F was determined by division of daily sap flux by an estimate of occupied ground area calculated from a vertical projection of the basal tree-crown perimeter.

trees was l m m a a y • greater m a n ~owen rano measurements oI plantation evaporation rate on 4 fine spring days in Australia (Hatton and Vertessy, 1990). Our comparative results obtained in a natural forest appear to agree even more closely than measurements using other sap-flow techniques in relatively uniform conifer plantations. 1 Y~

lrtlrIS'l)lrtlllOYl/~flVlYOflm~fll

lnlertICllOn3"

L, any tree sap-nux uenslty vaneu uy more than an oruer ol magmtutae over the 14 trees in the xylem sap-flow plot (Fig. 5). The sum of the 14 basal tree crown areas was 540 m 2, which exceeded plot ground area by a factor of 1.6, so that the tree sap-flux densities in Fig. 5 are not directly comparable with the plot sap-flux densities based on ground area given in Fig. 4. The range in tree sap-flux densities of 0.1-1.4 m m day -I on 13 March (mean of 0.7 m m day ~ and a standard deviation of 0 . 4 m m day - l ) is typical for a fine day. Variation in tree sap-flux density on 13 March appeared to be normally distributed (i.e. Gaussian) on a plot basis, with the median equal to the mean and a standard deviation equivalent to one-third of the range. However, variation in tree sap-flux density was not normally distributed in horizontal space (Fig. 5). This indicates the importance of placing eddy covariance instruments well above the forest to obtain adequate mixing of sampled air for flux measurements. When modelling evaporation and stand micro-climate, it has been useful to portray evaporation and even stand micro-climate as horizontally extensive averages in plantation forests (e.g. Kelliher et al., 1990). However, to model evaporation for the more variable natural forest, our results show that it is necessary to incorporate aspects of vertical and horizontal stand structure. Tree social position strongly affected an individual's contribution to plot sap-flux density (Fig. 6). The three emergent trees contributed about 50% of

66

F.M. KELLIHER

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Fig. 6. Half-hourly sap-flux densities (F) for 3 emergent (o), 5 canopy (o), and six subcanopy (,x) beech trees at Maruia on 12-17 March 1991 ((a)-(f) respectively). Sap-flux densities were determined by division of sap fluxes by vertical projections of basal tree-crown perimeters. The tree canopy was wet until 12:00h on 14 March.

daily plot sap-flux density. Particularly after rainfall (Fig. 6(c)), sap flux of emergent trees began about 1 h earlier than that of canopy and sub-canopy trees, suggesting faster canopy drying rates above the general canopy. There were 203 rain days and 2216 mm rainfall at Maruia during the year ending 31 March 1991. As canopy drying rate will be directly related to annual tree transpiration rate in this climate, this increases the importance of emergence to transpiration. Differences in transpiration between trees of the three social positions may be attributable, at least in part, to the state of the environment at different locations in the tree canopy. One important factor could be the frequency of gust penetration, which affects atmospheric mixing and removal of humid air accumulating near the crowns. Another environmental factor results from the complexity of canopy architecture in a natural forest, which leads to variable shading by adjacent larger trees, affecting stomatal aperture and thus trans-

TREE

TRANSPIRATION

IN A BROAD-LEAVED

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Fig. 7. Daytime measurements of half-hourly sap-flux density (F) for three emergent red beech trees and air saturation deficit (D) when net radiation> 150Wm 2 at Maruia on 12 (a) and 13 (b) March 1991. Three data points are labelled with time of day (hour). Sap-flux densities were determined by division of sap fluxes by vertical projections of basal tree-crown perimeters.

piration rate of smaller trees. In addition, maximum stomatal capacity may differ between trees of different social position. K6stner et al (1992) observed a positive correlation between maximum stomatal conductance and leafnitrogen content in Nothofagus at Maruia, as mediated through a low photosynthetic capacity (Schulze and Chapin, 1987). Emergent trees had foliage with a higher nitrogen content than their suppressed competitors (K6stner et al., 1992). Stand structure and nutrition appear to determine maximum tree-transpiration rate. However, during the course of a day, this capacity can be regulated by stomatal response to D. Stomatal conductance of artificially-illuminated red beech shoots, located near the top of the tree canopy, decreased by 50% as D increased from 0.5 to 1.2kPa (K6stner et al., 1992). Sap flux density of the three emergent trees increased with increasing D up to 0.50.6 kPa (Fig. 7). During the afternoon, sap-flux density gradually decreased, despite further increase of D indicating stomatal closure. In the late afternoon, further decreases in sap-flux density reflected decreasing radiation and

68

FM. KELLIHER ET AL.

shading up to sunset as D did not decrease significantly below 0.6 kPa until the end of the day. There was some evidence of hysteresis in the relationship between sap-flux density and D. Trees may have been under water stress late in the day, which reflects the adaptation of N o t h o f a g u s to a generally cool wet climate (Benecke and Evans, 1987). An independent meteorological means of examining the relative importance of radiative and advective energy in determining evaporation rate uses the correspondence of changes in net radiation (R.) and wet-bulb temperature (McNaughton and Jarvis, 1983). Wet-bulb temperature (Tw) is a measure of total heat content of the air proportional to the total heat flux, the sum of sensible and latent heat (i.e. evaporation) fluxes (Monteith and Unsworth, 1990). Although forest-floor evaporation was significant at Maruia, tree transpiration was the dominant component of total evaporation. It can be shown that the relative proportionality of Rn and Tw changes just above the forest reflect the aerodynamic isolation (i.e. conductance) of the stand from turbulent imposition of an air saturation deficit determined by larger-scale (e.g. regional) processes. This, in combination with canopy (stomatal) conductance, sets the contributions of radiative and advective energy to treetranspiration rate (McNaughton and Jarvis, 1983). However, exact quantitative comparisons are difficult because an additional contribution to the extremity of R, variation under intermittently cloudy conditions results from strong forward scattering of radiation by water droplets at the edge of clouds ('silver linings', Monteith and Unsworth, 1990). Aerodynamic conductance for m o m e n t u m transfer (gam) of the forest was proportional to wind speed (u) with a coefficient of about 0.1 (Fig. 8). Similar results were obtained for seven other plant canopies including three wind tunnel models, two stands of corn, and two forests (Raupach, 1989). For coniferous plantations, Jarvis et al (1976) also concluded from a literature review that a convenient approximation for gain is given by the quantity 0.1 *u. One would intuitively surmise that our rough canopy, with up to an approximate 15 m variation in tree height, would enhance mechanical turbulence contributing to a higher gain than might be expected from a more uniform plant stand created by management. However, wake turbulence and horizontal heterogeneity, which would be expected to arise from small-scale structural diversity, appear to be of considerably less importance than shear turbulence at the canopy scale (Raupach, 1989). Atmospheric stability may influence variability of gain at a given wind speed. The Monin-Obukhov length (L) was used to classify stability, in an attempt to incorporate surface boundary conditions (Stull, 1991), with L = { - u . 3 / [ k ( g / T , ~ ) w - ~ . ~ ] } where k is the von Karman constant, g is acceleration due to gravity, and /~ is °K (Stull, 1988). We used the classification system L < - 200m = near neutral, - 2 0 0 m < L < 0 = unstable, and L > = stable, after Anderson et al. (1986). The M o n i n - O b u k h o v length,

TREE T R A N S P I R A T I O N IN A BROAD-LEAVED FOREST

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Fig. 8. The relationship between aerodynamic conductance for momentum (gain) and wind speed (u), derived from three-dimensional sonic anemometer measurements during selected daytime hours on 6 fine days in March 1991 at Maruia. Data are stratified into three atmospheric stability classes based on the Monin-Obukhov length (L, m) as L > - 200 (near neutral, e), 200 < L < 0 (unstable, O), and L > 0 (stable, zx). The solid line is gain = 0.1 u.

which varied from - 1 7 4 0 to 830m, appeared to be somewhat influential although other factors probably also contribute to the variability in g~,m(Stull, 1991). Total forest aerodynamic conductance for water vapour transfer also includes leaf boundary-layer conductance. Boundary-layer conductances for water vapour transfer were 0.04 and 0.02 m s ~(on a one-sided area basis) for typical silver and red beech shoots, respectively, that were coated with gypsum, wetted, and placed in a ventilated porometer chamber with a wind speed of 0.25-0.50 m s ~ (David Whitehead, personal communication, 1991). These estimates suggest an approximate similarity in the contributions of g~,m and the boundary layer component to total aerodynamic conductance with canopy scaling of the shoot boundary-layer conductances using a leaf area index of 7 and the inevitability of low wind speed (like that in the porometer chamber) within the tree canopy. On a clear day (Fig. 9(a)), the greatest short-term change in Rn occurred within 20 min, soon after sunrise, from - 2 to 207 W m-2, with a corresponding increase in T,~ of 0.9°K for a relative sensitivity of 4.3 x 10 3°KW -~ m 2. These relative changes in R n and Tware similar to those obtained over pasture during the afternoon of a partly cloudy day (McNaughton and Jarvis, 1983). Later in the morning, Tw changed substantially less per unit change in Rn (1.2 x 10 3°KW ~m2). Wind speed was about 0.7ms 1 in the early morning and 1.9 m s i in the later morning. Consequently, the relative sensitivity of R, and Tw reflected significant differences in wind speed, aerodynamic conductance, and thus the dynamic nature of coupling between our forest and the air saturation deficit.

70

F.M. KELLIHER ET AL.

On an intermittently cloudy day (Fig. 9(b)), results were also dynamic. The two most significant cloud-induced events in the morning caused 1.1 × 1 0 - 3 ° K W - I m 2 over 5min and 3.3 × 10 3 ° K W - t m 2 over 10min, respectively. Before 10:00h, wind speeds were < l m s -I and gain was 0.030.10ms -~ After 10:00h, wind speed increased to 3 - 4 m s -~ and g,m was > 0.10 m s ~. During the afternoon, R, again changed dramatically over short periods, but corresponding changes in Tw were much smaller than during the morning and were even opposite in sign on some occasions. When the tree canopy was wet (Fig. 9(c)), the maximum relative sensitivity of R, and Tw was about 2.5 × 1 0 - 3 ° K W - t m 2 during a 15min event when wind speed was about 1.0 m s -t . This moderate relative sensitivity suggests that our forest can be closely coupled to the air saturation deficit even when the tree canopy is wet. Under these conditions, wind speed and the air saturation deficit itself would strongly affect tree canopy drying rate and radiative energy would be of lesser importance (although it is recognised that air saturation deficit and radiation are not entirely independent). This is supported by measurements in a mixed Nothofagus forest by Pearce et al. (1980) that showed day and night-time evaporation rates from a wet canopy were similar. Wet canopy evaporation rate significantly influences when tree transpiration can take place. This may be a particularly important determinant of annual tree transpiration rate at Maruia because rain falls there about 200 days of the year. CONCLUSIONS

Beech forest evaporation could be measured by the eddy covariance technique with an accuracy of order 10% based on agreement with independent energy balance measurements. Total evaporation rates were 1.7-2.4mm day t on 6 consecutive late-summer days with different weather conditions. Lysimeter measurements indicated that forest floor evaporation was 10-20% of total evaporation, with rates up to 0.5 m m d a y -t . Xylem sap flow measurements showed that tree social position, assessed by emergence of crown from the general canopy level, strongly affected an individual's contribution to tree canopy transpiration. About 50% of daily transpiration emanated from the emergent class of red beech comprised of only 30-40 trees ha For a dry tree canopy, daytime Bowen ratios were generally about 1-2 during clear periods and were variable, in proportion to the available energy, under partly cloudy conditions. Daytime Bowen ratios were somewhat less at about 0.8 when the tree canopy was partially wet. The unexpectedly high Bowen ratios in this broad-leaved forest corresponded to aspects of tree canopy structure and stand interaction with the environment. The reason for this is two-fold; sensible heat transfer was high because of the small size of beech leaves and stomatal conductance was low because of low photosynthet-

TREE TRANSPIRATION IN A BROAD-LEAVED FOREST

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ic capacity associated with low nitrogen nutrition. This indicates that it is difficult to generalise transpiration characteristics for vegetation types. This New Zealand broad-leaved forest generally behaved more like a conifer plantation than other broad-leaved forests studied in the temperate zone and the tropics. Leaf characteristics and physiology appear to be more important to transpiration than vegetation physiognomy. ACKNOWLEDGMENTS

We are grateful to Drs David Whitehead and Glenn Stewart for kindly allowing us to use their shoot boundary layer and harvested tree data. Drs Dennis Baldocchi and Robert McMillen helped us enormously by providing advice and software for processing 3-dimensional eddy covariance data. Financial support in New Zealand was provided by grants from the Founda-

72

F.M. KELLIHERET AL.

tion for Research, Science and Technology and the Department of Conservation. German funding was provided by Bundesminister fur Landwirtschaft un Forsten and the Bundesminister fur Forschung und Technologie. Our colleages Drs. Peter Clinton, Udo Benecke, and Joanna Orwin and an anonymous reviewer provided constructive criticism of the manuscript. Tom Pearson and Tom Farndon did the final 'mouse work' on the Figures. REFERENCES Abbott, P.F. and Tabony, R.C., 1985. The estimation of humidity parameters. Meteorol M ag., 114: 49-56. Anderson, D.E., Verma, S.B., Clement, R.J., Baldocchi, D.D. and Matt, D.R., 1986. Turbulence spectra of CO~, water vapor, temperature and velocity over a deciduous forest. Agric. For. MeteoroL 38: 81-99. Baldocchi, D.D., McMillen, R.T., Matt, D.R. and Hutchison, B.A., 1985. Evapotranspiration from an oak-hickory forest. In: Proc. National Conf. on Advances in Evapotranspiration. Chicago, IL, 16-17 December 1985, pp. 414-422. Baldocchi, D.D., Hicks, B.B. and Myers, T.P., 1988. Measuring biosphere-atmosphere exchanges of biologically related gases with micrometeorological methods. Ecology, 69: 1331-1340. Beets, P.N., 1980. Amount and distribution of dry matter in a mature beech/podocarp community. N.Z,J. For Sci., 2: 395-418. Benecke, U. and Evans, G., 1987. Growth and water use in Nothofagus truncata (Hard beech) in temperate hill country, Nelson, New Zealand. Institute of Terrestrial Ecology, Merlewood Research Station, UK. ITE Symp., 20: 131-140. Bernhofer, Ch. and Gay, L.W., 1989. Evapotranspiration from an oak forest infested with mistletoe. Agric. For. Meteorol., 48: 205-223. Black, T.A. and Kelliher, F.M., 1989. Processes controlling understorey evapotranspiration. Philos. Trans. R. Soc. Lond., (Ser. B.) 324: 207-231. Cermak, J., 1989. Solar equivalent leaf area: an efficient biometrical parameter of individual leaves, trees and stands. Tree Physiol., 5: 269-289. Diawara, A., Loustau, D. and Berbigier, P., 1991. Comparison of two methods for estimating the evaporation of a Pinus pinaster (Ait.) stand: sap flow and energy balance with sensible heat flux measurements by an eddy covariance method. Agric. For. Meteorol, 54: 49-66. Droppo, J.G. and Hamilton, H.L., 1973. Experimental variability in the determination of the energy balance in a deciduous forest. J. Appl. Meteorol, 12: 781-791. Finnegan, J.J. and Raupach, M.R., 1987. Transfer processes in plant canopies in relation to stomatal characteristics. In: E. Zieger, G.D. Farquhar and I. Cowan (Eds.) Stomatal Function. Stanford Univ. Press, Stanford, CA, pp. 385-429. Fitzjarrald, D.R., Stormwind, B.L., Fisch, G. and Cabral, O.M.R., 1988. Turbulent transport observed just above the Amazon forest. J. Geophys. Res., 93: 1551-1563. Granier, A., Bobay, V., Gash, J.H.C., Gelpe, J., Saugier, B. and Shuttleworth, W.J., 1990. Vapour flux density and transpiration rate comparisons in a stand of Maritime pine (Pinus pinaster Air.) in Les Landes forest. Agric. For. Meteorol, 51: 309-319. Hatton, T.J. and Vertessy, R.A., 1990. Transpiration of plantation Pinus radiata estimated by heat pulse method and Bowen ratio, Hydrol. Processes, 4: 289-290. Jarvis, P.G., James, G.B. and Landsberg, J.J., 1976. Coniferous forest. In: J.L. Monteith (Ed), Vegetation and the Atmosphere, Vol. 2, Academic Press, London, pp. 171-240. Kelliher, F.M., Whitehead, D., McAneney, K.J. and Judd, M.J., 1990. Partitioning evapotransiration into tree and understorey components in two young Pinus radiata D. Don stands. Agric. For. Meteorol, 50:211-227.

TREE TRANSPIRATIONIN A BROAD-LEAVEDFOREST

173

Korner, Ch., Bannister, P. and Mark, A.F., 1986. Altitudinal variation in stomatal conductance, nitrogen content and leaf anatomy in different plant life forms in New Zealand. Oecologia 69: 577-588. K6stner, B.M.M., Schulze, E.-D., Kelliher, F.M., Hollinger, D.Y., Byers, J.N., Hunt, J.E., McSeveny, T.M., Meserth, R. and Weir, P.L., 1992. Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: An analysis of xylem sap flow measurements. Oecologia, (in press). McMillen, R.T., 1988. An eddy correlation technique with extended applicability to non-simple terrain. Boundary-Layer Meteorol, 43: 231-245. McNaughton, K.G. and Jarvis, P.G., 1983. Predicting effects of vegetation changes on transpiration and evaporation. In: T.T. Kozlowski (Ed), Water Deficits and Plant Growth. Vol. 7. Academic Press, London, pp. 1-47. Monteith, J.L., 1965. Evaporation and environment. In: G.E. Fogg (Ed.) The State and Movement of Water in Living Organisms. Symp. Soc. Exp. Biol. No. 19. Academic Press, New York, USA. pp. 205-234. Monteith, J.L. and Unsworth, M.H., 1990. Principles of environmental physics. 2nd edn. Edward Arnold, London. 291 pp. Pearce, A.J., Rowe, L.K. and Stewart, J.B., 1980. Nighttime, wet canopy evaporation rates and the water balance of an evergreen mixed forest. Water Resourc. Res., 16: 955-959. Pearcy, R.W., Schulze, E.-D. and Zimmermann, R., 1989. Measurement of transpiration and leaf conductance. In: R.W. Pearcy, J. Erlinger, H.A. Mooney, and P.W. Rundel (Eds.) Plant Physiological Ecology: Field Methods and Instrumentation. Chapman Hall, London, UK, pp. 137 160. Raupach, M.R., 1989. Stand overstorey processes. Phil. Trans. Roy. Soc. London (B) 324:175 190. Schuepp, P.H., LeClerc, M.Y., MacPherson, J.I. and Desjardins, R.L., 1990. Footprint predictions of scalar fluxes from analytical solutions of the diffusion equation. Boundary-Layer Meteororol., 50: 355-373. Schulze, E.-D. and Chapin, I|I, F.S., 1987. Plant specialization to different resource availability. Ecol. Stud., 61: 120-148. Shuttleworth, W.J., Gash, J.H., Lloyd, C.R., Moore, C.J., Roberts, J., Filho, A.M., Fisch, G., Fihlo, V.S., Ribeiro, M.G., Molion, L.B., Sa, L.D., Nobre, J.C.A., Cabral, O.M.R., Patal, S.R., and E)e Moraes, J.C., 1984. Eddy correlation measurements of energy partition for Amazonian forest. Q. J. R. Meteorol Soc., 110: 1143-1162. Stewart, J.B., 1988. Modelling surface conductance of pine forest. Agric. For. Meteorol; 43: 19-35. Stewart, G.H. and Rose, A.B., 1990. The significance of life history strategies in the developmental history of mixed beech (Nothofagus) forests, New Zealand, Vegetatio, 87:101-114. Stull, R.B., 1988. An introduction to boundary layer meteorology. Kluwer Academic Publishers. Dordrecht, Netherlands, 666 pp. Stull, R.B., 1991. Static stability---An update. Bull. Am. Meteorol Soc., 72: 1521-1529. Verma, S.B., Baldocchi, D.D., Anderson, D.E., Matt, D.R. and Clement, R.J., 1986. Eddy fluxes of CO 2, water vapor, and sensible heat over a deciduous forest. Boudary-Layer Meteorol, 36:7 l-91. Weaver, H.L. and Campbell, G.S., 1985. Use of Peltier coolers as soil heat flux transducers. Soil Sci. Soc. Am. J., 49: 1065-1067. Webb, E.K., Pearman, G.I. and Leuning, R., 1980. Correction of flux measurements for density effects due to heat and water vapour transfer. Q. J. R. Meteorol Soc., 106:85 100.