Radiation balance, transpiration and photosynthesis of an

Agricultural and Forest Meteorology, 64 (1993) 201-221 201 Elsevier Science Publishers B.V., Amsterdam

Radiation balance, transpiration and photosynthesis of an isolated tree

S.R. Green The Horticulture and Food Research Institute of New Zealand, Private Bag 11 030, Palmerston North,

New Zealand

(Received 15 June 1992; revision accepted 20 November 1992)

ABSTRACT

Green, S.R., 1993. Radiation balance, transpiration and photosynthesis of an isolated tree. Agric. For. Meteorol., 64: 201-221.

The radiation balance of an isolated walnut tree was measured using an experimental Whirligig device. The total amount of all-wave radiation absorbed by the tree canopy was used to estimate transpiration rates using a Penman-Monteith model. The results compared favourably with tree water use, measured by the heat-pulse technique. The total amount of photosynthetically active radiation (PAR) absorbed by the tree canopy was combined with a photosynthetic light response curve to estimate net photosynthesis rates. The results compare favourably with published data from other tree canopies.

Daily energy balance calculations showed that on average, about two-thirds of the total radiant energy absorbed by the tree canopy was dissipated as latent heat in the form of transpiration. The dominant environmental variable influencing transpiration was the vapour pressure deficit of the air. Almost two-thirds of the net latent heat flux was attributable to the vapour pressure deficit component, with the remainder owing to the radiation component. Daily transpiration-assimilation ratios varied from day to day in response to changing environmental conditions, but generally decreased with increasing net photosynthesis and with increasing transpiration. This appears to be the first time that such a direct measurement of the energy balance and photosynthesis of a single tree has been made.

INTRODUCTION

One of the major factors influencing both transpiration and photosynthesis in plants is the amount of radiation intercepted by the foliage elements. In general, the amount of intercepted radiation depends not only on the density and distribution of the foliage, but also on the amount of incoming radiation and the proximity of neighbouring plants. In many agronomic, forestry and horticultural applications, the plants are widely separated and the vegetation

Correspondence to: S.R. Green, The Horticulture and Food Research Institute of New Zealand, Private Bag 11 030, Palmerston North, New Zealand.

0168-1923/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

202 s.R. GREEN

is best represented by isolated plants. Thus it is of practical and theoretical interest to know how single plants intercept and utilise radiation.

Many theoretical models have been developed to predict radiation absorption within continuous canopies (Norman, 1979; Ross, 1981), but only a few models have been developed to predict radiation interception by either individual plants in a stand (Grace et al., 1987; Wang and Jarvis, 1990) or by single, isolated plants (Charles-Edwards and Thornley, 1973; Thorpe et al., 1978; Fritschen et al., 1980). On the other hand, several experimental methods have been developed to measure the radiation balance of individual plants. Examples include the work of Van Elsacker et al. (1983) and Thorpe et al. (1978), who both used simple photographic methods for analysing the radiation interception by individual plants, and McNaughton et al. (1992) who developed a radiometric method to measure the radiation interception of a single, small tree.

The purpose of this paper is to report the results from an experimental study of the energy balance and photosynthesis of a single, isolated tree. The absorption of both all-wave radiation and photosynthetically active radiation (PAR) were measured using the radiometric method described by McNaughton et al. (1992). These data are used to estimate rates of transpiration and net photosynthesis by the tree canopy. Transpiration estimates are compared against tree water-use measured by the heat pulse technique and an energy balance is computed for the whole tree canopy. The relative importance of both radiation and vapour pressure deficit, in controlling tree water use, are investigated. Transpiration-assimilation ratios are calculated for the tree canopy using the ratio of moles of H 2 0 lost during transpiration to moles of CO2 taken up during photosynthesis.

THEORY

In this section the set of equations to predict transpiration rates are presented and the method of measuring the amount of radiation absorbed by the tree canopy is outlined.

Transpiration by a single leaf

The equation for predicting the transpiration per unit of leaf plan area for a single, hypostomatus leaf is given by Thorpe (1978) as

s R n q- 0 . 9 3 p c p D a / r a

2E = s+O.937(2+rs/ra) (1)

where 2 is the latent heat of vaporization (2.454 kJ g-~), E is the transpiration rate (g s -~ m -2 leaf plan area), R n is the net radiation flux density absorbed by the leaf (W m -2 leaf plan area), D a is the vapour pressure deficit of air (Pa),

RADIATION BALANCE, TRANSPIRATION AND PHOTOSYNTHESIS OF AN ISOLATED TREE 203

r a is the leaf boundary layer resistance (s m -~), rs is the leaf stomatal resistance (s m-~), s is the slope of the saturation vapour pressure curve (Pa °C-~) at the ambient air temperature T a, (°C), 7 is the psychrometric constant (66.1 Pa), p is the air density (kg m-3) " and Cp is the specific heat capacity of air at constant pressure (J kg -~ K 1). The factor 0.93 arises from the assumption that the boundary layer resistance for vapour is 0.93 times that for sensible heat. Equation (1) can be used for routine calculation of leaf transpiration if the necessary meteorological data are recorded (Rn, Da, Ta, u) and either measurements or estimates of leaf boundary layer resistance and stomatal resistance are available.

Transpiration by a whole plant

The transpiration rate for a whole plant can be found by summing eqn. (1) over the population of leaves on a plant. For a set (canopy) of symmetrical, hypostomatous leaves exposed to air with a common ambient saturation deficit, the total evaporation can be written as the sum of the evaporation flux from all of the i leaves in a canopy (layer), so that

~ . FsRni +0.93pcpD,/rai

where the f~ 's are the fractional area of each leaf expressed in terms of the total leaf area of the canopy (or canopy layer), and the remaining terms are as defined by eqn. (1). Equation (2) is the equivalent Penman-Montei th model for canopy transpiration and follows directly from the treatment of a single, hypostomatous leaf with water vapour transport from one side only (Jarvis and McNaughton, 1986). Expanding the left hand side of eqn. (2) yields

s f R n , i 0 . 9 3 f p c p D . Ira, i

2E = ~ s_+_O.937(2+rs,i/ra,i) -]-~i s+O.937(Z+rs,/ra,i) (3)

(I) (II)

where the total evaporation is written as the sum of a radiation component (I) and a vapour pressure deficit component (II), respectively. The task of calculating eqn. (3) then reduces to that of estimating the fractional leaf areas, f , and determining mean values for the resistances, r~. i and r,,i, and the corresponding radiation absorption, Rn,~, as the other meteorological variables can be measured directly.

For the purpose of calculation, eqn. (3) can be approximated by consider- ing the total canopy leaf area to be divided into a set of uniform sunlit leaves, which have a stomatal resistance appropriate to fully-sunlit leaves; the remaining fraction of shaded leaves absorb zero net radiation and have a stomatal resistance appropriate to fully shaded leaves. The fractional leaf areas and the corresponding radiation absorbed by sunlit and shaded leaves

2 0 4 SR. GREEN

can be estimated from the total amount of radiation intercepted by the whole plant canopy, as described below.

Leaf boundary layer resistances can be calculated from the empirical relation derived by Landsberg and Powell (1973), which accounts for the mutual sheltering of clustered leaves as

ra = 58p°S6(d/u) °5 (4)

where d is a characteristic leaf dimension (m), u is the mean windspeed (m s-~) across the leaf surface (taken at a mid canopy height), and p is a measure of foliage density 'seen' by the wind (p is the ratio of total leaf plan area to the area of the foliage projected onto a vertical plane).

Radiation absorption by a single plant

The net amount of radiation absorbed by a plant canopy can be found by integrating the net radiation flux passing inward through each point of a surface which encloses the plant. This relationship can be written formally as

= IsR, ds (5) Q.

where Qn is the total radiation flux absorbed by the plant, R, is the net inwards radiation flux density and ds is an elemental area of the enclosing surface, S. McNaughton et al. (1992) describe a method based on eqn. (5) for measuring radiation absorption, using a sphere to enclose a single tree. For a sphere of radius, Ls, eqn. (5) can be rewritten as

,', - - 2 /'n/2 Qn = zrcLs J-=/2 R.(O)cos(O)dO (6)

where Rn (0) is the average inward radiation flux density at latitude 0, and 0 is measured in radians. The integral in eqn. (6) is approximated using the weighted sum of radiation data recorded at discrete latitudes by a set of eight net radiometers. The mathematics for calculating the weighting factors used in approximating Qn is described fully in McNaughton et al. (1992). The experimental device for measuring these radiation totals is known as the 'Whirligig'. A modified form of this device was used in the present study, as described below.

M A T E R I A L S A N D M E T H O D S

Experimental procedure

Measurements were made of the radiation absorption and transpiration by an isolated walnut tree at the Massey University Research Orchard, near Palmerston North, New Zealand (latitude 40.2 S, longitude 175.4 E). Data


were collected continuously, every 20 min over a 9 day summer period (no rain) from the 17-25 January 1992. There was a 5 min break in data collection every other day to retrieve radiation data from the Whirligig. During the course of the experiment, a diurnal set of stomatal resistance data and a photosynthetic light response curve were measured using leaves from a neighbouring walnut tree. These data were collected for the purpose of calculating rates of tree transpiration and canopy photosynthesis.

Plant material

The tree was a 10 year old walnut growing on a Karapoti silt loam (Fig. 1). The tree was isolated, being north of and some 15 m away from the nearest neighbouring tree, and not shaded except at very low sun angles. The tree canopy was about 3.5 m high x 3.2 m wide. The total projected leaf area (one side only) of the tree was estimated to be 26.4 m 2, using a sub-sample of 10% of leaves whose area was measured in situ using a Licor portable leaf-area meter (model LI-3000, Licor, Lincoln, NE). The silhouette area of the foliage projected onto the vertical plane was estimated photographically to be about 6.5 m 2 , giving rise to a canopy foliage density, p, equal to about 4.1 (m 2 m-2). This estimate ofp was subsequently used in eqn. (4) to calculate leaf boundary layer resistances, based on a characteristic leaf dimension, d = 0.1 m, which was the mean leaf width. All leaf boundary layer resistances were taken to be equal.

Radiation absorption

The net amount of radiation absorbed by the walnut tree was measured using a modified version of the Whirligig device described by McNaughton et al. (1992). Briefly, the Whirligig consisted of a vertical, circular frame rotating on a level, horizontal turntable. A discrete set of eight net radiometers were mounted at equally spaced intervals on the circumference of the frame. As the frame rotated it described a sphere about the tree, with each radiometer following a horizontal, circular path at a fixed latitude on the sphere. All-wave radiation was measured using a set of eight matched REBS Q*6 net radiometers (Radiation and Energy Balance Systems, Seattle, WA) and PAR radiation was measured using a set of 16 matched PAR sensors similar in design to those described by Biggs et al. (1971). The total radiation absorbed by the tree canopy was approximated using the weighted sum of the radiation data recorded by each sensor.

A pair of Campbell CR21X data loggers (Campbell Scientific, Logan, UT) were used to measure signals from the radiometers once every second, and to compute a total for the absorption of all-wave radiation (W) and PAR radiation (#mol s -~) once every 20 min. The calculations were made after a

206 S.R. GREEN

Fig. 1. Photograph of the Whirligig device mounted around a 3.5 m tall walnut tree at the experimental site near Palmerston North, New Zealand.

whole number of revolutions of the Whirligig, rotating at about 3.2 rpm. Radiation data from the Whirligig were synchronised with the other meteorological measurements using pulses generated from a microswitch mounted on the stationary part of the turntable to the Whirligig.


The procedure for computing total radiation absorption was the same as that described by McNaughton et al. (1992). However, following the initial tests reported by McNaughton et al., the drive mechanism of the Whirligig was altered slightly to improve both the reliability of the measurements and the mechanical stability of the rotating frame. These modifications involved removing the central pole from the Whirligig, and driving the turntable directly by an electric motor, thereby leaving the inside of the Whirligig free of obstructions. The energy absorbed by the empty Whirligig frame was typically less than 5% of the total radiant energy absorbed by our test tree, and was subsequently ignored in the calculations.

Fractional leaf areas

The area of the sunlit leaves was estimated from the ratio of the total amount of PAR radiation absorbed by the canopy (/~mol s -~) divided by the incoming PAR flux density over grass (/tmol s -~ m-2). On a sunny day, this ratio was about 6.5 m 2 at solar noon, which is similar to the estimate of silhouette area of the tree canopy projected onto the vertical plane; the vertical projection of leaf area on the ground was assumed to be the same. As a first approximation a spherical leaf distribution was assumed, giving rise to a sunlit leaf area twice as large as the vertical projection on the ground. Thus, of the 26.4 m 2 of total leaf area some 13 m 2 were estimated to be sunlit and the remaining 13.4 m 2 of leaf were assumed to be in the shade.

Stomatal resistance

Stomatal resistance was measured on leaves from an adjacent walnut tree at 30 min intervals throughout the day of 21 January 1992, using a Licor LI1600 steady state porometer. Walnut leaves are hypostomatous, so measurements were made on the underside of the leaves only. Mean resistances were calculated from seven sunlit and seven shaded leaves chosen randomly from the sunlit and shaded parts of the tree canopy, respectively.

Tree transpiration by the Penman-Monteith model

The procedure for calculating tree transpiration was to divide the total canopy leaf area into fractions of sunlit and shaded leaves. Tree transpiration rates were then calculated using eqn. (3) by assuming the total net (all-wave) radiation absorbed by the canopy was absorbed by a set of uniform sunlit leaves, which had a total leaf area of 13 m 2 and a stomatal resistance appropriate to fully-sunlit leaves. The remaining shaded leaves were assumed to absorb zero net radiation (i.e. Term I in eqn. (3) was assumed to be zero), having stomatal resistances appropriate to fully shaded leaves. All leaf

208 S.R. GREEN

boundary layer resistances were taken to be equal, and calculated using eqn. (4).

Tree transpiration by the heat-pulse technique

Transpiration rates were also estimated from the rate of sap movement in the stem of the walnut tree using the compensation heat-pulse technique (Swanson and Whitfield, 1981). Three sets of heat-pulse probes, each com- prising a heater and two temperature probes, were installed into parallel holes drilled radially into the tree stem. The probe sets were installed at heights of between 0.25 and 0.50 m above the ground and were equally-spaced around the circumference of the tree stem. Sap velocity was measured as described by Green and Clothier (1988) using the theoretical calibrations of Swanson and Whitfield (1981) to account for the probe induced effects of wounding.

Each set of heat-pulse probes contained four sets of sensors to measure sap velocity at four radial depths. The sensor depths ranged from 0.5 to 4.8 cm below the cambium in a stem with a mean radius of 5.5 cm. By using three sets of heat-pulse probes sap velocity was measured at 12 locations in the sapwood. Volume flow rates were calculated from the integral over the sapwood cross-section, of a second-order least squares regression equation fitted to the sap velocity profile, given by

Eh = 2nf~rV(r)dr (7)

where E h is the measured transpiration rate (kg h -~) and V(r) is the sap velocity at a radial depth, r,in a stem of cambium radius R and heartwood radius, h.

A Campbell CR21X data logger was used to control the heat-pulse equip- ment and to measure sap flow rates automatically, once every 20 min. Between measurements, the data logger switched-off power to the heat-pulse equip- ment in order to conserve the batteries. The sap flow data were synchronized with the Whirligig measurements, by activating the heat-pulses at 10 min into every 20 min interval.

Tree photosynthesis

Net rates of photosynthesis for the whole tree canopy were calculated by combining photosynthetic light response measurements from single leaves with estimates of mean PAR irradiance of the canopy leaves.

The photosynthetic response to incoming photon flux was measured on three sunlit leaves from an adjacent walnut tree, using an LCA2 portable infrared gas analyser (ADC Ltd, Hoddesdon, UK) and natural sun as the light source. Output from the ADC system was logged on a Toshiba laptop computer. A range of photon flux densities was produced by placing neutral


density filters over the leaf chamber to progressively shade the leaf surface. Dark respiration rates were measured in zero light. In addition the absorbance ratio, which is the fraction of incoming PAR that is absorbed by the leaf, was measured on a few leaves to be 0.76, using a laboratory designed integrating sphere (Greer and Laing, 1992).

Photosynthetic rates for the tree canopy were calculated on the basis of the mean PAR flux densities for the sunlit and shaded leaves. Preliminary measurements on shaded leaves indicated the incident photon fluxes were only about 8% that of the sunlit leaves. On the basis of these measurements, mean PAR flux densities were calculated for sunlit and shaded leaves by assuming the total net PAR radiation absorbed by the canopy (Qp) was being absorbed by 13 m 2 of sunlit leaves and 13.4 m 2 of shaded leaves, the PAR absorption by shaded leaves being 0.76 times 8% that of sunlit leaves. So the mean PAR flux density of the sunlit leaves was calculated as Qp/(13.0 + 0.76"0.08" 13.4) #mol m -2 s -~ and the mean PAR flux density of the shaded leaves was calculated as 0.76"0.08"Qp/(13.0+0.76"0.08"13.4)/~mol m -2 s -~.

The procedure for calculating canopy photosynthetic rates was to add together the estimates of CO2 uptake by sunlit and shaded leaves, based on a photosynthetic light response curve measured at a single temperature of 25 °. However, since net photosynthetic rates are dependent on temperature, the calculations were corrected to compensate for temperature changes. This temperature correction involved adjusting the respiration rates using a Qt0 value of 2.0, which is equivalent to halving the respiration rate for a 10°C decrease in air temperature (see Jones, 1983). This procedure for calculating canopy photosynthesis is analogous to a big-leaf approximation since it assumes each leaf absorbs the average amount of PAR radiation and that each leaf has the same light response characteristics.

Meteorological measurements

PAR radiation, all-wave radiation, air temperature and vapour pressure deficit were measured once every 15 s and averages of these meteorological data plus mean windspeed were recorded once every 20 min using a Campbell CR21X data logger.

Net all-wave and PAR radiation balances over grass were measured using a REBS Q*6 radiometer and a pair of PAR sensors, respectively. These radiation sensors were the same type as those used on the Whirligig. The net PAR balance was taken as the difference between measured incoming and outgoing PAR radiation. The three radiation instruments were mounted beside each other at a height of about 4.5 m above the ground, on a mast approximately 10 m away from the Whirligig. Mean air temperature and vapour pressure deficit were measured using a ventilated psychrometer similar to that described by Black and McNaughton (1971). Wind speed was

210 S.R. GREEN

5000

4000

3000

= 2000

1000

0

i

0 6 12 1'8

_ _ 2 3 - , - , 2

600

400

200

- - - - 0

24

Time of day

Fig. 2. Diurnal course of the total amount of all-wave radiation (Q°) absorbed by an isolated walnut tree on 23 January 1992. The total projected leaf area of the tree was 26.4 m 2. Also shown is the flux density of net all-wave radiation (R,g) measured over a level, grassed area nearby.

measured using a small, three-cup anemometer. Both the psychrometer and anemometer were mounted on the instrument mast at a height of 2.5 m above the ground, which was about equal to mid canopy in the walnut tree.

R E S U L T S A N D D I S C U S S I O N

Net all-wave radiation balance

The pattern of net all-wave radiation absorption by the walnut tree on an almost cloud free day shows a much more flat-topped curve for the absorption of radiation compared to incoming radiation flux (Fig. 2). This is exactly what would be expected for an isolated tree; the canopy intercepts more radiation at low sun angles and the decline in fractional light interception towards solar noon is counter balanced by the increased incident radiation. Similar patterns of net radiation absorption for a single tree were found by McNaughton et al. (1992) and for a hedgerow of apple trees by Landsberg et al. (1974).

During the middle part of the day about 4300 W of radiant energy was absorbed by the tree canopy. This equates to an average flux density of about 160 W m -2 leaf plan area, which is similar to measurements on apple leaves reported by both Landsberg et al. (1974) and Butler (1976), showing a rather flat topped curve with a net radiation per unit leaf area of about 140 W m - 2 .

R A D I A T I O N BALANCE, T R A N S P I R A T I O N A N D PHOTOSYNTHESIS OF AN ISOLATED TREE 211

10.0

8 .0

T 6.0 ¢n

E E

~-~ 4.0

2.0

- - s u n n y leaves ....... s h a d e leaves ~ 23-1-92

÷ ÷

~" A

0.0 0 6 12 18 24 Time of day

Fig. 3. Time course of stomatal conductance for sunlit and shaded leaves. Each point is the mean of seven leaves; the errors bars are + 1 SD. Each line is a logistic function fitted to the resistance data using a non-linear least squares regression technique.

Tree transpiration rates

The time course in stomatal conductance (gs = 1/rs), is shown in Fig. 3. A logistic function was fitted to this data using a non-linear least squares regression technique, in order to generate values o fg s synchronized with other meteorological data used in the calculation of 2E. Separate curves were fitted to the sunlit and shaded leaves, as shown in Fig. 3. In fitting a logistic equation to the conductance data, it was simply assumed the time course was sym- metric, with a maximum value at solar noon and a minimum value during the middle of the night• Expressing gs as a function of time was a rather crude approximation, made necessary because insufficient data were collected to express g~ as a function of the main driving variables (i.e. PAR and Da). Nevertheless this approximation produced a reasonable fit to day-time con- ductances, albeit with a small overestimation of stomatal conductance in the late afternoon as the stomata presumably began to close in response to higher vapour pressure deficits.

Transpiration rates predicted by the Penman-Monte i th model for 23 January 1992 compare well with instantaneous sap flow measured by the heat-pulse method (Fig. 4), given the likely errors involved in both methods. Integrating these data over the whole day (midnight to midnight) yields a predicted tree water use of 38.3 1, compared with a measured sap flow of 39.61.

212 S.R. GREEN

5.0

I ...c o~ 4.0

v

"~ 3.0 ¢-

. 2 "4,'- o 2.0

. _ EL (/1 c e 1.o

I.-

o.o o

Penman-Montei th ~ ' ~ 2 3 - 1 - 9 2 - 5 : H e a t - p u l s ~ I / f ~

2 ),, 6 12 18 24

Time of Day

Fig. 4. Comparison of the diurnal course in tree transpiration predicted using a Penmon-Monteith model (solid line) and measured using the heat-pulse technique (broken line).

These results show not only that the transpiration rates predicted by the Penman-Monteith model are reasonable, but also they increase our confi- dence in the radiation absorption data from the Whirligig, since Q, is an important term in the calculations.

Tree energy balance

The radiant energy balance of the tree canopy was calculated using Q, = H + 2E, where Qn was measured using the Whirligig and 2E was estimated by the Penman-Monteith model. The sensible heat flux, H, was not measured. Instead, H was simply computed as the difference between Qn and 2E.

The energy balance for an almost cloud free day showed that in the early part of the day, H exceeded 2E so that most of the absorbed radiant energy was dissipated as sensible heat (Fig. 5). H peaked around midmorning (09:00 h) and then declined progressively through the remainder of the day, becoming negative shortly after sunset, presumably as the tree canopy cooled. In contrast, the diurnal pattern of 2E demonstrated a steady increase through the day, peaking around mid-afternoon (15:30 h) and declining thereafter as both radiation and vapour pressure deficit decreased. It should be noted that 2E remained positive but small, i.e. transpiration continued for a few hours after sunset, since the vapour pressure deficit of the air remained elevated and the leaf stomata remained partially open (Fig. 3). Green et al. (1989) observed


5000

4000

3000

= 2000

1000

0

0 6 12 18 24

Time of day

Fig. 5. Energy balance of the isolated walnut tree Q, is the total all-wave radiation absorbed by the tree canopy, 2E is the latent heat flux calculated using a Penman-Monte i th model and H is the sensible heat flux calculated by taking the difference between Q. and 2E.

night-time transpiration in apple trees and kiwifruit vines under similar environmental conditions.

Latent heat fluxes exceeded sensible heat fluxes throughout most of the day so that over the 24-h period (midnight to midnight), approximately 64% of the total radiant energy absorbed by the tree was dissipated as latent heat. The daily integral indicated that of the 148 MJ of energy absorbed by the tree canopy, some 94 MJ of energy was dissipated as latent heat and only 54 MJ was dissipated as sensible heat. These results are counter to the energy balance data presented for an isolated Scots pine by Fritschen et al. (1980), where the sensible heat flux was almost as large as Qn throughout most of the day and very little transpiration occurred.

The Penman-Monte i th model provides a direct estimate of the relative importance of radiation and vapour pressure deficit in controlling tree transpiration, as indicated by Terms I and II of eqn. (3), respectively (Fig. 6). In the early part of the day, the radiation component was larger than the deficit component, whereas in the later part of the day the reverse was true and transpiration became more strongly influenced by vapour pressure deficit. The daily integrated totals (midnight to midnight) for this day indicated that of the 94 MJ of latent heat energy, some 51 MJ (i.e. 54%) was derived from the vapour pressure deficit component and only 43 MJ (i.e. 46%) was derived from the radiation component. The pattern illustrated in Fig. 6 was typical for

214 S.R. GREEN

v

c o

o - -

o - - 1:: t~ O.

O) 5 -

¢.-

W

3000

2 0 0 0

,~E J' - - Q-term - - D-term

1000 ~ ., f / /

0 ' ' '8 0 6 12 1 24

Time of Day Fig. 6. The relative importance of the radiation and vapour pressure deficit in controlling tree transpiration. 2E is the latent heat flux calculated using eqn. (3) and the lines denoted by Q and D are the corresponding radiation and vapour pressure deficit components (Terms I and II of eqn. (3), respectively),

the sunny days of the experiment, whereas on cloudy days (e.g. 20 January) the radiation component was typically less than the deficit component throughout most of the day.

Daily energy balance totals for each day of the experiment are presented in Table 1 and the corresponding mean meteorological data are presented in Table 2. On average, 64% of the radiant energy absorbed by the tree canopy was dissipated as latent heat and only 36% of the absorbed radiation was dissipated as sensible heat. The vapour pressure deficit was the dominant environmental variable influencing transpiration of the walnut tree. Accord- ing to the Penman-Monte i th model, the vapour pressure deficit component was about 58% of the latent heat flux, whereas the radiation component was only about 42% of the latent heat flux integrated over the day (midnight to midnight). A similar observation was made for apple trees by Thorpe (1978) who found that the radiation component seldom exceeded 30% of the latent heat flux.

On average, the daily estimates of tree transpiration by the Penman- Monteith model are within about 10% of the rates of sap flow measured by the heat-pulse technique (Table 1). This result further helps to validate the use of a Penman-Monte i th model to predict canopy transpiration over the course of the experiment.

There were a few days, however, where predicted transpiration was less


TABLE I

Individual components of the daily energy balance of an isolated walnut tree. Qn is the total radiant energy absorbed by the tree canopy, hE is the latent heat flux estimated by eqn. (3), and H is the sensible heat flux calculated by taking the difference between Qn and hE. Q-term and D-term refer to the radiation and vapour pressure deficit components of hE (Terms I and II of eqn (3), respectively), hE h is the daily volumetric sap flow measured by the heat-pulse technique

Day Qn H 2E Q-term D-term hE h January, 1992 (MJ) (MJ) (MJ) (M J) (M J) (M J)

17 53.2 20.2 33.0 15.5 17.5 36.2 18 111.8 45.9 65.9 29.2 36.6 74.3 19 128.3 51.7 76.6 26.9 49.6 95.3 20 96.7 24.2 72.5 23.9 48.6 84.8 21 140.8 55.6 85.3 37.8 47.4 87.3 22 102.0 32.0 70.0 32.7 37.3 76.1 23 147.6 53.8 93.8 43.2 50.6 96.9 24 77.2 27.4 49.8 17.6 32.2 57.5 25 62.6 24.0 38.6 17.5 21.1 43.9

consistent with the measured sap flow. For example, on the 19 and 20 January, predicted transpiration rates were consistently between 15 and 25% lower than measured sap flow rates. Many factors could help to explain this anomaly, although it is quite likely that the stomatal response on these days was different from the response measured on a neighbouring tree some 4 days later. It is well known that stomata respond to different environmental conditions (Jarvis, 1981). So, in the case of changing environmental conditions, canopy resistance is not a simple function of time as has been assumed here. A better approximation may well have been to determine r s as a function of the main driving variables such as radiation and vapour pressure deficit.

TABLE 2

Mean daily environmental data over the experimental period 17-25 January 1992. Rng is the net all-wave radiation flux density over grass, Rpg is the net PAR radiation flux density over grass, T a is the air temperature, D a is the saturation deficit of the air and U is the mean wind speed

Day Rng Rpg T~ D a U January 1992 ( M J m -2) ~umol m -2) (C) (Pa) (ms -1 )

17 5.8 18.8 15.7 175 0.73 18 14.0 44.0 15.3 290 1.38 19 16.2 57.0 12.8 398 2.20 20 10.3 40.5 11.0 436 1.34 21 16.6 58.4 12.2 421 0.97 22 12.4 41.0 14.7 355 0.79 23 17.2 55.9 15.0 429 1.11 24 9.5 20.8 16.6 234 1.55 25 6.7 15.9 16.8 206 1.07

Qp " ~ ' "" . 23-1-92 RI~ J

15000

10000

0 "~ 5000

2 1 6 S.R. GREEN

0 0 6 12 18 24.

2000

1500 "~"~'~ ? E

1000 "~

g 500 n-

Time of doy

Fig. 7. Diurnal course of the total amount of PAR radiation (Qo) absorbed by an isolated walnut tree on 23 January 1992. The broken line shows the net flux density of PAR radiation (R.g) measured over an adjacent horizontal grassed area.

However this was not possible because the necessary data were not collected at the same time as the stomatal resistance measurements were made.

The net sensible heat flux for 20 January was relatively small, being only about 25°,/0 of the net all-wave radiation absorbed by the tree canopy (Table 1). This low value for H resulted from a combination of small daytime sensible heat fluxes as a result of overcast sky conditions, and relatively large negative sensible heat fluxes at night (i.e. leaf temperature less than air temperature). Assuming night-time transpiration is small, a negative sensible heat flux is expected at night because His approximately equal to Qn (which is always less than or equal to zero). However, high vapour pressure deficits and a warm, windy night gave significant nocturnal transpiration on 20 January, as con- firmed by the heat-pulse results. This night-time transpiration gave rise to an increased negative sensible heat flux at night. The apparently low sensible heat flux on 22 January is also partly explained by a large negative heat flux at night owing to the combination of a high nocturnal radiation loss and some night-time transpiration.

PAR absorption and tree photosynthesis

The diurnal pattern of PAR absorption, Qp, w a s relatively constant during the day compared with the incoming PAR radiation over a horizontal grassed surface (Fig. 7). This is similar to the pattern of absorbed all-wave radiation


,•" I 0 data 15.0 model 23- I -~92

I:: o o o

lo.o[ 8

t g s.oj 0

~- 0.0 Z i i i

0 500 1000 1500

Photon f lux density (/~mol m -2 s - I )

Fig. 8. The photosynthetic light response data measured on three leaves from a neighbouring walnut tree on 23 January 1992 (D. Greer, personal communication, 1992). The curve drawn through the data points is a hyperbolic tangent function described by Laing (1985).

(Fig. 2), except that the measured net PAR balance at night-time is always near zero.

Approximately 13 mmol s -~ of PAR energy was absorbed by the tree canopy during the middle part of the day, and this equates to a mean absorption per unit canopy leaf area of about 492 #mol m -2 s -~ This value is similar to the absorbed photon flux density of 580/~mol m -2 s -~ measured on an isolated apple tree (leaf area of 1.26 m E) under similar ambient conditions (Thorpe et al., 1978). It is worth noting that the mean absorption per unit leaf area peaked at about 940#mol m -2 s -~ for the sunlit leaves and was only about 60#mol m -2 s -J for the shaded leaves.

The photosynthetic light response for walnut was measured at a single temperature of 25°C and a relative humidity of about 50%, using three well-exposed leaves from a neighbouring walnut tree (Fig. 8). Net photosynthetic rates for the tree canopy were calculated from this curve by using the mean PAR flux density for the sunlit and shaded leaves and correcting the respiration rates for temperatures other than 25°C (Fig. 9).

Net photosynthetic rates for the whole tree canopy peaked at around 5.5, #mol m -2 s-~, with a corresponding dark respiration rate of about - 1.0/~mol m -2 s -~ during the night (Fig. 9). The average net photosynthetic rates for the sunlit leaves were much higher, peaking at around 11.0 #mol m -2 s - l , whereas net photosynthetic rates for the shade leaves peaked at around 0.0/~mol m -2

218 S.R. GREEN

T U)

t N I E

m o E ::I.

o I , -

01 n

6.0

4.0

2.0

0.0

- - R p g 2 5 - 1 - 9 2

t l I/ ~t ~ /

0 6 12 18 24

2000

1500 i

I

looo E o E :::1.

500 ~ "

O. c~

Time of Day

Fig. 9. Diurnal course of canopy photosynthesis for 23 January 1992 calculated from PAR absorption (Fig. 7) and the photosynthetic light response curve (Fig. 8) (see text).

s -j . So very little net photosynthesis was occurring in the shaded leaves of the canopy. The diurnal pattern illustrated by Fig. 9 is similar to that of an isolated apple tree (Thorpe et al., 1978) showing a flat topped curve with a relatively broad maximum of around 7/~mol m -2 s -] .

The procedure for calculating photosynthetic rates using average photon fluence rates means that photosynthesis is approximately linear with PAR since the average irradiance levels on sunlit and shaded leaves almost always lies below the upward shoulder of the photosynthesis-light response curve. Flattening of the photosynthesis curve (Fig. 9) relative to the PAR absorption curve (Fig. 7) was therefore attributable to the temperature response of respiration.

Integrating the photosynthetic rates over a full day (midnight to midnight) produced a daily carbon balance for the tree canopy of about 5.1 mol day -j (i.e. about 224 g day-l). This estimate did not include the carbon lost through root respiration so that the daily carbon balance for the whole tree was probably somewhat less than 5.1 mol day -~ . These measurements are similar to daily photosynthetic rates of 4 m tall orange trees reported by Cohen and Fuchs (1987) in a hedgerow orchard under similar ambient conditions.

The daily carbon balance of the tree canopy for each day of the experiment is presented in Table 3, along with daily measurements of tree transpiration measured by the heat-pulse technique. Daily estimates of the transpiration- assimilation ratio (E/A) defined on a mole for mole basis as the ratio of the water lost through transpiration to the net CO2 taken up during photosynthe-


TABLE 3

Daily totals (midnight to midnight) of tree water use by the heat-pulse technique (E), and estimates of the daily carbon balance (A) and transpiration-assimilation ratio (E/A) of the tree canopy

Day E A E/A January 1992 (molday -I) (molday l) (molmol-~)

17 822 1.70 484 18 1683 3.83 440 19 2161 5.08 426 20 1922 4.39 438 21 1978 5.50 359 22 1728 3.91 442 23 2200 5.09 432 24 1306 1.92 678 25 994 1.80 554

sis, ranged between about 430-680 mol mol -~ with a mean value of 470 mol mol -~. These results are in broad agreement with measured daily transpiration-assimilation ratios of other tree species. For example, Cohen and Fuchs (1987) reported an E/A ratio of about 400 mol mol-I in 4 m tall orange trees and Price and Black (1991) reported an E/A ratio of about 560 mol mol- in stands of Douglas-fir, under ambient conditions similar to this experiment. Under advective conditions on the other hand, Baldocchi et al. (1981) found lower E/A ratio of around 200 mol mol -z in alfalfa.

In general, E/A in the walnut tree tended to decrease both with increasing net photosynthesis (r 2 = 0.59) and with increasing transpiration ( r 2 = 0.39), although the relationship was only weak. So the ratio of transpiration to assimilation was not constant, but rather E/A reflected the changing environmental conditions during the course of the experiment. The highest values of E/A were observed on days having lower radiation and lower vapour pressure deficits. Higher E/A ratios on humid, cloudy days have also been reported in alfalfa (Baldocchi et al., 1981) and Douglas fir (Price and Black, 1991).

C O N C L U S I O N S

The Whirligig device described by McNaughton et al. (1992) was found to be an excellent research tool for experimental studies of the energy balance and photosynthesis of small, single trees. Data from the Whirligig were incorporated effectively into a Penman-Montei th model to produce estimates of canopy transpiration which agreed with volumetric rates of sap flow measured in the tree stem. The PAR absorption data were combined with a photosynthetic light response curve of leaves to produce estimates of net photosynthesis by the tree canopy which were similar to published data from other tree canopies. This appears to be the first time that such a direct

220 S.R. GREEN

measurement of the energy balance and photosynthesis of a single tree have been made.

Daily energy balance calculations showed that on average about two thirds of the total radiant energy absorbed by the tree canopy was dissipated as latent heat in the form of tree transpiration. The dominant environmental factor influencing transpiration was found to be the vapour pressure deficit of the air. Almost two-thirds of the net latent heat flux was due to the vapour pressure deficit component with the remainder owing to the radiation component. Daily E/A ratios varied from day to day in response to changing environmental conditions, but generally decreased with increasing net photosynthesis and with increasing transpiration.

ACKNOWLEDGEMENTS

The author wishes to thank Dennis Greer for the photosynthetic light response and PAR absorbance data and John Julian for redesigning and rebuilding the drive mechanism of the Whirligig and assisting in collecting the leaf-area data. Many useful discussions were held with Keith McNaughton, who provided the initial enthusiasm and stimulus for this work.

REFERENCES

Baldocchi, D.D., Verma, S.B. and Rosenberg, N.J., 1981. Environmental effects on the CO2 flux and CO2-water flux ratio of alfalfa. Agric. Meteorol., 24: 175-184.

Biggs, W.W., Edison, A.R., Eastin, J.W., Brown, J.W., Maranville, J.W. and Clegg, M.D., 1971. Photosynthesis light sensor and meter. Ecology, 52: 126-131.

Black, T.A. and McNaughton, T.A., 1971. Psychrometric apparatus for Bowen-ratio determination over forests. Boundary-Layer Meteorol., 2: 246-254.

Butler, D.R., 1976. Estimation of the transpiration rate in an apple orchard from net radiation and vapour pressure deficit measurements. Agric. Meteorol., 16: 277-289.

Charles-Edwards, D.A. and Thornley, J.H.M., 1973. Light interception by an isolated plant: A simple model. Ann. Bot., 37: 919-928.

Cohen, S., Fuchs, M., Moreshet, S. and Cohen, Y., 1987. The distribution of leaf area, radiation, photosynthesis and transpiration in a shamouti orange hedgerow orchard. Part II. Photosynthe- sis, transpiration, and the effect of row shape and direction. Agric. For. Meteorol., 40: 145-162.

Fritschen, L.J., Walker, R.B. and Hsia, J., 1980. Energy balance of an isolated Scots pine. Int. J. Biometeorol., 24: 293-300.

Grace, J.C., Jarvis, P.G. and Norman, J.M., 1987. Modelling the interception of solar radiant energy in intensively managed stands. N.Z.J. For. Sci., 17: 193-209.

Green, S.R. and Clothier, B.E., 1988. Water use of kiwi fruit vines and apple trees by the heat-pulse technique. J. Exp. Bot., 39: 115-123.

Green, S.R., McNaughton, K.G. and Clothier, B.E., 1989. Observations of nighttime water-use in kiwi fruit and apple trees. Agric. For. Meteorol., 48: 251-261.

Greer, D.H. and Laing, W.A., 1992. Photoinhibition of photosynthesis in intact kiwi fruit (Aetinidia deliciosa) leaves: changes in susceptibility to photo inhibition and recovery during the growth season. Planta, 186: 418425.


Jarvis, P.G., 1981. Stomatal conductance, gaseous exchange and transpiration. In: J. Grace, E.D. Ford and P.G. Jarvis (Eds), Plants and their Atmospheric Environment. Blackwell, Oxford, pp. 175-204.

Jarvis, P.G. and McNaughton, K.G., 1.986. Stomatal control of transpiration: scaling up from leaf to region. Adv. Ecol. Res., 15: 1-49.

Jones, H.G., 1983. Plants and microclimate. Cambridge University Press, Cambridge, 323 pp. Laing, W.A., 1985. Temperature and light response curves for photosynthesis in kiwi fruit (Actinidia

chinensis) cv. Hayward., N.Z.J. Agr. Res., 28:117-124. Landsberg, J.J. and Powell, D.B.B., 1973. Surface exchange characteristics of leaves subject to

mutual interference. Agric. Meteorol., 13: 169-184. Landsberg, J.J., Beadle, C.L., Biscoe, P.V., Butler, D.R., Davidson, et al., 1974. Diurnal energy,

water and CO: exchanges in an apple (Malus pumila) orchard. J. Appl. Ecol., 12: 659-684. McNaughton, K.G., Green, S.R., Black, T.A., Tynan, B.R. and Edwards, W.R.N., 1992. Direct

measurement of net radiation and PAR absorbed by a single tree. Agric. For. Meteorol., 62: 87-107.

Norman, J.M., 1979. Modelling the complete crop canopy. In: B.J. Barfield and J.F. Gerber (Eds), Modification of the Aerial Environment of Crops. Am. Soc. Agric. Eng. Monogr. No. 2, ASAE, St. Joseph, MI, pp. 249-277.

Price, D.T. and Black, T.A., 1991. Effects of summertime changes in weather and root-zone soil water storage on canopy CO2 flux and evaporation of two juvenile Douglas-fir stands. Agric. For. Meteorol., 53: 303-323.

Ross, J., 1981. The radiation regime and architecture of plant stands. W. Junk, The Hague, 391 pp. Swanson, R.H. and Whitfield, D.W.A., 1981. A numerical analysis of heat-pulse velocity theory and

practise. J. Exp. Bot., 32: 221-239. Thorpe, M.R., 1978. Net radiation and transpiration of apple trees in rows. Agric. Meteorol., 19:

41-57. Thorpe, M.R., Saugier, B., Auger, S., Berger, A. and Methy, M., 1978. Photosynthesis and transpira-

tion of an isolated tree: model and validation. Plant, Cell Environ., 1: 269-277. Van Elsacker, P., Keppens, H. and Impens, I., 1983. A simple photographical method for analyzing

the radiation interception by an individual tree. Agric. Meteorol., 29: 285-298. Wang, Y.P. and Jarvis, P.G., 1990. Description and validation of an array model, MAESTRO.

Agric. For. Meteorol., 51: 257-280.

Documents

Radiation balance, transpiration and photosynthesis of an