Radiation interception measurement in poplar: sample size and comparison between tube solarimeters and quantum sensors

AGRICULTURAL AND FOREST METEOROLOGY ELSEVIER

Agricultural and Forest Meteorology 85 (1997) 209-216

Radiation interception measurement in poplar: sample size and comparison between tube solarimeters and quantum sensors M. Sattin a,*, R. Milne h, J.D. Deans b, P.G. Jarvis c a Centro di Studio sulla Biologia ed il Controllo delle Piante Infestanti--C.N.R., AGRIPOLIS, 35020 Legnaro (Padova), Italy b Institute of Terrestrial Ecology, Bush Estate, Penicuik EH26 OQB, UK c Institute of Ecology and Resource Management, University of Edinburgh, Mayfield Rd., Edinburgh EH9 3JU, UK Received 25 January 1996; accepted 24 September 1996

Abstract Monitoring radiation interception requires frequent measurements of incoming and transmitted radiation through the canopy. When planning and setting up such an experiment, the following must be taken into consideration: type of sensor to be used, how many measurements (in space and time) to use, and the statistical reliability of the collected data. To answer these and other questions, an experiment was carried out in Scotland using container-grown stands of three poplar clones. The transmittance data were not normally distributed. The mean and variance of hourly, daily and weekly values of transmittance, calculated using tube solarimeters or quantum sensors, were computed. The relationship between the mean and the variance, for both types of sensor, showed a good fit to the model S 2 = ax b (where S 2 is the variance, x is the mean, and a and b are equation parameters). The parameters of this equation have been used to determine the required sample size for the two types of sensor on the basis of the standard error of the mean. The reliability of the average transmittance does not change when considering the hourly, daily or weekly data. The standard error stabilises with 2-3 tube solarimeters and with 3-5 quantum sensors. There are high standard errors only with very high transmittance using the quantum sensors. The average seasonal ratio between the photosynthetic photon flux density (PPFD) and solar irradiance (SI) of the incoming radiation was 1.90 mmol J - l. With closed canopies (LAI > 2.5), the transmittance measured with tube solarimeters was as much as 4 - 8 times higher than that measured with quantum sensors. As a consequence, the fraction of intercepted solar irradiance was lower than the fractional PPFD interception. The seasonal average radiation conversion ratio based on PPFD and $I were 0.54 g m o l - l and 1.21 g MJ-u, respectively. © 1997 Elsevier Science B.V. Keywords: Radiation interception; Tube solarimeters; Quantum sensors

1. Introduction Over the last 20 years, much attention has been paid to the amount of radiation intercepted by agri-

* Corresponding author.

cultural crops and tree stands, and since Monteith (1972, Monteith, 1977) first proposed the hypothesis that dry mass production was proportional to the interception o f solar radiation, many workers (e.g. Gallagher and Biscoe, 1978; Bonhomme et al., 1982; Sivakumar and Virmani, 1984; Unsworth et al., 1984; Cannell et al., 1987, Cannell et al., 1988; W a n g et al., 1991; Corlett et al., 1992) have demonstrated the

0168-1923/97/$17.00 ~,) 1997 Elsevier Science B.V. All rights reserved. PII SO 1 6 8 - 1 9 2 3 ( 9 6 ) 1 ) 2 4 0 1 -X

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M. Sattin et al. /Agricultural and Forest Meteorology 85 (1997) 209-216

validity of this relationship for different crops and stands. The monitoring of radiation interception by the canopy of interest is, therefore, a fundamental step for this type of growth analysis, necessitating frequent measurements of the incoming and transmitted radiation through the canopy (Russell et al., 1989; Monteith, 1994). In the design of a field radiation-interception experiment, the following points need to be considered: (1) the type of sensor to be used; (2) how many measurements, in space and time, are required; (3) how to analyse the data statistically. Good reviews of the main methods of measuring intercepted radiation have been written by Sheehy (1985) and Pearcy (1989). Tube solarimeters sensitive to radiant energy over the whole solar spectrum (solar irradiance, SI) and quantum sensors, which respond to the number of photons in the wavelength band from 400 to 700 nm (the photosynthetic photon flux density, PPFD), are the most commonly used. Conversion between these measurements taken in the open is simple, given the fairly constant PPFD/SI ratio (Meek et al., 1984), but beneath canopies conversion is more difficult because of selective absorption of PPFD, and there is a lack of experimental data on this subject. Ideally, intercepted radiation should be measured continuously for the whole life of the crop. However, the need for replication makes this approach expensive and limits the number of canopies scanned. Generally, the type and number of sensors and the frequency of measurements are a compromise between available equipment and the statistical reliability of the data. There are papers that analyse the spatial distribution of radiation (e.g. Reifsnyder et al., 1971; Norman and Jarvis, 1974; Norman and Jarvis, 1975; Pukkala et al., 1991; Toumebize and Sinoquet, 1995) or the relationship between photosynthetically active radiation (PAR) and Red:Far-Red ratio (Sattin et al., 1994) beneath certain canopies, but there is a general lack of information about the number of sensors or length of path that should be used to obtain reliable measurements of transmitted radiation instantaneously or time integrated. The aims of this paper are: (1) to determine the required number of tube solarimeters and quantum sensors that should be used in relation to variability

in SI and PPFD canopy transmittance; (2) to compare the radiation transmittance in terms of PPFD and SI for three poplar clones during the season; (3) to obtain information about the PPFD/SI ratio of the incoming radiation and the radiation transmitted through poplar canopies; (4) to compare the biomass/radiation conversion ratio calculated in terms of energy or quanta.

2. Materials and methods

2.1. Experimental site and design An experiment was carried out in Scotland using container-grown stands of three poplar clones with contrasting canopy architectures. Uniform plots of three container-grown poplar clones, 'Beaupr6' (Populus trichocarpa Torr. and Gray V-235 × P. deltoides Bartr S. 1-173, an International clone), 'Fritzi Pauley' (P. trichocarpa Torr. and Gray V235) and 'Robusta' ( P. deltoides X P. nigra, a Euroamerican clone also known as P. × euramericana (Dode) Guinier), were established on level ground at 185m above sea-level at Bush Estate, near Edinburgh, Scotland (56°51'N, 3°12'W). Cuttings were planted singly in 10 dm 3 pots, which were arranged in plots of 40 X 15 pots for each of the three clones, comprising a single block of 40 X 45 pots. The variability of growth potential between individual plants of each clone was limited as far as possible (Milne et al., 1992). The plots were set out on capillary matting on 13 April 1989 and cuttings which failed to develop were replaced on 25 May by others which had been similarly maintained adjacent to the experimental block. After setting out the plots, the capillary matting was soaked by a fixed network of hoses and the cuttings were watered from above to allow capillary rise from the matting through the pots. The capillary matting was maintained at full water capacity throughout the growing cycle and during warm dry weather additional water was supplied by overhead sprinklers. As the plants grew, a green net barrier was progressively raised around the perimeter of the experimental plot. The net was 50% permeable to light and wind, reducing edge effects while providing protection from wind damage.

M. Sattin et al. /Agricultural and Forest Meteorology 85 (1997) 209-216

2.2. Radiation measurements Solar radiation flux was measured using tube solarimeters of 1.0 m length (TSL tube solarimeters, wavelength range 300-3000nm, Delta-T Devices, Cambridge). Two tube solarimeters were placed beneath each clone arm one in the open nearby. A dome solarimeter was placed beside this latter sensor to provide measurements which could be used to make adjustments for the non-uniform angular response of the tube solafimeters. The photosynthetic photon flux density was measured using 34 home-made (P.G. Jarvis, A.P. Sandford, F. Sinclair, D. Mackenzie, Institute of Ecology and Resource Management, Schools of Forestry and Ecological Science, University of Edinburgh) PPFD point sensors (wave]length range 400-700nm) with very similar characteristics to the LI-190 quantum sensors (LI-COR, Inc., Lincoln, NE, USA). Before setting up the experiment all the sensors were intercalibrated against a new LI-190. After the sensors had been placed in position, they were left running for 10 days before any interception of radiation started and the data from this period were used to crosscalibrate them once more. A random check on five sensors (against a new LI-190) at the end of the experiment showed no particular drift from the original calibration. Eight PPFD sensors were placed beneath each clone and two in the open nearby. The signals from solariraeters and PPFD sensors were recorded by a data logger (Delta logger, Delta-T Devices) and stored as hourly totals which were subsequently downloaded to the mainframe using a portable computer. These hourly totals were subsequently used to produce daily and weekly totals of intercepted solar radiation for each clone, and the

211

hourly, daily and weekly transmittances (%) were then computed. Intercepted solar radiation ( F i) was calculated as

Fi= F o - Fg where F 0 is the radiation above the canopy and Fg is the radiation at ground level. The starting date for growth and data acquisition was 5 June for 'Beaupr6' and 'Fritzi Pauley' and 15 June for 'Robusta'. Further experimental details have been given by Milne et al. (1992).

2.3. Calculation of conversion ratios For each clone and period between two consecutive harvests, the biomass/radiation conversion ratio ( e ) was calculated, in terms of both intercepted solar radiation and quantum flux. Considering that no obvious trends of E with time were observed (although variation occurred), the average seasonal for each clone was calculated as the slope of the regression between the accumulated dry mass and intercepted solar radiation in terms of energy and quanta. This procedure, although criticised by Demetriades-Shaw et al. (1992), is still considered appropriate by other workers (Monteith, 1994; Arkebauer et al., 1994).

2.4. Statistical treatment The distributions of all the hourly, daily or weekly values from the two different types of sensor were found not to be normally distributed using the Pearson test (Snedecor and Cochran, 1980). As the measurements were taken throughout the growing cycle, it can be expected that when the crowns are further

Table 1 SI and PPFD transmittance (%) and leaf area index (LAD for the three poplar clones on various dates during the growing cycle Date

'Beaupr6'

'Fritzi Pauley'

Transmittance (%)

11 July 14 August 4 September 2 October 6 November

SI

PPFD

26 14 16 19 39

13 5 5 8 31

LAI

2.8 5.6 3.3 2.8 0.0

Transmittance (%) SI

PPFD

32 12 13 20 36

15 2 3 8 21

'Robusta' LAI

2.0 5.1 3.4 2.7 0.7

Transmittance (%) SI

PPFD

80 t5 13 20 52

83 3 3 8 21

LAI

1.0 6.2 4.2 3.3 0.0

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apart and the transmittances are higher, the variability will be larger. The mean and the variance of hourly, daily or weekly percentage transmittances were computed and the relationship between these two statistical parameters was investigated. As there were only two tube solarimeters for each clone and the leaf area indices (LAIs) and light transmittances for 'Beaupr6' and 'Fritzi Pauley' were similar (Table 1), the means and the variances for the tube solarimeter measurements have been calculated putting together the data from these two clones, but excluding clone 'Robusta', which developed later. This allowed a better estimate of the variances because the calculations were then based on four replicates rather than on two. The data from the last month (when most of the light was intercepted by branches) and those from the first 2 h and last 2 h of the day for the first 10days after the plants began to intercept radiation (when the sun was low in the sky, causing very low but unusually uniform interception) were excluded from the calculation of the mean-variance relationships.

3. Results 3.1. Sensors' sample sizes

The data from both types of sensor (Fig. 1) showed a good fit to the model S 2 = ax b

-5 ...."". -t

1

2

3

4

-1

0

1

2

3

4

In(mean) Fig. 1. Mean (ln)-variance (In) relationship of the hourly percentage of transmittance for (a) the quantum sensors (equation of fitted straight line is y = - 1 . 8 2 + 1.78x) and (b) tube solarimeters (equation of fitted straight line is y = - 4 . 1 7 + 1.94x). The data from the last month, and those from the first and last 2 h of the first 10days after interception began, have been excluded.

25

8 20

a

g 15 i1)

E cd

- ----

90%transmittance 20% " 5% '

t

b

- ----

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5 2

4

6

8

10

2

4

6

8

10

Number of sensors Fig. 2. Relationship between sample size (number of quantum sensors (a); number of tube solarimeters (b)) and the standard errors (SE) for different mean transmittance percentage (hourly values).

(Taylor, 1961) or ln(S 2 ) = l n a + b l n x where S z is the variance, x is the mean, and a and b are equation parameters. The above relationships have then been used, on the one hand, to determine the required sample sizes on the basis of the standard error of the means (standard error, SE = o ' / n 1/2 o r S E = ( ( a x b ) / / n ) 1/2) for a required level of accuracy and, on the other, to give an approximate value of the reliability of the mean transmittances. This approach has been proved to be valid in determining sample sizes in other types of experiments (Barralis et al., 1986; Borin and Berti, 1991). The relationships between sample size (i.e. number of sensors) and standard errors for different mean transmittance percentages are shown in Fig. 2. There are high standard errors (greater than 10%) only at very high transmittances, especially for the quantum sensors (Fig. 2(a)). At medium-low transmittance, present during most of the growing cycle, the standard errors are similar for both types of sensor and they stabilise with 3 - 5 quantum sensors (Fig. 2(a)) and 2 - 3 tube solarimeters (Fig. 2(b)). The reliability of the average transmittance does not change with accumulated daily or weekly values (Fig. 3). It should be noted that the use of different percentage transmittances for the two types of sensor in Fig. 2(a) and Fig. 2(b) and in Fig. 3(a) and Fig. 3(b) is because of the influence of the different wavelength ranges of the two types of sensor on the detected transmittance: 20% and 5% mean transmittances measured with quantum sensors roughly correspond to 30% and 15% mean transmittance measured with tube solafimeters.

M. Sattin et al. /Agricultural and Forest Meteorology 85 (1997) 209-216

213

2000

20 - -

15 |

hourly values

---deiy

;

- -

t

h0udy values

-_-_ daily :

~n

1500

::~:.~....

E 1000

/~

0)

E 03 0 ~A

2

4

6

8 10 122 4 6 N u m b e r of sensors

8

E3 u_ El.

10

Fig. 3. Relationship between sample size (number of quantum sensors (a); number of tttbe solarimeters (b)) and the standard errors (SE) for hourly, dail[y and weekly values of mean transmittance. A mean transmittance of 20% for the quantum sensors (a) and 30% for the tube solarimeters (b) was considered.

The relationships between the standard errors (calculated as above) and transmittances for the sample sizes used in the experiment were very similar, the standard errors for the quantum sensors being slightly larger than the standard errors for the tube solarimeters and practically linear. The standard errors were very low (less than two) for transmittances less than 20% and even at very high transmittances, recorded before the canopies closed, the standard errors did not differ greatly between the two types of sensor and their values were always below 10%. Furthermore, owing to the influence of the different wavelength ranges on transmittance detected by the two sensor types, the standard errors were practically identical for a given transmittance, demonstrating the comparability of the average transmittances at any given time.

500

13._

0

,

i

0

,

400

L

,

600

i

,

800

1000

SI (W m-2)

Fig. 4. Relationshipbetweenincomingphotosyntheticphoton flux density (PPFD) and solar irradiance(SI) (hourly values). Regression line has been forcedthroughthe origin.

varied according to canopy density, leaf optical properties (e.g. as affected by leaf age) and clone. SI and PPFD were transmitted differently by the three poplar clones: the transmittances started to diverge very early in 'Fritzi Pauley' (when the mean transmittance was about 70%, Fig. 5(b)) whereas in 'Beaupr6' the two transmittances remained similar until transmittance reached about 30% (Fig. 5(a)). The coefficient for 'Robusta' showed intermediate behaviour. With closed canopies (LAI > 2.5) the transmittance

90 ~,~. "~:

5O

©

o

90

PPFD SI

.~....,.....

.--

, "~---

~

£-

Another aspect that has been investigated is the relationship between incoming radiation measured in terms of PPFD and of SI (Fig. 4). As other workers have pointed out, the two types of measurement have a very close proportionality. (R 2 = 0.987 for the fitted straight line of the hourly data). The slope (b = 1.90) is, however, slightly lower than those previously reported (Cannell et al., 1987; Grace et al., 1987). The slopes of the fitted straight lines for the daily and weekly data (data not shown) are not statistically different from that for the hourly data. The coefficient of proportionality between PPFD and SI was not the same beneath the canopies and

2652

b = 1.90

J

200

, ,

3.2. PPFD and SI transmittance

,

n

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, , ,

b

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50

~,.,,.--?,..~, , ~ ,

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~..~\

::

o .oo

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=

,o

.

150

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.

.

C

,

.

200

250

300

Julian day Fig. 5. PPFD and SI transmittances throughout the growing season for the three poplar clones: a, 'Beaupr6'; b, 'Fritzi Pauley'; c, 'Robusta' (daily values).

214

M. Sattin et al. / Agricultural and Forest Meteorology 85 (1997) 209-216

_o 2.5

1.5

$1

o

a

c\;.

6

V:o ,,';¥;,

4.,( ._d 2

D.. 0.5 13- 0i ................ 150 200 250 300 200 250 300 200 250 300

Julian day Fig. 6. PPFD/SI ratio of transmitted radiation (m) and LAI ( 0 ) throughout the growing season for the three poplar clones: a, 'Beaupr6'; b, 'Fritzi Pauley'; c, 'Robusta' (daily values). The horizontal line represents the average value of the PPFD/SI ratio for the incoming radiation (1.90 p,molJ-I ; see Fig. 3).

of SI was, on average, much higher than the PPFD transmittance (15-20% vs. 2-8%) (Fig. 5). The rise in transmittance for 'BeauprE, and partially for 'Fritzi Pauley', between Day 220 and 250 could be attributed to the modification of the canopies as a result of strong winds and high rainfall during that period. The scattering of the data recorded in the last part of the growing cycle was probably the result of the ageing and shedding of the leaves. In particular, a gale on Day 263 strongly modified canopy architecture. It should be noted that despite the negligible LAIs measured at the end of the season, the transmittance was still fairly low (about 40%) and, therefore, mainly the result of absorption by the branches as well as lower solar elevation. 2 R2=0.744

'\\ m

O3 C~ Ii

1.5

''

y=1.85 e(-0.2486x) X, \ \ \

•

1 \\

0_ G_ 0.5

0o

2

4

6

LAI Fig. 7. Relationship between LAI and PPFD/SI ratio (average of three days centred on the day LAI was recorded). The data points relative to the last LAI determination have been excluded because most interception was due to the branches. The intercept with the y-axis has been fixed at 1.901xmolJ -I (PPFD/SI value of the incoming radiation). 0 , 'Beanpr6'; *, 'Fritzi Pauley'; A, 'Robusta'.

The ratio between transmitted PPFD and SI for the three clones shows a high variability during the season, with the minimum between Days 200 and 230 (20 July-beginning of September) and reaching values as low as 0.15-0.3, depending on the clone (Fig. 6). This ratio also shows a strong dependence on LAI, although the clones behave differently, 'Robusta' (with reddish leaves) showing, in fact, values not as low as the other two clones despite the highest value of LAI (Fig. 6(c)). Despite these differences, given the low number of data points available for each clone, a common relationship for the clones was calculated (Fig. 7). A negative exponential equation showed a good fit to the data ( R E = 0.774) indicating that the higher the LAI is, the lower the rate of decrease of the PPFD/SI ratio.

4. Discussion The statistical method used proved to be a useful tool for determining the number of sensors required to obtain a reliable estimate of SI or PPFD transmittance while minimising the cost. With the uniform canopies present in this experiment, the number is low: 2-3 tube solarimeters and 3-5 quantum sensors appear to be enough to obtain reliable transmittance data. The number of quantum sensors seems to be inadequate only with very scattered canopies, where tube solarimeters gave better results (at least as far as the variability of the measurements is concerned). These considerations might be somewhat different for row-crops or spaced plantations of large trees (Wang et al., 1991). The seasonal average PPFD/SI ratio of the incoming radiation was somewhat smaller than has been reported in the literature (Varlet-Grancher et al., 1981; Pereira et al., 1982; Meek et al., 1984; Zhou et al., 1984; Weiss and Norman, 1985) or calculated through models (Lanciani and Ponticello, 1993), showing that care must be taken when this ratio is used in modelling studies. The small PPFD/SI ratio recorded in this study is likely to result from the frequent overcast days during the experiment. Blackburn and Proctor (1983) recorded very similar values for the ratio under cloudy skies in Canada, and other workers (Stitger and Mus-

M. Sattin et al. / Agricultural and Forest Meteorology 85 (1997) 209-216

215

Table 2 Dry matter production, seasonal fractional interception and conversion ratio of SI and PPFD (4- standard error) for the three poplar clones Clone

'Beanprr' 'Fritzi Pauley' 'Robusta' Mean

Dry matter production (g m-2 )

1526:1- 122 1405:1- 155 1216 :t- 158 1382

Fractional interception

Conversion ratio (¢)

SI

PPFD

SI (g M J - l )

PPFD (g mol- i )

0.64 + 0.02 0.62 + 0.01 0.45 4- 0.002 0.57

0.73 + 0.01 0.74 4- 0.01 0.55 4- 0,02 0.67

1.19 + 0.07 1.09 + 0.07 1.35 + 0.11 1.21

0.53 + 0.03 0.49 + 0.03 0.60 + 0.05 0.54

The data relate to the period from 5 June to harvest date (6 November) for 'Beauprr' and 'Fritzi Panley', and from 15 June to harvest for 'Robusta'.

abilha, 1982; Slomka and Slomka, 1986) have found even lower values in overcast conditions. The transmittance of SI and PPFD proved to be very different under dose, dense canopies. The trends of PPFD/SI ratio were closely mirrored by the LAI trends, but there was no relation between the maximum LAI value and the lowest PPFD/SI ratio of the three clones. Different leaf coloration (i.e. different optical properties) might account for the different PPFD/SI ratios for the three clones during the midpart of the season. This makes the conversion from one type of measurement to the other beneath canopies difficult and leads to the conclusion that, in these circumstances, PPFD sensors are more suitable given their better wavelength range of sensitivity, which is similar to the active spectrum of photosynthesis (McCree, 1973). It is interesting that, following strong wind and rain, the planophile clone 'Beauprr' and, to a lesser extent, the intermediate 'Fritzi Pauley', both with large leaves, showed a sharp rise in values of the PPFD/SI ratio between Days 220 and 240, whereas 'Robusta', more erectophile and with small leaves, was almost insensitive. As a consequence of the different transmittances, the fraction of intercepted SI was lower than that of PPFD (Table 2). As PPFD is the most appropriate measurement of radiation in relation to plant productivity, it may be concluded that SI measurements 'underestimate' canopy interception, thereby causing an 'overestimation' of e. The average e per period between two consecutive harvests, in terms of both energy (Milne et al., 1992) and quanta, diid not show any obvious trend during the season, ;although variation occurred in relation to weather conditions. During the third pe-

riod (between Days 226 and 247), when the weather was not favourable, e only reached values between 0.11 and 0.38gmo1-1 (or between 0.26 and 0.84 g MJ-l). With better weather conditions, during the following period, e reached values as high as 3 - 4 times those of the previous period. The seasonal average 8 value in terms of quanta (0.54 g mol- 1), similar to that obtained by Wang et al. (1991) for Sitka spruce, shows that the energy conversion efficiency obtained in this study is near the maximum obtainable for tree stands in Scotland.

Acknowledgements Thanks are due to Bob Astles, Ray Otfley and Frank Harvey for setting up and maintaining the tree stand at ITE. Dr. A. Berti at CNR-Padova gave much helpful advice on the statistics. The Italian National Research Council (CNR) provided the funding for M. Sattin to visit Edinburgh University.

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