1The applicability of a color acetate film for estimating photosynthetic photon flux2

density in a forest understory

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Koji Kawamura, Michiko Cho, and Hiroshi Takeda

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Laboratory of Forest Ecology, Graduate School of Agriculture, Kyoto University,

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Kyoto 606-8502, Japan

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Author for correspondence:

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Koji Kawamura

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Laboratory of Forest Ecology, Graduate School of Agriculture

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Kyoto University, Kyoto 606-8502, Japan

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Tel/Fax: +81-075-753-6080

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e-mail: koji@kais.kyoto-u.ac.jp

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Short communication (article type); Forest biology and ecology (subject area)

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10 pages of text, 0 tables, 1 page of figure legends, and 2 figures211

12Abstract
The applicability of a color acetate film (R-2D, Taisei Chemical Industries, Tokyo,

3Japan) for estimating photosynthetic photon flux density (PPFD) in a forest understory

4was examined. The fading ratio of the film (F), the total PPFD (PPFDtotal) to which the5film was exposed, and the average daily maximum temperature during exposure (T)6were obtained from measurements at multiple sampling points throughout an entire year

7within a natural secondary forest (N = 42). The ranges of the recorded values were as

8follows: F (35-99%), PPFDtotal (1.4-28.3 mol m-2), and T (6-32°C). PPFDtotal was

9regressed by F and T with a high r2 (= 0.94; P < 0.0001): PPFDtotal = (100 - F) / (1.08510+ 0.051 T). The absolute error (|estimated PPFDtotal – measured PPFDtotal|) averaged 1.311with a maximum of 5.7 (mol m-2), indicating a good fit. These results indicated the12broad applicability of the film, both spatially and temporally, for estimating forest

13understory PPFD.

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Key words: color acetate film, light measurement, PPFD, forest understory

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Introduction
Assessments of photosynthetic photon flux density (PPFD) provide important

3information about the growing conditions of forest understory plants. Direct

4measurements using quantum sensors can be effective, but these devices are expensive,5

cumbersome, and impractical for multiple-point sampling. Simple, indirect methods for

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accurately estimating PPFD in the forest understory are needed.

7A color acetate film (R-2D, Taisei Chemical Industries, Tokyo, Japan) is useful.

8The film is impregnated with azo dyes and fades as the dyes are decomposed by

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exposure to sunlight (Yoshimura et al. 1989). Fukushima (1998) confirmed that the10fading of the film was highly correlated with the integrated PPFD in a forest understory,

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although the study was limited to a single sampling point during one month of the year.

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The spectral sensitivity of the color film is maximal in the green light band (52113nm, Yoshimura et al. 1989), so it does not correspond with that of the quantum sensor

14(400-700 nm) exactly. As leaves can act as a spectral filter, the spectrum of light under15

the forest canopy varies with canopy structure and density (Muraoka et al. 2001).

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Owing to differences in the spectral sensitivities of the film and the quantum sensor, the17relationship between the fading of the film and the integrated PPFD measured using the18

sensor can vary even within a forest. Hence, if film is used at multiple points in a forest19understory, we may need to establish a general relationship between the fading of the20

film and the integrated PPFD, including the possible variation across sampling points.

21In addition, because the fading reaction of the film depends on temperature (Yoshimura

22et al. 1989; Akiyama et al. 2000; Fukushima 2000), we should correct for the effect of3

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temperature if film is used year-round. In this study, to confirm the broad applicability

2of film as a tool for estimating PPFD in forest understory, we examined the relation of3fading of the film, temperature, and the integrated PPFD based on data obtained at

4multiple points throughout an entire year in a natural secondary forest.

56Materials & Methods

789Study Site
The study was conducted in a natural secondary forest at the Kamigamo

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Experimental Forest Station of Kyoto University (35°04'N, 135°46'E; altitude 150 masl),

11Japan. A 15 × 20-m plot was established in the forest. The density of overstory trees

12(with a diameter at breast height ≥ 5 cm) was 1,600 trees/ha; the mean dbh was 11.1 cm;13and the mean height was 8.2 m. The dominant species were the evergreen trees Ilex

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pedunculosa Miq. and Chamaecyparis obtusa Endl., while there were some deciduous

15trees, such as Quercus serrata Thunb. ex Murray. The density of subcanopy and16

understory trees (with dbh < 5 cm and height > 0.5 m) was 10,433 trees/ha. Species of17

Ericaceae (Rhododendron macrosepalum Maxim. and Lyonia ovalifolia Drude var.

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elliptica Hand.-Mazz.) and Theaceae (Eurya japonica Thunb. and Cleyera japonica

19Thunb.) were dominant.

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Color acetate film
Color acetate films impregnated with azo dyes (R-2D, Taisei Chemical Industries,4

1Tokyo, Japan) that fade as solar radiation is absorbed were used in the study. The

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optical transmittance (O) of the film was determined using a portable photometer

3(THS470, Taisei Chemical Industries). The absorbance (D) of the film was calculated

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from the transmittance, O, as follows:

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D = a + b (-log (O / 100))

Eq. 16

where a and b are meter-specific constants (a = -0.2683, b = 1.7456). From the7absorbance of the film before use (D0) and after exposure (D), the fading ratio (F) was

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calculated using the following formula:

9F ＝ 100 (D / D0)

Eq. 2

1011Field measurements

12Two films were put on the side of a quantum sensor (LI-190SA, Li-Cor, NE,13USA), which was then placed at a sampling point in the understory of the established

14plot. The sensor was connected to a datalogger (LIT-DR 190SZ-7600, HOGA, Kyoto,

15Japan), and instantaneous PPFDs were recorded at 2-min intervals. The measurements

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were collected for 7 to 10 days, and subsequently the absorbances of the film after

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exposure (D) were determined. These measurements were conducted repeatedly

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between March 2003 and March 2004 (1-4 points per month). The sampling points were19

arbitrarily located at 1-3 m above the ground within the plot (15 × 20 m), e.g., under20

canopy-gaps, beneath closed-canopy, or within tree crowns, to include a wide range of21

light quality environment. A total of 42 points were sampled.

2251Data analysis

2The fading ratio (F) was calculated for each film using the absorbance of the film

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before use (D0) and after exposure (D) (Eqs. 1-2). The total PPFD (PPFDtotal, mol m-2)

4during the exposure period was calculated as the integral of the instantaneous PPFD

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recorded by the sensor.

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We assumed the following relationship between F and PPFDtotal:

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F = -K (PPFDtotal) + 100

Eq. 3

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where K is a reaction rate constant. The constant K changes with temperature

9(Yoshimura et al. 1989), since the fading rate of each dye is dependent on temperature.

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To characterize the change of K with temperature, we assumed the following

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relationship:

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K=v+wTEq. 4

13where v and w are constants, and T is the average daily maximum temperature during

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the measurement period. Daily records of maximum temperature were obtained from15

the Kamigamo Experimental Forest Station.

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Note that according to a preliminary analysis of our data, the linear functions

17(Eqs. 3 and 4) had higher coefficients of determination than previously used non-linear

18functions, such as exponential (Yoshimura et al. 1989; Fukushima 2000) or19

second-order polynomial (Akiyama et al. 2000) functions. The choice of function to20

characterize the relationship of PPFDtotal to F and T should be examined in each study.

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From Eqs. 3 and 4, the following regression of PPFDtotal on F and T was

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calculated:
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PPFDtotal = (100 - F) / (v + w T)

Eq. 5

2Parameters v and w, and the coefficient of determination, r2, were estimated using the3

Quasi-Newton method with nonlinear least squares (SYSTAT 1992). The estimated

4PPFDtotal was then calculated using Eq. 5, the estimated values of v and w, and the5measured values of F and T, for each measurement. The relationship of the estimated

6PPFDtotal to the PPFDtotal measured directly using the quantum sensor was examined to

7validate the applicability of films for estimating PPFDtotal in a forest understory.

89Results

10For the 42 measurements, the ranges of the recorded values were as follows: F11(35-99%), PPFDtotal (1.4-28.3 mol m-2), and T (6-32°C). The difference in F between

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the two films in each measurement averaged 2.1% (SD = 2.9). The daily PPFDtotal on

13days with clear skies averaged 1.7 mol m-2 (range: 0.2-4.8).

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Figure 1 plots F against PPFDtotal. The data are categorized into two classes of T

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and the data for these two classes segregated. At a given PPFDtotal, F was smaller at

16high temperature class, indicating that high temperatures accelarated the rate of film

17fading. When PPFDtotal was regressed by (100 - F)/K, and the effect of temperature was18

not included (i.e., K is constant), the coefficient of determination (r2) was 0.84. The

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error of estimation (= the estimated PPFDtotal – the measured PPFDtotal) was20

significantly positively correlated with T (Error = 0.29 T - 6.75; r = 0.74; P < 0.0001).

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Using Eq. 5, we regressed PPFDtotal on F including the effects of T as follows:

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PPFDtotal = (100 - F) / (1.085 + 0.051 T)
7Eq. 6

1The regression was statistically significant (P < 0.0001, N =84), with a high r2 (= 0.94).

2Fig. 2 shows the relationship between the estimated and measured PPFDtotal. Since Eq.6

3includes the effect of T, the data in Fig. 2 are not segregated for the two classes of T.

4The errors of estimation were not significantly correlated with T (r = 0.01; P = 0.91),

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indicating a good correction for the effect of temperature. The errors of estimation were6

also not significantly correlated with PPFDtotal (r = -0.21; P = 0.06), and the absolute

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value of the errors averaged 1.3 mol m-2 and ranged from 0.0 to 5.7 mol m-2, indicating

8a good fit.

910Discussion

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In this paper, we examined the applicability of color acetate film for evaluating12

the forest understory light environment. A good relationship between PPFDtotal and F

13and T was obtained from multiple-point sampling throughout the year. The relationship14in Eq. 6 corrects for the effects of temperature and includes possible variation in the15

relationship between PPFDtotal and F across sampling points that might differ in light

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quality. Therefore, the method assured the broad applicability of the film, for evaluating17

the forest light environment, with a high accuracy of estimation. Since the regression

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constants will change, calibration is required for each forest.

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Fukushima (1998) reported that the fading ratio of film changed irregularly

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until the integrated PPFD exceeded 5 mol m-2. In our study, however, for low values of21

PPFDtotal (1.4-5.0 mol m-2), F was strongly correlated with PPFDtotal (r = -0.75; P <220.001; N = 28). Therefore, there was no evidence to suggest an increased error of8

1estimation in low-light environments.

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The average of the absolute errors of estimation (1.3 mol m-2) could limit the3ability of the film to distinguish differences in a narrow range of light environments

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accurately. However, since PPFDtotal in the understory varied up to 20-fold from 1.4 to

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28.3 (mol m-2), the film method is well suited to ranking understory light conditions

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across a large number of sites. Therefore, the accuracy and low cost of color acetate

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film make it a good option for estimating the integrated PPFD in a forest understory.

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Acknowledgments
We thank the staff of Kamigamo Experimental Forest Station, who kindly granted

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us permission to conduct field studies in the Kamigamo Forest and to use their

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meteorological data. Financial support for the first and second authors was provided by

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the Japan Student Services Organization.

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Literature Cited

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Akiyama, T., Sakai, T., Jia, S., Shinoda, S., and Fukuo, A. (2000) Examination of17

film-solarimeters for field use. Photogrammetry and Remote Sensing 39 (4):13-18.

18[In Japanese with English summary]

19

Fukushima, S. (1998) An estimation of the light environment under forest canopy by a20

simple recording film for measuring integrated global radiation-A relationship21

between fading of color acetate films and integrated PPFD-Trans. 109th Mtg. Jpn.

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For. Soc.: 289-290. [In Japanese]9

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Fukushima, S. (2000) An estimation of integrated PPFD by a simple recording film -A

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relationship between fading of color acetate films and temperature-Trans. 111th

3Mtg. Jpn. For. Soc. 143. [In Japanese]

4Muraoka, H., Hirota, H., Matsumoto, J., Nishimura, S., Tang, Y., Koizumi, H., and5Washitani, I. (2001) On the convertibility of different microsite light availability

6indices, relative illuminance and relative photon flux density. Funct. Ecol. 15:

7798-803.

8SYSTAT (1992) Statistics, version 5.2 edition. SYSTAT, Evanston, Illinois.

9Yoshimura, T., Komiyama, K., and Ishikawa, T. (1989) Simple recording film for

10measurement of integrated global solar radiation. Solar Energy 15 (5): 47-53. [In11Japanese with English summary]

10Figure Captions

Figure 1. The relationship between the fading ratio (F) of color acetate film and thePPFDtotal measured using a quantum sensor. Data were categorized into two classes ofaverage daily maximum temperature (T): Open dots, 6-22ºC (N = 36); filled dots,
22-32ºC (N = 48). The boundary (22ºC) was defined as the average T of all the data
points.

Figure 2. The relationship of the PPFDtotal estimated using film (Y) to the PPFDtotal
measured using a quantum sensor (X). Data were categorized into two classes of T:
Open dots, 6-22ºC (N = 36); filled dots, 22-32ºC (N = 48). The line Y=X is shown forcomparison.

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Kawamura et al (SC04003)

Figure 1100908070605040
300510152025

Measured PPFD total (mol/m2)

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Kawamura et al (SC04003)

Figure 23025201510500510152025

Measured PPFD total (mol/m2)

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