Reflectance Measurements m a. Citrus Canopy on Ground. Marfa-Amparo Gilabert and Joaqufn Melid. Departament de Termodinitmica, Facultat de Ffsica, ...
REMOTE SENS. ENVIRON. 45:281-293 (1993)
Solar Angle and Sky Light Effects on Ground Reflectance Measurements m a Citrus Canopy Marfa-Amparo Gilabert and Joaqufn Melid Departament de Termodinitmica, Facultat de Ffsica, Universitat de Valencia, Valkncia, Spain
G r o u n d radiometry was used to gather spectral data from different targets of a citrus canopy, in order to analyze the effect of solar zenith angle and proportion of diffuse radiation on spectral reflectance. Results have shown that the variation in solar angle causes significant changes in nadirsensed reflectance from vegetation, which exhibits a marked diurnal pattern with a minimum slightly shifted from the solar noon. This fact is more noticeable in the near-infrared and middle-infrared regions of the spectrum. Furthermore, the visible part of the spectrum has resulted in being highly influenced by the diffuse radiation incident on the canopy, which has been quantified by two different physical parameters: the proportion of diffuse irradiance k~ and the sky clearness e. It has been shown that the reflectance factor increases linearly with increasing diffuse radiation, but only below a threshold value, above which the reflectance remains constant. On the other hand, the reflectance dependence on the parameter e has allowed us to identify three well-defined zones of sky light conditions, in which reflectance presents a different behavior.
Address correspondence to Maria-Amparo Gilabert, Departament de Termodin~tmica, Facultat de Ffsica, Universitat de Valencia, 46100 Burjassot, Valencia, Spain. Received 15 May 1991; revised 24 October 1992. 0034-4257 / 93 / $6.00 ©Elsevier Science Publishing Co. Inc., 1993 655 Avenue of the Americas, New York, NY 10010
INTRODUCTION
Vegetation canopies exhibit generally non-Lambertian properties, so that the bidirectional reflectance factor changes as the view and solar angles and canopy characteristics change (Colwell, 1974; Smith, 1983). Several investigators have studied the influence of Sun and view angles on reflectance from agricultural canopies. Shybayama and Wiegand (1985) developed a simple model to describe the bidirectional characteristics of a wheat canopy in order to estimate the reflectance factor at nadir view position from off-nadir measurements. Jackson et al. (1990) have reported an extensive series of bidirectional reflectance factor measurements over different surfaces. Results of these measurements at different view angles demonstrate the need to account for non-Lambertian properties of surfaces when oblique views are obtained from satellite sensors. For the nadir view, a variation of the spectral reflectance factor with the sun angle is also reported and attributed to shadows created by the different surface features. Not only the agricultural canopies show nonLambertian effects (Pinter, 1986; Deering and Eck, 1987) but also natural canopies do (Ranson et al., 1985; Gross et al., 1988). Kock et al. (1990) found that the Sun zenith angle has a decisive influence on the spectral reflection value measured above forest trees, even for solar angle variations of less than 10%. Kriebel (1978) has
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reported reflectance values at 0.52 /~m wavelength of four natural surfaces: savannah, bog, pasture land, and coniferous forest. For all these surfaces, the anisotropy (ratio of the highest to the lowest reflectance value) increases with increasing zenith angle of incidence from about a factor of 3 to about a factor of 10 or more. The azimuthal anisotropy must increase with increasing zenith angle of incidence due to the shadowing effects produced by the vertical structure of vegetation canopies. Kriebel (1976) has also examined the influence of changes in atmospheric turbidity together with the solar zenith angle on the radiant flux reflected by vegetated surfaces. He concluded that, due to the angular anisotropy of the reflection properties of natural surfaces, the diurnal reflected radiation at 0.52/~m may change by 1% per degree change of the solar zenith angle, and by 1% per 6 % change of the spectral atmospheric turbidity factor (which accounts for the optical depth of the atmosphere). Other studies have also reported the reflectance change of vegetation due to the proportion of the sky light incident on the canopy. Deering and Eck (1987) found that variations in the solar irradiance distribution lead to significant differences in the bidirectional reflected radiation of vegetation canopies as measured at the ground. An increase in atmospheric optical depth, and the accompanying increase in diffuse sky irradiance, result in a decrease in background shadowing and also in specular reflection. Both effects are largely dependent on the wavelength, which can produce differences in the normalized difference vegetation index (NDVI) under hazy and clear sky conditions. Robinson and Biehl (1979) have also reported changes in the reflectance factor of vegetation as a consequence of sky light. The present study was undertaken to quantify the effects of illumination (solar zenith angle and proportion of diffuse radiation) on the nadir measurements of the reflectance factors taken over an orange tree and over soil covered by herbacious vegetation (Oxalis sp.), which is the typical background of the citrus parcels in winter time. These measurements were used to obtain some of the radiometric inputs for a geometrical canopy model (article in preparation) that has been developed to characterize the spectral behavior of citrus or-
chards as observed from satellite (Gilabert and MeliL 1990). The aim of our study is to provide well-determined values for the reflectance of the aforementioned targets on the Thematic Mapper (TM) wavelength intervals. Given the influence of solar zenith angle on the reflectance of vegetation, it is necessary to know the dependence of the reflectance of our targets with the solar angle in order to find the appropriate values which could reproduce the illumination conditions of a given image. On the other hand, reflectance changes due to the proportion of sky light incident on the canopy have also been analyzed, since, as the literature reports, this kind of changes in the reflected radiation could lead to misinterpretation if they were considered to be due to changes of the surface where in fact only the distribution of the incoming radiation is different.
INSTRUMENTATION AND METHODS
The experimental field used in this study was located at the IVIA Center (39.6°N, 0.4°W). The data collection developed mainly during February, although some days of January and March were also measured. The data were collected under cloudless sky conditions for days which presented different sky irradiance conditions. The total shortwave irradiance was taken into account to document illumination conditions throughout the data collection period. These solar radiation data were measured about 10 km away from the experimental field, under similar conditions of atmospheric turbidity. Therefore, the spatial extrapolation can be accepted as a good approximation (Hay and Hanson, 1985). The reflectance factor has been defined as the ratio of the radiant flux reflected by a target to that which would be reflected into the same reflected beam geometry by an ideal diffuse reference surface irradiated in the same way as the sample (Milton, 1987). Reflectance factors of the targets under study have been measured (from 400 nm to 2500 nm) by using a GER Single fieldof-view IRIS (SIRIS)1 spectroradiometer, which I Trademarks are included for the benefit of the reader and imply no endorsement or preferential treatment by the Department of Thermodynamics of the University of Valencia.
Solar Angle and Sky Light Effects 283
~
(a)
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Figure i. Experimental design used for acquiring reflectance data of the tree: a) reference measurement; b) target measurement.
(b)
Figure 2. Experimental design used for ac-
(a)
quiring reflectance data of the herb background: a) reference measurement; b) target measurement.
(b)
is a grating instrument that enables us to obtain the spectrum of the reflected flux from any surface. The standard panel Spectralon has been used as the ideal diffuse reference surface. The one used here was 99% reflective over the visible and near infrared ranges and about 96 % over the middle infrared region. Spectral reflectance of the target relative to the reference is calculated by using the GER software package. The target has been always observed with a nadir angle of view. Two platforms for acquiring spectral data were employed according to the height of the different targets. For the tree, data were acquired with a tower system erected over it (Fig. 1). The height of this tower was 6 m, being the distance between the optical head and the top of the tree of about 1.5 m. Calibration measurements were made with the reference panel resting on a movable platform hanging 1 m below the optical head. For the low herb cover of the soil, the optical head of the spectroradiometer was mounted on a tripod (Fig. 2). In this case, the reference panel was resting on a lower tripod. Reference panel measurements were col-
lected immediately before the radiance measurements from the targets. All the data presented in this study were collected under essentially Figure 3. Reflectance factors coefficients for our standard panel to correct the reflectance values obtained in this campaign, as a function of the solar zenith angle, according to the method proposed by Jackson et al. (1992). 1,05
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cloud-free conditions. On cloud-free days, irradiance changes were not as m u c h a problem as on partly cloudy days. Nevertheless, to minimize the possible errors due to irradiance fluctuations (Milton, 1987; Duggin, 1982) and given that the time delay between the reference and target measurements is about 2 min, each set of measurements (reference/target) was replicated three times. The final value assigned to each reflectance factor is the average of these three values, with an error at each wavelength given by the maximum deviation. Since our interest is focussed on interpreting Landsat-5 TM values, filters reproducing the relative spectral response of the Thematic Mapper bands, f(A) (Markham and Barker, 1985; Markham, personal communication), have been applied to each spectrum,
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,
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where R(2) is the spectrum corresponding to the reflectance factor; f(2) is the relative spectral response into the TM band within the wavelengths ),i and 2j, and Rzi_~j(% ) is the reflectance factor (expressed in %) which corresponds to each TM band. In next sections we will call Rai_~j(%) as T M I ( % ) . • "TM7(%). The data for each wavelength band were calibrated for the correction factor of the panel according to the m e t h o d proposed by Jackson et al. (1992). These authors calibrated 11 molded halon reference panels (similar to the one used in this work) and found that, although these panels
Solar Angle and Sky Light Effects 285
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to identify the dependence of the tree reflectance on the different parameters (physiological evolution of the tree, solar zenith angle, and proportion of diffuse irradiance), we have taken the angles before noon as negatives and measurements corresponding to different days have been plotted using different symbols. In addition, and due to the fact that solar zenith angles in this time were always smaller than 50 °, a pair of graphs have been drawn for each spectral band: one for the before-noon (a.m.) measurements and another for the afternoon (p.m.) ones. The associated errors have not been plotted because they are, in most of cases, less than or equal to symbol heights (Table 1). Some other analyses show that these errors do not exhibit any particular dependence with the day or the hour. However, a larger dispersion was observed in TM7, which can be attributed to the larger noise inherent to the instrumentation in this wavelength interval. The analysis of Figure 4 lets us identify two characteristics in the tree reflectance:
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differed in their directional/hemispherical and directional/directional reflectance, the differences were sufficiently small that "general" calibration equations could be developed for molded halon panels like the Spectralon (Jackson et al., 1992). In this way, the spectral reflectance of the panel in each wavelength (which is not 100%) and its non-Lambertian properties were taken into account. Figure 3 shows the values corresponding to the reflectance factor coefficients for the correction of the Spectralon panel used in this campaign as a function of solar zenith angle. Only the values corresponding to TM Bands 4, 5, and 7 are represented since the values for the visible bands hardly differ from the values in TM4. RESULTS AND DISCUSSION Tree Reflectance
Figure 4 shows the measured reflectance values for the tree in each simulated TM band. In order
i. The change in the solar zenith angle results in a significant diurnal variation of the reflectance, mainly in the infrared bands (TM4, TM5, and TM7), with a reflectance minimum whose situation cannot be determined in those graphs. ii. In the near-infrared band (TM4) and also in the mid-infrared bands (TM5 and TMT) the dispersion among values corresponding to different days is low, and it is possible to identify a typical diurnal pattern of the tree reflectance. However, in the visible bands (TM1, TM2, and TM3), the reflectance values for different days exhibit different behavior, and it is not possible to find a single law of variation. The aforementioned minimum in the diurnal pattern of the tree reflectance can be more clearly determined if we plot the same values as a function of the solar azimuth angle (Fig. 5). Effectively, this minimum is not situated at the solar noon ((0~= 0°), conferring a certain asymmetry to the
Table 1. Order of Magnitude of the Relative Errors cr (%) Associated with the Tree Reflectance Values for Each Spectral Band Band
TM1
TM2
TM3
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TM5
TM7
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patterns. This asymmetry registered in the diurnal variation of the tree spectral response has been reported frequently in the literature but related to the study of the whole canopy reflectance. Some authors (Kondratiev et al., 1981) explain this effect as due to the variations of plants' physiological properties, while others (Pinter, 1986) invoke the presence of dew on the leaves in the morning. In our case, both factors may be acting simultaneously. On the one hand, physiological changes are inherent to the evolution of the tree
along the day. On the other hand, the presence of dew is typical in winter time, when the measurements were taken. The distributions of the reflectance values corresponding to Bands TM4, TM5, and TM7 could be phenomenologically reproduced by means of a quadratic law. A regression analysis gives correlation coefficients (r) of 0.77, 0.81, and 0.73, respectively. The first derivatives of the obtained curves allow us to estimate the reflectance minima: (0s- 14 ° (TM4), q~s- 17 ° (TM5),
Solar Angle and Sky Light Effects 287
and ~0s-10 ° (TMT). The regressions for reflectance in the visible bands present correlation coefficients very low (0.40 for TM1, 0.15 for TM2, and 0.06 for TM3). The great dispersion observed in the visible bands could be attributed to the influence of diffuse irradiance. Although all the data were collected under cloudless sky conditions, significative changes in the proportion of diffuse irradiance were registered. The presence of sky light has an influence on the spectral response of vegetation (Kondratyev et al., 1981; Milton, 1987; Robinson and Biehl, 1979; Kriebel, 1979) and also on the reference measurements over an standard panel (Jackson et al., 1987; Kimes and Kirchner, 1982). Robinson and Biehl (1979) have shown that the effect of sky light decreases with increasing wavelength. They have also shown that the reflectance factor of vegetation can vary up to a 3 % due to the presence of sky light (for a visibility of about 8 km). Deering and Eck (1987) have also pointed out the importance of the proportion of diffuse to total radiation on the interpretation of the bidirectional spectral reflectance of vegetation, and they report an increase of reflectance by approximately 30% and 10% for the red and near-infrared bands, respectively. Jackson et al. (1987) have shown that the nonisotropic nature of the diffuse irradiance can implicate an error on the measured reflectance factor that is difficult to evaluate. Two different physical parameters have been used in order to analyze the effects of sky light on the tree reflectance. The first one is the proportion of diffuse irradiance kd, defined as
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(2)
where IT is the total irradiance, Ib the direct (beam) irradiance, and Id the diffuse irradiance. Then, ke can vary between 0 and 1. The second parameter is the sky clearness e, which was introduced by P~rez et al. (1986) to parameterize the sky conditions in the development of models of diffuse irradiance (P~rez et al., 1986; 1987):
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horizontal plane. Obviously, these two parameters are related by the solar zenith angle 0s: e- 1
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In the first place, we have analyzed the relationship between the fraction of diffuse irradiance kd and the tree reflectance (Fig. 6). Only the data within an interval centered at solar noon have been plotted (solar zenith angle less than 53 ° , solar azimuth angles between - 15 ° and + 15°), in order to avoid reflectance variations due to the changes with the solar elevation. In this interval, the reflectance variations as a consequence of the solar elevation were negligible with respect to those originated in the whole range of solar angles (about a 20% in TM1, a 12% in TM2, and a 4% in TM3). Figure 6 reveals the fact that reflectance is strongly correlated with the diffuse irradiance, which is more noticeable in the visible bands. In the infrared bands, which do not appear in the figure, the dispersion among reflectance values is higher and, besides, the relative variation observed in the whole range of values as a consequence of diffuse irradiance is less than 50%, whereas that variation in visible bands is always higher than this percentage. A fit of the reflectance curves from Figure 6 has not been found to be representative because not every environmental factor that could have an influence (like dew point) has been completely controlled throughout the experience. However, the most striking fact is the abrupt law of variation of reflectance, which exhibits a linear variation for kd lesser than 0.2 (with correlation coefficient r 2 greater than 0.9 in visible bands), and immediately above this value the reflectance remains nearly constant. Therefore, there would be two possibilities for the reflectance variation with kd: a two-step one (linear below a certain value of kd and constant above this value) or, more likely, a continuous law of variation, showing a linear behavior for low values of kd and saturation for high values. In relation to sky clearness, Ineichen et al. (1990) have analyzed the influence of a series of factors related to soil and illumination conditions on the albedo values. They show that the highest
288
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variations come from changes in the sky clearness. Although their works (Ineichen et al., 1987; 1990) are referred to albedo and not to spectral reflectance, we have found it interesting to analyze in this article the possible relationship between reflectance values and sky clearness. All the tree reflectance values obtained have been analyzed as a function of the sky clearness e for the visible spectral bands (Fig. 7). In all the spectral ranges, two different zones can be identified, correspond-
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ing to e lower and higher than 20, respectively. The first one presents a high dispersion among the reflectance values; meanwhile, in the second one, the tree reflectance tends to be constant. This constant zone can be easily explained because e values higher than 20 correspond to a very good sky conditions (ku lesser than 0.10). Otherwise, these graphs confirm that reflectance varies strongly with diffuse illumination but only below a certain degree of diffuse irradiance. The plots of Figure 7 present dispersions between 80% and 100% for the visible bands (the
Solar Angle and Sky Light Effects 289
fact points out once again the stronger influence of diffuse radiation on the visible reflectance, while its effect is of about the same order of magnitude as that due to solar elevation in the near-infrared region.
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dispersions were between 60% and 70% for the infrared ones). Figure 8 details more specifically the interval corresponding to e values lesser or equal than 20, but only those reflectances measured around solar noon (solar zenith angle lesser than 53 ° ) have been represented, in order to eliminate the influence of the solar zenith angle. The maximum dispersions for the visible bands are of the same order of magnitude as those observed in Figure 7 (whereas they are much lower for the infrared bands: 50% lower). This
Figure 9 shows the measured reflectance for the herb (Oxalis sp) in each simulated TM band. The information related to the associated errors can be found in Table 2. Despite fewer measurements, we can observe clearly the diurnal pattern of the reflectances, which is also more regular in TM4, TM5, and TM7 bands, as for tree reflectance. The most noticeable fact to point out on the evolution of herb reflectance is the sharper variations observed in the morning in comparison to those observed in the tree reflectance patterns. This abrupt decrease with the solar zenith angle may be attributed to the physiological change which takes place in the herb cover, as can be observed in Figure 10. On the early morning, leaves were covered by dew and were horizontally extended, presenting the maximum effective surface to intercept the incident solar radiation. However, in a few hours, their intercepting surface diminished to avoid transpiration, by means of a heliotropic movement. This fast evolution of the herb physiology makes it difficult to determine its reflectance at the Landsat overpass time. A change in the solar zenith angle from 70 ° to 60 ° in the morning (solar angle interval which perfectly could correspond to a winter image) results in a variation in the reflectance as high as 100%. Therefore, the satellite overpass time is critical in determining with accuracy the herb reflectance by ground measurements, as used for input in a reflectance canopy model. Concerning the dispersion observed in visible bands, a similar analysis to that of the tree re-
Table 2. Order of Magnitude of the Relative Errors a (%) Associated with the Herb Reflectance Values for Each Spectral Band Band
TM1
TM2
TM3
TM4
TM5
TM7
tr(%)
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Figure 9. Measured reflectance data for the herb, in each TM band, as a function of the solar zenith angle. Angles before noon have been taken as negative. Different symbols correspond to measurements taken in different days.
flectance has been carried out. No correlation has been found between reflectance and diffuse radiation. This dispersion cannot be explained in terms of the atmospheric sky light because this is not the only source of diffuse radiation incident on the herb background. Effectively, as a consequence of multiple reflections inside the canopy (herb background is surrounded by trees), the solar radiation should have been measured at ground level to be able to analyze the influence of diffuse irradiance on herb reflectance. For this reason, definitive conclusions related to this point cannot be established in this article.
COMMENTS This study has demonstrated a strong physical correlation (within statistical limits) between the
reflectance of the trees and some external factors such as the solar zenith angle and the solar diffuse irradiance. For herb reflectance, only the influence of solar angle has been analyzed. In both cases, Band TM4, TM5, and TM7 reflectance values present a precise diurnal pattern with zenith angle, which can be accounted for approximately by a quadratic law around solar noon. The tree reflectance values for different days in visible bands suggest that the reflectance factor varies enormously with sky conditions. This conclusion has been tested by the examination of the correlations between the reflectance values and the proportion of diffuse irradiance incident on the upper of the canopy for the period analyzed. Results clearly confirm that a certain amount of diffuse irradiance produces considerably higher reflectance values in the visible region of the spectrum. The dependence of the visible re-
Solar Angle and Sky Light Effects 291 p.m.
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Figure 10.
Aspect of the h e r b b a c k g r o u n d (Oxalis sp.) in the early m o r n i n g and towards the noon. Drastic changes can b e observed.
flectance on kd also suggests the existence of a diffuse irradiance threshold, above which reflectance is insensitive to sky condition fluctuations. This threshold seems to lie about ka = 0.20. However, this value must not be considered as definitive because not all the environmental factors which could be affecting the reflectance were measured. Below this critical value the tree reflectance for visible bands increases linearly with increasing ka. In our opinion, the representation of reflectance as a function of e, R(e), gives more qualitative information than the R(kd) one. The law of variation of R(e) (Fig. 7) is more complicated, but clearly exhibits three well-defined zones: i. The zone of higher values (e > 20), where the reflectance tends to saturation. We can label it as "clear-day conditions." ii. A transition zone, for e values between 8 and 20, approximately, in which the tree reflectance changes drastically. It can be labeled as "variable-sky-light conditions." This
transition zone is much less abrupt in the infrared bands (TM4, TM5, and TM7). iii. The last one, for e lower than 8, more or less, can be labeled as "perfectly diffuse conditions." For the visible bands, the tree reflectance in this zone tends to saturate at a value higher than that obtained in clear conditions. Otherwise, the tree reflectance for the infrared bands seems to present values similar to those reached in clear conditions. The influence of sky light should be taken into account to analyze and interpret not only ground data but also remotely sensed spectral measurements. Since changes due to diffuse irradiance fluctuations are much less in the near infrared band than in the red one, the influence of sky light cannot be removed, although a traditional vegetation index (as the NDVI) is used. A simulation procedure, based on a geometrical canopy model (article in preparation), can be
292 Gilabert and Meli6
useful to d e t e r m i n e t h e sensitivity o f t h e N D V I , c a l c u l a t e d for the w h o l e c a n o p y , to c h a n g e s in i l l u m i n a t i o n conditions. As was e x p e c t e d , t h e inf l u e n c e o f diffuse i r r a d i a n c e is s t r o n g e r as t h e t r e e c o v e r increases. A d i s p e r s i o n o f u p to a 7 0 % can b e r e a c h e d in T M 3 b a n d , w h i l e it is r e d u c e d to 1 2 % in t h e N D V I . A l t h o u g h t h e i n f l u e n c e of sky light is less i m p o r t a n t w h e n using t h e vegetation index, it is still affecting t h e data. T h u s , an i n c r e a s e in t h e v e g e t a t i o n i n d e x values c o u l d b e a t t r i b u t e d to a c h a n g e in t h e v e g e t a t i o n w h e n it w o u l d b e o n l y t h e c o n s e q u e n c e of a h i g h e r a m o u n t o f diffuse irradiance. The authors would like to express their appreciation to M. T. Younis for his significant contribution to the field treatment preparations and measurement activities and to M. P. Utrillas for her work processing the solar irradiance data used in this work. Special thanks are given to Dr. J. A. Martinez-Lozano (Solar Energy Group from this university) and Dr. M. S. Moran (USDA / ARS, U.S. Water Conservation Laboratory, Phoenix) for their critical review of the manuscript. Dr. B. L. Markham, from Goddard Space Flight Center, kindly provided the relative spectral response tables of Landsat-5 TM bands. We also thank the IVIA Center for the facilities given to take the measurements in the orchard canopy. M. A. Gilabert held, during this research, a fellowship (PFP1, 1987)from the Ministry for Science Education (Spain).
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