K. J. Ranson, C. S. T. Daughtry, and L. L. BiehlLaboratory for Applications of Remote Sensing, Purdue University, West Lafayette, IN 47907

Sun Angle, View Angle, andBackground Effects on SpectralResponse of Simulated Balsam FirCanopies

ABSTRACT: An experiment is described that examines the effects of solar zenith angle andbackground reflectance on the composite scene reflectance of small balsam fir (Abies balsamea(L.) Mill.) arranged in different densities. In this study, the shape, density, and, conse­quently, the needle area index and phytomass of the canopies, as well as the backgroundreflectance, were controlled.The effects of sun angle, view angle, and background reflectance on the multispectral re­sponse of small balsam fir trees were significant. Regression models relating spectral vege­tation indices (i.e., normalized difference (NO) and greenness (GR) to phytomass showedvery poor relationships (W = 0.0) for balsam fir canopies with a grass background. However,strong linear relationships were found for NO (R2 = 0.9) and GR (R2 = 0.8) with phytomassfor a background that simulated the reflectance of snow. Changing solar zenith angle sig­nificantly affected the models relating NO to phytomass for the snow background, but wasnot significant in the model relating GR to phytomass for the snow background.The spectral properties of the background may confound changes in the spectral responseof the overstory vegetation. Based on our results, winter-time data, when snow masks theunderstory, would provide better estimates of overstory phytomass in coniferous forests thanwould summer-time data.

0099-1112/86/5205-649$02.25/0©1986 American Society for Photogrammetry

and Remote Sensing

vary with solar illumination angle, viewing angle,understory or background reflectance, atmosphericeffects, as well as intrinsic canopy parameters ofphytomass and LA!. Early studies using broad-bandsolarimeters showed increased reflectance as solarzenith angle increased (Stewart, 1971; Jarvis et aI.,1976). Jarvis et al. (1976) also noted that the diurnalvariation of Sitka spruce (Picea sitchensis (Bong.) Corr.)was less than that reported for Scotch pine (Pinussylvestris L.). Greater surface roughness and, thus,greater ligilt-trapping efficiency of the spire-shapedcrowns of sitka spruce versus the rounded crownsof Scotch pine was cited as the cause of this differ­ence. Diurnal reflectance studies by Kriebel (1979)and Kimes et al. (1980) showed a dependence of thereflectance from coniferous forests on solar zenithangle. Kriebel (1978) reported an increase in reflec­tance as zenith view angle increased for four veg­etated surfaces including a coniferous forest. Overall,data for the bidirectional reflectance of forest can­opies is scarce.

The understory or background reflectance can alsobe important to the composite scene reflectance fromforests. In a study of a deciduous forest in northernMinnesota, Badhwar et al. (1983) found that the un­derstory reflectance limited their attempts to infer

INTRODUCTION

T HE WORLD'S FORESTS are important for storageof carbon and exchange of CO2 with the atmos­

phere. Large tracts of forests are being harvested atan alarming rate to provide fuel and fiber and tomake room for agricultural development. Some for­ests downwind from industrial areas are declining,presumably due to acidic precipitation and/or ozone(Lefohn and Brockson, 1984). The effects of large­scale losses in forests on the global ecosystem areunknown at present. It is increasingly important tobe able to quantify the present status and potentialproductivity of global forest resources.

The usefulness of remote sensing data for quan­tifying the leaf area index (ratio of one-sided greenleaf area per unit of ground area) and phytomasshas been demonstrated for agricultural canopies (eg.,Kollenkark et aI., 1982; Asrar et aI., 1984). Becauseleaf area index (LAI) and phytomass are key deter­minants of net primary production (NPP) in plantcanopies, it follows that remote sensing technologymay contribute valuable information for under­standing the changes in our global ecosystem (Bot­kin and Pecan, 1982).

The reflectance measured from forest scenes may

PHOTOGRAMMETRIC ENGINEERING AND REMOTE SENSING,Vol. 52, No.5, May 1986, pp. 649--658.

PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1986

MMR Band TM Band Wavelength

fJ.ffi1 1 0.45 - 0.522 2 0.52 - 0.603 3 0.63 - 0.69 8.9m

4 4 0.76 - 0.905 1.15 - 1.306 5 1.55 - 1.757 7 2.08 - 2.358 6 10.60 - 12.50

8/i/

Sensor (60' off-Nadir)

Sensor~---L.-.J (Nadir)

FIG. 1. Arrangement of sensors for balsam fir turntable ex­periment.

Turntable

was placed on the boom of an aerial-tower truckand positioned to view the scene at off-nadir angles(Figure 1). The third instrument was mounted on atripod to view a 1.2 by 1.2-m reference panel paintedwith barium sulfate. Periodically, this instrumentwas also used to measure the reflectance of repre­sentative samples of the background materials. Es­timates of the amount of diffuse irradiance werealso obtained by measuring the reflectance of thereference panel shielded from direct sunlight.

With a background in place beneath the trees,radiometric measurements were made for each 10°rotation of the turntable (0 to 360°). Each of the 36different angles of the turntable presented a slightlydifferent balsam fir canopy to the sensors. A com­plete rotation of the turntable took about 5 minutes.Changing backgrounds required about 8 minutes.A complete set of 36 observations for all three back­grounds was completed in less than 45 minutes.The maximum change in solar zenith angle was lessthan 2° for a set of observations for a given back­ground and about 5° for a group of all three back­grounds.

Spectral measurements were collected over thereference panel every 30 seconds and used to con­vert the voltages measured by the other instrumentsto reflectance factor (RF). (RF is defined as the ratioof the radiant flux reflected by a sample surface totha~ which would be reflected into the same re­flected-beam geometry by an ideal (lossless) per­fectly diffuse (Lambertian) reference surface irradiatedin the same way as the sample (Nicodemus et aI.,1977)). The reference data were corrected to com­pensate for the non-Lambertian properties of bar­ium sulfate paint at illumination angles greater than55°, as noted by Kimes and Kirchner (1982). Cor-

TABLE 1. DESCRIPTION OF WAVELENGTH BANDS OF

BARNES MODULAR MULTIBAND RADIOMETER (MMR) ANDCORRESPONDING LANDSAT THEMATIC MAPPER CHANNELS.

MMR BAND 8 WAS NOT USED IN THIS ANALYSIS.

650

LA! of aspen (Populus tremuloides Michx.).The success of studies designed to examine the

effects of sun angle, view angle, and backgroundreflectance on remote sensing data is often relatedto the experimenter's ability to obtain detailed mea­surements of the scene. If the scene could be con­trolled to a good degree of certainty, those factorsof interest could be examined with increased con­fidence. For this reason an experiment was de­signed to examine the effects of solar zenith angleand background reflectance on the composite scenereflectance of small balsam fir (Abies balsamea (L.)Mill.) arranged in different densities. A linear modelrelating phytomass to spectral vegetation indices wasalso evaluated. In this study, the shape, density,and, consequently, the LA! and phytomass of ourcanopies, as well as the background type, were con­trolled. All reflectance data were acquired under clearskies and short path length so that atmospheric ef­fects were negligible.

A series of four experiments was initiated in thefall of 1983 and continued during the summer of1184. Seven-year-old balsam fir trees planted in 28­litre pots were purchased from a nursery. Balsamfir has a symmetrical, pyramidal crown and linear­flattened needles and is considered the principal bo­real forest fir of North America (Spurr, 1964). Thetrees were arranged on a 4.0-m diameter rotatingplatform (turntable) at three densities as describedbelow. Backgrounds of Kentucky bluegrass (Poa pra­tensis L.) sod, and white- and black-painted boardswere alternately placed beneath the trees and spec­tral reflectance was measured.

Three multiband radiometers (Barnes Engineer­ing Company Model 12-1000) were used for spectralmeasurements. This instrument is sensitive to vis­ible, near-infrared, middle-infrared, and thermal ra­diation as described in Table 1. The thermal bandwas not used in our experiment. One radiometerwas attached to a boom and leveled to view thescene in the nadir direction. A second radiometer

METHODS

SUN ANGLE, VIEW ANGLE, AND BACKGROUND EFFECTS

rection factors were derived from goniometric mea­surements of the reference panel made in thelaboratory. The three instruments were correlatedto each other from simultaneous measurements ofthe reference panel.

We were unable to measure the spectral reflec­tance of the small balsam fir trees directly so, forcomparison purposes, we estimated the reflectanceof sunlit trees using a simple model: Le.,

RT = (Rc - RBAB)/AT (1)

where Rc is the composite scene RF, RB is the RF ofthe background, and AT and A B are the proportionsof the scene occupied by trees and background, re­spectively.

Two spectral vegetation indices were utilized inthis study. The normalized vegetation index or nor­malized difference (ND) was calculated as (RF4-RF3)/(RF4+ RF3) where RF3 and RF4 are Band 3 (Red) andBand 4 (near-IR) RFs. The Tasseled Cap featuregreenness (GR) (Kauth and Thomas, 1976) was cal­culated from coefficients derived by Crist et al. (1984):i.e.,

GR = -0.1603(RF1) - 0.2819(RF2) - 0.4934(RF3)+ 0.7940(RF4) - 0.0002(RF6) - 0.1446(RF7)

(2)

where RFl, RF2, ... , RF7 are the RFs in Barnes radi­ometer bands (Table 1). A constant of 20.0 was addedto each calculated GR value to avoid negative values.

In addition to reflectance factor, several measure­ments of the balsam fir trees were also made. Theseincluded height, crown diameter, percent cover, dryphytomass, and needle area index (NAl). Tree heightswere measured from the background surface to thetop of the leader; crown diameters represent theaverage of two orthogonal measurements across thebase of the crown. Percent cover (AT in Equation 1multiplied by 100) was estimated by projecting scenephotographs on a dot grid and computing the per­centage of dots hitting foliage. Phytomass was es­timated by weighing oven-dried (70° for 48 h) needlesand branches separately for ten trees. Mean needlearea per tree was estimated as the total dry weightof needles multiplied by the needle area to dry weightratio. The projected areas of 32 samples of 50 needleswere determined by planimetering photographs ofthe needles. The average needle area per tree wasmultiplied by the density of trees for a given ex­periment to obtain the needle area index. The mea­sured canopy characteristics for our experiments arelisted in Table 2.

EXPERIMENT I

The first experiment was conducted on 27 October1983. The balsam fir trees were placed on theturntable in an equidistant arrangement with abetween-tree spacing of 0.76 m. The trees had anaverage height of 1.05 m, but varied between 0.67and 1.37 m. Average crown diameter was 0.75 mand percent cover was 74 percent (Table 2). For this

651

experiment, nadir reflectance measurements wereacquired with a 15° field of view. Spectral data werelimited by the large solar zenith angles that occurat our latitude (40° 30'N) at this time of year (Table3).

EXPERIMENT II

A second experiment was conducted on 25 June1984. Newly acquired balsam fir trees were placedon the turntable with a spacing of 0.61 m, resultingin a percent cover of 91 percent (Figure 2a). Thetrees were pruned to a uniform height of 1.05 m.

In addition to the nadir zenith view angle usedin Experiment I, spectral data were also acquired ata 60° view zenith angle and with view azimuth angleswithin 30° of the sun's principal place for bothforward- and backscattering directions. The additionof the off-nadir view zenith angle required a decreasein the angular FaY of the radiometers to 10°.

EXPERIMENT III

On 19 July 1984, the turntable was populated withtrees at a spacing of 0.76 m, as in Experiment I. Dotgrid estimates of average percent cover were 70percent (Figure 2b). Spectral data were acquired frommid-morning to early afternoon with the sameprocedures used in Experiment II.

EXPERIMENT IV

A final experiment was performed on 18 September1984 with trees arranged on the turntable with aspacing of 0.91 m and 52 percent cover (Figure 2c).At this rather wide spacing of the trees, a noticeablerow effect was present, especially at larger solarzenith angles and with the white background. Theequidistant arrangement of the trees resulted in alarge amount of sunlit background whenever a rowof trees lined up with the sun. This resulted in higherreflectances at 60° intervals of turntable rotation. Tc:minimize this "row" effect, these data points weredeleted. This effect was most noticeable for the highlyreflecting white background. However, to comparebackground effects, we imposed this constraint onmeasurements with grass and black backgroundsfor the low-density canopy as well. The overall effectwas reduced mean reflectance and decreasedvariation. A similar effect was present in our datafor off-nadir reflectance measurements. Thus, wealso deleted those data values acquired when thesensor was looking directly down a row. Table 3summarizes the spectral data collected over thecourse of the four experiments.

RESULTS AND DISCUSSION

BACKGROUND REFLECTANCE

The balsam fir canopies, as arranged on the turn­table and viewed by a sensor, consisted of a mixtureof trees and background. The amount of each com-

652 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1986

TABLE 2. MEAN BALSAM FIR CANOPY CHARACTERISTICS FOR FOUR EXPERIMENTS. STANDARD DEVIATIONS ARE SHOWN IN

PARENTHESIS.

NeedleCrown Area Canopy

Experiment Date Spacing Height Diameter Phytomass Index Cover

m kg/m2 %27 Oct 1983 0.76 1.04 0.73 1.50 3.5 74

(0.13) (0.10) (0.38) (0.9) (3)

II 25 Jun 1984 0.61 1.05 0.72 3.04 7.4 91(0.04) (0.12) (0.49) (0.9) (1)

III 17 Jul 1984 0.76 1.05 0.72 1.95 4.8 70(0.04) (0.12) (0.31) (0.6) (4)

IV 19 Sep 1984 0.91 1.05 0.72 1.35 3.3 52(0.04) (0.12) (0.22) (0.4) (3)

TABLE 3. SOLAR ANGLES AND NUMBER OF REFLECTANCE DATA SETS FOR BALSAM FIR CANOPIES.

Sun Angle Range No. of Sets' ZenithView

Experiment Date Zenith Azimuth G W B Angles

-- degrees -- degreesI 27 Oct 1983 57-76 205-239 3 3 3 0

II 25 Jun 1984 17~5 113-280 9 8 8 0,60III 17 Jul 1984 20-34 115-209 4 4 4 0,60IV 19 Sep 1984 39-79 150-263 7 7 7 0,60

'G = Trees with grass backgroundW = Trees with white backgroundB = Trees with black background

~ "'~;.,f

SUN ANGLE, VIEW ANGLE, AND BACKGROUND EFFECTS 653

background, NO values were high (> 0.8) for bothcanopies and all illumination and view angles. Thenadir-viewed NO increased as solar zenith angleincreased for both canopies and all backgrounds.NO measured at 60° view zenith in the forward­scattered direction did not vary significantly withsolar zenith angle for most cases. The exception wasthe black background where NO decreased between65° and 79°. NO in the backscatter direction tendedto increase for the low density canopy (Figures 5a,c, and e) and decreased for the high density canopy(Figures 5b, d, and f).

Greenness (GR) responded differently than NO tochanges in background, solar zenith angle, andviewing angle (Figure 6). Nadir GR decreased as solarzenith angle increased for both canopies with grassand for the high density canopy with white andblack backgrounds (Figures 6a, b, d, and f). For thelow density canopy with white and blackbackgrounds, nadir GR showed a slight linear increaseas solar zenith angle increases (Figure 6c and e).Significant increased in GR were observed for bothbackscattered and forward-scattered directions forthe 60° view zenith angle for most cases. Theexceptions were the low-density white and blackbackgrounds where GR in the backscattered directiondid not vary significantly with sun angle because ofthe large variances of the measurements (Figures 6cand e).

The data presented above indicate the complexityof interpreting spectral response from heterogeneousscenes. However, if the spectral response of the sceneis considered as a composite effect of vegetation andbackground, each with sunlit and shadedcomponents, the nature of the trends becomesapparent.

The light-scattering behavior of green vegetationis strongly wavelength dependent (Knipling, 1967).Green, chlorophyll-containing leaves strongly absorbred sunlight and reflect and transmit a largeproportion in the near-JR. Thus, homogeneouscanopies with large amounts of green leaves arehighly reflective in the near-IR, but reflect very littlein the red region. The NO for a homogeneous densecanopy will approach a value of 1.0. Forheterogeneous scenes, the effects of backgroundreflectance and shadows cast by the trees must alsobe considered.

For example, consider the trends of NO with solarzenith angle for nadir measurements. The observedNOs for both high and low density canopies (Figures5a and b) were only slightly greater than NO of thegrass background alone (Figure 4). This is becauseof the similar red and near-IR reflectance of treesand grass (Figure 3) in our experiments. The increasein NO as solar zenith angle increased occurred becauseof the differential scattering of red and near-IRradiance by the canopy and background. When thesun was higher above the horizon (small solar zenithangle), the nadir-viewing sensor received more

7

;!!. 80a:: 6-_-O__o-_-o....W~hite boards

ot; 60

~

~ 40 GrassZ A:----G..~ fr '~"u 20 /j . D....~ .f' Balsam fir~.....

~ Black boards . '0a:: O~~~~=S:==",,;;:~:;;;:;;;;,:o::::~;t)

I

1.0 50

0.8 ----- 40

Gl

0.6 30 ~0 fTIZ Z

04 20 ~(f)(f)

0.2 10

0.0 015 25 35 45 55 65

SOLAR ZENITH ANGLE,deg.

2 3 4 5 6MMR WAVELENGTH BAND

FIG. 3. Reflectance factors (AF) of grass, white, and blackbackgrounds in MMA wavebands. Estimated AF of balsamfir tree is also shown. Solar zenith angle during measure­ments was about 18°.

FIG. 4. Changes in normalized difference (NO) and green­ness (GA) of grass, white, and black backgrounds as a func­tion solar zenith angle.

at varying solar zenith angles are shown in Figure4. For NO, a general increase in response with solarzenith angle was apparent for tl-.' grass and blackbackgrounds while the white background responseremained essentially zero. GR for black backgroundresponse was constant through the day, but GR forwhite background increased slightly at solar zenithangles greater than 50°. The initial slight increaseand then decrease in both GR and NO for grass wasprobably related to the condition of the grass sadand not to extrinsic factors such as varying irradi­ance. Examination of calibration panel data and to­tal incidence pyranometer data revealed no significantchanges in irradiance that would account for theobserved variation.

COMPOSITE SCENE REFLECTANCE

Figure 5 illustrates the behavior of NO to changesin solar zenith angle for nadir and 60° view zenithin the forward and backscatter directions for lowand high density canopies (Experiments IV and II,respectively) for the three backgrounds. The behaviorof NO for the medium density canopies wasintermediate and was omitted for clarity. With grass

b.

and NO increased. This was especially evident forthe low denSity canopy with white background(Figure 5c) where NO increased from 0.3 at a sunangle of 35° to over 0.6 at a solar zenith angle of 65°.These results are consistent with the greater sun­angle dependence of reflectance from agriculturalcanopies with row structure reported by Pinter etal. (1983) and Ranson (1983).

The generally high values of NO for off-nadirmeasurements were a result of the sensor viewingmore canopy and less background. When the viewdirection is toward the sun (forward-scattering), alarge proportion of the scene is shaded and NO ishigh. Note that when the solar zenith angle wasvery large (e.g., > 75°) NO decreased (Figures Sa, c,and e). The probable cause of this was an increasein the proportion of red band irradiance in thespectral distribution of skylight at very large solarzenith angles.

In the back-scattered direction, the proportion ofshadows is reduced and the sensor viewed primarily

Grass

f.

Black

d.

0--0 00 View Zenith

0--1] 600 View ZenithBackscatter

/:r'-t:J. 60 0 View ZenithForward Scatter

White

~~tF='--{J ~

I ~:.n.:~_.6-·-·~.:..o·-.6 ~-D

7

Low Density Conop'Y_

1.0

0.8

0.6

0.4

0.2 e.0.0

15 35

1.0

0.8

0.6

0.4

0.2 a.0.0

1.0

0.8

0 0.6Z

0.4

0.2C.

0.0

PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1986654

reflected light from directly illuminated areas of thescene. As the solar zenith angle increased, theprobability of tree foliage intercepting solar irradiancealso increased. Because near-IR transmittance bygreen leaves is an order of magnitude greater thanthat of the red band, red reflectance decreases at ahigher rate than near-IR. Thus, the shaded portionsof the scene could be expected to have higher NOvalues than directly illuminated areas.

The trends for the highly reflecting whitebackground dramatized the effects of shadows onNO (Figures 5c and d). At the highest sun positions(i.e, smallest zenith angles), contributions of directlyilluminated background to the scene reflectance weregreatest and NO values were at the observedminimum for both high and low canopies. As sunzenith angles and shadows increased, the proportionof directly illuminated background decreased andthe contribution of the background to scenereflectance also decreased. However, the absolutedifference between red and near-IR RFs increased,

SOLAR ZENITH ANGLE, deg.

FIG. 5. Mean normalized difference (ND) variation with solar zenith angle for low and high density balsam fir canopies withdifferent back grounds and sensor viewing geometries. Vertical bars indicate one standard deviation. For data pointswithout vertical bars, standard deviations were less than or equal to symbol height.

655

7555

f.

d.

..tiigh Density ConoJ'.y_

... / .........0

~b.

with white and black backgrounds was caused by anearly constant decrease in RFs for all bands as sunangle increased (Figures 6c and e).

Viewing the canopies in the back-scattereddirection resulted in increased GR as sun angleincreased. This is related to the increased amountof directly illuminated scene components as the solarzenith angle approached the hot spot.

Our results demonstrate that careful considerationof sun angle, view angle, and backgroundcharacteristics is necessary to understand the spectralreflectance from heterogeneous scenes such asforests. However, of keen interest to resourcemanagers is understanding how to interpretmultispectral data and to infer specific attributes ofthe forest canopy. For example, the relationships ofnormalized difference and greenness to green LAIand phytomass of agricultural crops are well known(Tucker, 1979; Kollenkark et aI., 1982; Pinter et aI.,1983). Tucker et ai. (1985) have even related NO tophytomass over the entire African continent.

Black

Grass

~ .-'~.~

35 55 75 15 35

SOLAR ZENITH ANGLE, deg.

Low Density Conop.L

o-----D

o.

20 '----'-_....1.-_'----'-_--'------'

15

e.30

50

c.20 '----'---'--'----'------'-------'

White

60

30

0--0 0° View Zenith

G-{] 60° View Zenith60 Backscatter

tr°--fJ. 60° VIew ZenIthForward ScaTter .-8

6---

I. .-1,

~

20 '------'---'--'----'---'-------'

50

40

60

40

SUN ANGLE, VIEW ANGLE, AND BACKGROUND EFFECTS

~ 50wzQ 40W

ffi 30

FIG. 6. Mean greenness variation with solar zenith angle for low and high density balsam fir canopies with differentbackgrounds and sensor viewing geometries. Vertical bars indicate one standard deviation. For data points without verticalbars, standard deviations were less than or equal to symbol height.

directly illuminated needles, branches, andbackground. This caused the NO response todecrease, as shown in Figures 5b, d, and f. Also, itis possible that at solar zenith angles greater thanthe "hot spot" angle (i.e., for our data, solar zenith= view zenith = 60°) NO may increase because ofincreased shadows in the scene. This may explainthe increase shown in Figures 5c and e. However,there are insufficient data to directly support thisstatement.

The effect of increasing solar zenith angle on thegreenness (GR) transformation also appeared to bedependent on shadowing. As the sun angleincreased, the increased shadows caused an overalldarkening of the scene. Because GR is a function ofthe magnitude of the reflectance and not relativedifferences as in NO, there was an overall decreaseof GR with increasing sun angle for the nadir view(Figures 6a, b, d, and e). This also explains the lowGR values measured for the forward-scatter direction.The increase in GR noted for the low density canopy

PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1986

The effects of sun angle, view angle, and back­ground type on the multispectral response of smallbalsam fir trees are significant. Observed increases

CONCLUSIONS

has nearly Lambertian reflectance characteristics(Suits, 1978) although Knowles Middleton andMungall (1952) suggest that this is true only for zenithangles of incidence less than 45°. For our purposes,we will assume that spectral reflectance of thesimulated snow background is a reasonableapproximation of freshly fallen snow.

Multiple linear regression analysis was employedto examine the effect of the backgrounds on thespectral response. The solar zenith angle (as)isincluded in the model to account for the dependenceobserved in Figures 5 and 6. The model used was

Y = bo + bI X + b2 as (3)

where Y is the estimated spectral response of eitherNO or GR, X is phytomass, and bo, bI' and b2 are themultiple regression coefficients. Reflectance factorsmeasured from the backgrounds alone (Figure 4 andFigure 7) were used to represent cases of no balsamfir phytomass.

For the grass background the relationship betweenthe spectral variables and phytomass was very poor;i.e., R2 = 0.36 (Table 4). Regression coefficients forphytomass (b I ) were not significant (ex = 0.20), asshown in Table 4. Figures 8a and c graphicallyillustrate these results, that is, the spectrally similarbackground masks changes due to the density ofthe trees. Badhwar et al. (1984) described a similareffect for an aspen forest.

The high contrast between trees and "snow"background improved the relationships betweenphytomass and the spectral variables. Regressioncoefficients for phytomass (b I ) were both highlysignificant (ex = 0.01). R2 values were also greaterthan 0.80 in both cases (Table 4). The effects of solarzenith angle on NO were significant for phytomass(Table 4). However, for GR the regression coefficientsrelated to solar zenith angle (b2) were not Significantlydifferent from zero (ex = 0.20) (Figures 8b and d).

Figure 9 presents the relationships of nadir NOand phytomass for data restricted to solar zenithangles between 35 and 45°. The relationship obtainedfor the grass background was extremely poor (r2 =0.0) with NO constant over the range of phytomass.On the other hand, the relationship for the "snow"background is strongly linear (r2 = 0.99).

These results demonstrate the potential forestimating forest canopy variables from remotesensing data. Very little information can be extractedwhen the background spectral response is similarto the overstory. However, when there is highercontrast between the overstory and the understory,both NO and GR may perform well.

76

\ Measured Snow\

Simul ated snow't>..:....

cJ. 80

a::

~ 60U

~

~ 40z;:!~ 20...Jl.L.~ 0 L-_---l.__...L_.--JL-_--'--__L-_-....J

I

656

2 3 4 5

MMR WAVELENGTH BANDFIG. 7. Comparison of spectra for snow-covered lake andsimulated snow derived by combining spectral reflectancemeasurements for white and grass backgrounds. Snowspectra provided by G. Badhwar, NASA Johnson Space Cen­ter, Houston, Texas.

RELATIONSHIP OF SPECTRAL VARIABLES TO CANOPY

PHYTOMASS

In order to examine some relationships of spectralresponse to phytomass for balsam fir, the nadir­laeasured data for all four experiments werecombined. To evaluate these relationships forconditions that approximate balsam fir and naturalbackgrounds, we analyzed composite scenereflectance data from the grass background. Inaddition, we also created a new data set for thebalsam fir canopy reflectance with a background thatapproximated the spectral characteristics of freshlyfallen snow. Published values of the nadir reflectanceof snow (Dirmhirn et aI., 1979; Obrien and Munis,1975) indicate that the reflectance factors for freshsnow are high in the visible and near-IR (> 70percent), but low « 20 percent) in the middle-IRregions. None of the three backgrounds individuallyapproximated the spectra of snow. However, usingthe visible and near-IR RFs from the white backgroundwith middle-IR RFs measured from the grassbackground provided spectra that approximate snow.Figure 7 presents an example of simulated snowbackground RFs compared with data measured overa snow-covered lake on 3 December 1983 in northernMinnesota (G. Badhwar, personal communication).Because composite scene reflectance data for thebalsam fir canopies were acquired from grass andwhite background scenes under similar solar zenithangles, we combined visible and near-IR RF data fromcanopies with white background with middle-IR RFdata from canopies with the grass background tocreate a new data set to simulate the spectral responseof canopies with a snow background. With thisprocedure, NO values for canopies over simulatedsnow will be the same as those for the whitebackground. GR values, however, will be slightlyhigher than for the white background because oflower middle-IR RFs and the negative coefficients forthese bands (see Equation 2). Freshly fallen snow

TABLE 4. MULTIPLE REGRESSION COEFFICIENTS FOR RELATIONSHIPS OF NORMALIZED DIFFERENCE (NO) AND GREENNESS

(GR) TO PHYTOMASS (X) AND SOLAR ZENITH ANGLE (65 ), MODEL USED WAS Y = bo + b,X + ~6s.

657

7555

0--0--0--0 --0-----0

d.

ctP-.-o-~-A.-O--.-o--.~.---1.

09---0----0---0--..(,

ACKNOWLEDGMENTS

This work was supported by NASA Contract NAS9-

not totally describe the situation encountered in anatural forest. However, the basic principles of thespectral response from small balsam fir with respectto solar zenith angle, view zenith angle, and back­ground reflectance should provide insight for stud­ies of the natural forest environment.

Experiments such as the one described here canprovide significant useful information for under­standing the factors that affect the spectral responsefrom forest scenes. In addition, controlled experi­ments can provide data that may serve as "test beds"for canopy modeling studies. In the future we planto increase the range of viewing angles in our ex­periments and also extend our analysis to micro­wave sensors.

Grass

Phylomoss(kg 1m2) t'iAI

o 0.0 0-01.35 3.3 0- --()I. 50 3.5 b---61.95 4.8 A---.t.3.04 7.4 0-.-0

c.

1.0 o._~_ -..0

0.8' 6_6-6

0.0 L-_'-----'_----'-_----'--_-'----'

5Q

0.2

SUN ANGLE, VIEW ANGLE, AND BACKGROUND EFFECTS

10

0.6oz

0.4

(j)40~-o~ 30zz~ 20a::Cl

SpectralVaria-

ble bo b, b2R2 SE, SE2

NO 0.78" 0.007 0.001" 0.36 0.005 0.0003GR 45.97" -0.544" -0.087" 0.28 0.406 0.024

NO -0.15" 0.274** 0.005" 0.92 0.015 0.001GR 10.62" 10.790" 0.074 0.82 0.957 0.055

OL.---'---..I..--..I..--..I..--..I..---'15 35 55 75 15 35

SOLAR ZENITH ANGLE,deg.

FIG. 8. Changes in normalized difference (NO) and greenness (GR) as a function of solar zenith angle for five densities ofbalsam fir trees with grass and "snow" backgrounds.

in nadir measurements of NO as solar zenith angleincreased appear to be caused by the proportion­ately higher near-IR component within the shadowscast by the trees. This effect was also observed foroff-nadir measurements. The effect of the increasedred irradiance component present in skylight at verylarge sun zenith angles may also be important. Caremust be taken when interpreting NO data when sunand view angles vary.

The spectral properties of the background mayconfound changes in the spectral response of theoverstory vegetation. Based on our results, winter­time data, when snow masks the understory, wouldprovide better estimates of overstory phytomass inconiferous forests then summer-time data. Whenextending these principles to actual forests, care mustbe taken to avoid situations when snow is on thetrees.

Obviously, the controlled nature of this study may

Grass

Back­ground

"Snow"

"Coefficient significant at ()( = 0.01.

0.8

REFERENCES

(Received 29 August 1985; accepted 22 October 1985; re­vised 13 December 1985)

Univ., W. Lafayette, Indiana, 47906-1399, June 29-July1. Pp. 4B/41-51.

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Kimes, D. S., J. A. Smith, and K. J. Ranson, 1980. Vege­tation reflectance measurements as a function of solarzenith angle. Photogrammetric Engineering and RemoteSensing 46:1563-1573.

Knipling, E. B., 1967. Physical and physiological basis fordifferences in reflectance of healthy and diseasedplants, in Workshop on infra-red color photography in theplant sciences, Florida Dept. Agric. Winter Haven, Fla.March 2-3.

Knowles Middleton, W. E., and A. G. Mungall, 1952. Theluminous directional reflectance of snow. J. Opt. Soc.Am. 42:572-579.

Kollenkark, J. c., C. S. T. Daughtry, M. E. Bauer, and T.L. Housley, 1982. Effects of cultural practices on ag­ronomic and reflectance characteristics of soybeancanopies. Agron. J. 74:751-758.

Kriebel, K. T., 1978. Measured spectral bidirectional re­flection properties of four vegetated surfaces. AppliedOptics 17:253-260.

--, ~979. Albedo of vegetated surfaces: Its variabilitywith differing irradiances. Remote Sens. Environ. 8:283­290.

Lefohn, A. S., and R. W. Brockson, 1984. Acid rain effectsresearch - a status report. J. Air Pollution Control Assoc.34:1005-1013.

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Obrien, H. W., and R. H. Munis, 1975. Red and near­infrared spectral reflectance of snow, in A. Rango (ed.),Operational Applications of Satellite Snowcover Opera­tions. Workshop Proc., NASA Special Publication SP­391, Washington, DC.

Pinter, P. J., R. D. Jackson, S. B. Idso, and R. J. Reginato,1983. Diurnal patterns of wheat spectral reflectances.IEEE Trans. Geosci. Remote Sens. GE-21:156-163.

Ranson, K. J., 1983. Angular reflectance characteristics of cornand soybean canopies. Ph. D. Thesis, Purdue Uillv., Uillv.Microfilms, (Diss. Abstr. 84-07598), Ann Arbor, Mich.,168 p.

Spurr, S. H., 1964. Forest Ecology. Roland Press, New York,N.Y.

Stewart, J. B., 1971. The albedo of pine forest. Quart. J. R.Met. Soc. 97:561-564.

Suits, G. H., 1978. Natural sources, in W. L. Wolfe andG. J. Zissis (ed.), The Infrared Handbook. Office of NavalResearch, Dept of the Navy, Washington, D. C.

Tucker, C. J., 1979. Red and photographic infrared linearcombinations for monitoring vegetation. Remote Sens.Environ. 8:127-150.

Tucker, C. J., J. G. R. Townshend, and T. E. Goff, 1985.African land-cover classification using satellite data.Science 227:369-375.

PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1986

Y= -0.005+0.274X r 2 =0.99

0.4

0.2

658

oz

Asrar, G., M. Fuchs, E. T. Kanemasu, and J. L. Hatfield,1984. Estimating absorbed photosynthetic radiation andleaf area index from spectral reflectance of wheat.Agron. J. 76:300-306.

Badhwar, G. D., R. B. MacDonald, and F. G. Hall, 1984.Spectral characterization of biophysical characteristicsin a boreal forest: Relationship between ThematicMapper band reflectance and leaf area index for as­pen. IGARSS'84 1:111-115, Strasbourg, France, 27-30August.

Botkin, D. B., and E. Y. Pecan, 1982. Habitability of theearth: Land-air interactions, world food production, and netprimary production. Report prepared for National Aer­onautics and Space Administration from conf. at Uruv.of Calif., Santa Barbara, May 1982. 39 p.

Crist, E. P., R. Laurin, J. E. Colwell, and R. J. Kauth, 1984.Investigations of vegetation and soils information containedin Landsat thematic mapper and multispectral scanner data.ERIM Tech. Rep. 160300-1O-F, Ann Arbor, Michigan.

Dirmhirn, I., and F. D. Eaton, 1979. Reflected irradianceindica trices of natural surfaces and their effect on al­bedo. App/. Optics 18:994-1008.

Jarvis, P. G., G. B. James, and J. J. Landsberg, 1976. Con­iferous forest, in J. L. Monteith (ed.), Vegetation andthe Atmosphere, Vol. 2 Case studies. Academic Press,New York, N.Y.

Kauth, R. J., and G. S. Thomas, 1976. The tasseled cap­graphic description of spectral temporal developmentof agricultural crops as seen by Landsat. Proc. Symp.Machine Processing Remotely Sensed Data, LARSlPurdue

16528 under the Characterization of Vegetation WithRemote Sensors (COYER) project. The authors ap­preciate the field assistance provided by D. Locks,D. Lindman, and P. Mercer. We gratefully acknowl­edge the capable assistance of Mary Rice for typingthe manuscript.

0.0 t::L..__---L -l.-__...---J

o I 2 3

PHYTOMASS, Kg/m 2

FIG. 9. Relationship of nadir normalized difference (NO) andphytomass for balsam fir canopies with grass (squares) andsimulated snow (circles) backgrounds. Data were acquiredwith solar zenith angles between 35° and 45°.

0.6

1.0 Y= 0.862 -0.004X r2 =0.0

-------~--LT-----