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Thermal infrared reflectance of dry plant materials: 2.5–20.0 μm
REMOTE SENSING OF ENVIRONMENT 26:265-285 (1988)
265
Thermal Infrared Reflectance of Dry Plant Materials: 2.5-20.0
CHRISTOPHER D. ELVIDGE Desert Research Institute and Agricultural Experiment Station, University of Nevada System, Reno, Nevada 89506-0220
Thermal infrared reflectance spectra were measured for suites of plant materials from seven species. All of the spectra have low overall reflectance (0.0-6.0%). Dry plant materials generally yield spectra characterized by lignin and holocellulose spectral features, denoted by reflectance increases in the 4.0-5.5/~m region, at 6.4 #m, and between 10.0 and 14.0 #m.
documentation have yet to be achieved for vegetation. Information regarding the thermal inRecent high spectral resolution refrared (TIR) spectral characteristics of flectance measurements of green leaves earth surface materials is important for in the TIR by Salisbury (1986) and understanding and modeling the ex- Salisbury and Milton (1986; 1988) rechange of radiant energy between the vealed a wide range of distinctive spectral biosphere, lithosphere, and atmosphere. features. While the number of species In addition, this spectral region contains measured remains small, the work of many fundamental absorptions associated Salisbury and Milton indicates that there with specific molecular bonds. The devel- is considerable potential in remote sensopment of advanced TIR sensors de- ing for discriminating vegetation characsigned to identify and measure the inten- teristics with either active or passive TIR sity of these spectral features will provide sensors having high spectral resolution. significant advance in our abilities to To date, TIR reflectance spectra have identify earth surface materials using re- been measured for a small number of mote sensing. species, and generally only green leaves The TIR spectral characteristics of all have been measured. Green leaves are major mineral species have been studied clearly of major importance in most plant intensively and catalogued in works such canopies. However, most plant canopies as Lyon (1963) and Salisbury et al. (1987). also contain dry plant materials such as The TIR emittance features for silicate dry leaves, dry reproductive structures, minerals in the 8-12 #m region are of bark, and wood. As components of the sufficient magnitude to permit litholo- plant canopy and associated litter, dry gic discriminations using the airborne plant materials contribute to the upThermal Infrared Multispectral Scanner welling TIR radiance in terrestrial envi(TIMS) (Kahle and Rowan, 1980). The ronments. The level of contribution from TIR absorption and transmission charac- the various components of plant canopies teristics of the atmosphere have also been may be expected to vary both spatially extensively detailed (e.g., Kneizys et al., and seasonally, providing detail in re1983). Similar levels of understanding and motely sensed data. Introduction
©Elsevier Science Publishing Co., Inc., 1988 655 Avenue of the Americas, New York, NY 10010
0034-4257/88/$3.50
C. D. ELVIDGE
266
This paper presents the results of a preliminary examination of the TIR reflectance features of dry plant materials from plant canopies and associated plant litters. The approach has been to measure the TIR reflectance spectra for a suite of plant canopy and litter components from individual species. Suites of spectra are presented for seven species. In addition, TIR reflectance spectra are presented for major compounds found in plant canopies and associated litter.
Previous Investigations A large number of broadband TIR emission studies of crops and other vegetation types can be found in the literature. Green vegetation is essentially a blackbody when broadbands are employed. As a result, broadband TIR (8-12 /zm) emission measurements are routinely used to survey radiant temperatures of plant canopies. Such measurements are used to estimate evapotranspiration (e.g., Reginato et al., 1985) or green biomass when combined with visible-near infrared data (Hatfield, 1983). There have been relatively few studies of the spectral characteristics of plant materials in the TIR. Early reflectance measurements of green leaves were reported in tabular form due to the coarse spectral resolution employed. Gates and Tantraporn (1952) report specular reflectance measurements for 27 species at 3.0, 5.0, 7.5, 10.0, and 15.0 /xm. Wong and Blevin (1967) measured diffuse plus specular reflectance of green leaves from 15 species with bands 0.1-0.3 ~tm wide centered at 2.8, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, and 14.0/zm. These early studies found that green leaves had low reflectance (0.0 to 10.0%)
in the TIR. The low reflectance of green leaves was attributed to the intense TIR absorption of liquid water present in the leaf tissue. Higher reflectance values were generally found on leaves having copious hair or thick waxy cuticles. Because of the broadbands used in making the measurements the spectral features of the leaves could not be resolved. Continuous narrow band TIR reflectance spectra of plant materials have only recently appeared in the literature. Gates (1980) provides TIR spectra of green leaves for three tree species over the range 2.5-25.0/~m. The three species were found to have unique spectral features and a reflectance range of 0.0-6.0%. Salisbury (1986) and Salisbury and Milton (1986) conducted high spectral resolution specular reflectance (biconical) measurements of green leaves in the 8-14 /lm region for 13 species. Salisbury and Milton (1988) report directional hemispheric reflectance spectra for green leaves from six tree species. The spectra of Salisbury and Milton provide the first well-documented set of TIR reflectance spectra of green leaves. Their results confirm the earlier discovery of species specific spectral features noted by Gates (1980). Because of the specular nature of the reflectance features observed in smooth (hairless) green leaves, Salisbury (1986) hypothesized that these features originated from compounds in the waxy cuticular layer. The species specific differences in the TIR would then be expressions of the variation in cuticle compositions between species.
Experimental Reflectance spectra were measured using an Analect Instruments Model 6200
THERMAL INFRARED REFLECTANCE OF DRY PLANTS
Fourier transform interferometer (FTIR) using a biconical reflectance attachment composed of two off-axis paraboloid mirrors positioned above a horizontal sample. Measurements of diffuse plus specular reflectance were made with the mirror carriage in the horizontal position. In addition, diffuse reflectance measurements were made with the carriage rotated 15 ° from horizontal. Except for the diffuse reflectance of water, only the spectra of diffuse plus specular reflectance are presented here. An aluminum-coated mirror was used as a standard for the diffuse plus specular reflectance measurements. Vapor deposited gold on a fine sandpaper (600 grit) was used as a standard for the diffuse reflectance measurements [as recommended by Nash (1986)]. The spectral resolution of the instrument was four wavenumbers. In terms of micrometers, spectral resolution ranges from 0.0025/~m at 2.5 /am to 0.159 ~tm at 20 #m. The diameter of the beam focus was 1 mm. Two hundred spectra were acquired and averaged from a single location on the sample material. Sets of 10 spectra were acquired at several points on the sample surface in order to select the representative point from which the 200 spectra were acquired. Suites of samples from seven plant species were collected and measured on the FTIR. The seven plant species are bigberry manzanita (ArctostaphyIos glauca), California buckwheat (Er/ogonum fasciculatum), big sagebrush ( Artemisia tridentata), pinyon pine (Pinus monophylla), giant wildrye ( Elymus condensatus), Mormon tea (Ephedra nevadensis), and white peppermint (Eucalyptus pulchella ). Green leaves were collected in the field, placed in plastic bags, and kept cool on ice until
267 TABLE 1 Weight Percent Water
Arctostaphylos glauca Green lea/ Wood Red bark Pericarp Gray lea/ Brown lea/
48.89 7.26 8.40 10.86 7.19 5.45
Eriogonum fasciculatum Green lea/ Gray bark Wood Brown flowers Gray leaves Brown leaves
60.58 5.81 7.08 6.59 9.14 6.71
Artemisia tridentata Green leaves Gray bark Wood Gray leaves Brown flowers Senesced leaves
55.63 6.44 5.78 6.56 7.67 8.33
Pinus monophylla Green needles Gray bark Wood Brown cone Black needles Brown needles Sap
53.71 7.03 7.26 7.59 8.71 6.42 2.09
Elgmus condensatus Green lea/ Green stem Gray stem Gray leaf Yellow stem Yellow leaf Brown lea/
72.77 78.69 6.52 5.10 6.27 5.22 6.12
Ephedra nevadensis Green stems Gray bark Wood Gray stems Yellow stems
45.16 7.26 7.10 7.94 5.80
Eucalyptus pulchella Green leaf Brown bark Tan bark Wood Seed capsules Gray lea/ Brown leaf
50.27 7.87 7.30 8.46 9.16 7.35 5.65
.9.68
(;. D. ELV1D(;I~ I
1
1
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0.44 SPRUCE CELLULOSE
? ~ t ~
0.32
COTTON CELLULOSE
U,J
o z
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LOBLOLLY PINE LIGNIN
_
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rr"
4
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SPECULAR M . .
0.44 WATER
b
2.5
0.03
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6.0
8.0
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DIFFUSE
1
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12.0 14.0 10.0 WAVELENGTH (pro)
I .....
16.0
I
18.0
~
I
20.0
FIGURE 1. Reflectance spectra of cellulose, lignin, and water. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0/zm is provided for each spectrum. Both the diffuse and diffuse plus specular reflectance spectra of water are presented.
269
THERMAL INFRARED REFLECTANCE OF DRY PLANTS
the measurements were made. Dry plant materials were collected in the field and measured without additional drying or preparation. After being measured on the FTIR, both the green and dry plant materials were dried to constant weight at 60°C to determine their water contents, which are listed in Table 1. Powdered samples of a number of compounds known to be plant material constituents were available in purified form. The compounds measured were: spruce and cotton cellulose, loblolly pine lignin, sweetgum lignin, water, B-glucan, xylan, arabinogalactan, starch, citrus and apple pectin, carnauba wax, D-ribulose 1-5-diphosphate carboxylase, humic acid, and tannic acid.
Spectra of Plant Compounds
TABLE 2
Cellulose Absorption Features a
Wavelength (~m) 2.90 3.03 3.15 3.37 3.38 3.41 3.44 3.46 3.48 3.51 6.12 6.80 6.94 7.06 7.27 7.33 7.49 7.60 7.83 7.96 8.16 8.33 8.66 9.03
Water (Fig. 1) accounts for 40-80% of the fresh weight of green leaves. The diffiase reflectance of water is near zero throughout the TIR. However, the specular reflectance of water is characterized by reflectance peaks at 3.08 and 6.16/~m, with a broad rise in reflectance from 12 to 20 /~m. While water is a major constituent of green leaves, it is only a minor constituent of the dry plant materials found in plant canopies (Table 1). Cellulose (Fig. 1) is the most abundant organic compound in terrestrial ecosystems, forming one third to one half of the dry weight of most plants (Colvin, 1980). Cellulose is a polysaccharide, formed from the straight linkage of D-glucose units. It persists in natural environments due to its insolubility in water and organic solvents and its resistance to biologic decay. The spectra for cellulose derived from cotton and from spruce are presented in Fig. 1.
9.28 9.43 9.66 9.80 9.95 10.04 10.36 11.21 12.50 13.16 14.29 15.38
IDENTIFICATION OH stretching same same C - - H stretching same CH 2 asymmetric stretching C - - H stretching same same CH 2 symmetric stretching adsorbed water OH in-plane bending CH 2 symmetric bending C - - H bending same OH in-plane bending CH 2 wagging C - - H bending
OH in-plane bending asymmetric bridge C - - O - - C stretching asymmetric in-phase ring stretching skeletal vibrations involving C - - O stretching same same same same same same asymmetric out-of-phase stretching ring breathing CH 2 rocking OH out-of-phase bending same
aFrom Blackweil and Marchessault (1971) and IAang
(1972). The absorption features of cellulose identified by Blackwell and Marchessault (1971) and Liang (1972) are listed in Table 2. While the absorption features in Table 2 were derived from transmission spectra, they are still usehd guides in the
270
C. D. ELVIDGE
identification of the absorption features found in reflectance spectra. Cellulose has an intense absorption in the 2.9-3.2/zm region due to OH stretching. There is a minor reflectance peak at 3.28 gm. At 3.45 gm there is an intense absorption from C - - H stretching. Cellulose reflectance rises from 3.5 to a major reflectance peak at 4.29 gm. There are minor absorptions at 4.45, 4.66, and 4.85 ftm. The highest reflectance for the cellulose powders occurs at 5.31 gm. Following this there is an absorption at 6.08 gm plus a reflectance peak at 6.42 ftm. The spectral range from 6.9 to 10.1 gm is characterized by a series of overlapping absorptions. There is a broad rise in reflectance from 10.2 to 13.4 /zm, interrupted by a prominent absorption at 11.11 ftm. This final absorption corresponds to TABLE 3
Lignin Absorption Features"
WAVELENCTH (p,m) 2.92 3.42 3.48 3.55 5.76 5.81 6.02 6.23 6.62 6.80 6.85 6.99 7.30 7.52 7.87 8.13 8.77 8.85 9.22 9.66 10.31 11.80 12.27 13.00
the 11.21 asymmetric out-of-phase absorption listed on Table 3. The 13.4-20.0 gm region is characterized by intense absorption. Lignin (Fig. 1) is a complex polymer of phenylpropanoid units which encrusts and penetrates fibrils of cellulosic material. The quantity of lignin in plant materials ranges from 10 to 35% of the dry weight (Crawford, 1981). Lignin is one of the most persistent of all nab/rally occuring hydrocarbons. Two milled wood lignin samples were measured: lobloUy pine and sweetgum. The mechanical milling procedure is preferable over extraction techniques involving solvents, which tend to alter the spectrum of the lignin residue (Hergert, 1971). The lignin absorption features identified by Hergert (1971), Chua and Wayman (1979), and
IDENTIFICATION OH stretching C - - H stretching in methyl and methylene groups same same C = O stretching in unconjugated acids and esters C = O stretching in unconjugated ketone and carboxyl groups C = O stretching in para substituted aryl ketones aromatic skeletal vibrations same C - - H deformations from methyl and methylene groups salne
aromatic skeletal C - - H symmetric deformation syringyl ring breathing with C - - O stretching guaiacyl ring breathing with C - - O stretching syringyl and guaiacyl ring breathing with C - - O stretching aromatic C - - H in-plane deformation, guaiacyl type aromatic C - - H in-plane deformation, syringal type C - - O deformation in secondary alcohols and aliphatic ethers aromatic C - - H in-plane deformation, guaiacyl type and C - - O deformation in primary alcohols = C H out-of-plane deformations same SalTle saine
"From Hergert (1971), Chua mad Wayman (1979), and Rodriguez and Suty (1983).
FI-IERMAL INFRARED REFLECTANCE OF DRY PLANTS
i
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271
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POLYSACCHARIDES
m
0.1 1
XYLAN (OAT)
O. 1 9
APPLE
t w Z
w _.1 u. iii ¢,r
---
2.5
PECTIN
I|
1 4.0
I 6.0
I 8.0
I I I 0.0 12.0 WAVELENGTH (IJm)
I 14.0
I 16.0
I 18.0
20.0
FIGURE 2. Reflectance spectra of polysacchaxides. The spectra have been displac~d vertically to avoid overlap. The percent reflectance at 10.0/~m is provided [or each spectrum.
272
Rodriquez and Suty (1983) are listed in Table 3. Lignin has a major absorption in the 2.9-3.0 ~m region, attributable to OH stretching. There is a sharp reflectance peak at 3.25 /~m. At 3.37 /zm there is an intense absorption associated with C - - H stretching. Lignin reflectance rises abruptly from 3.6 /zm to its reflectance peak at 4.25/zm. There is a dip in reflectance centered in the vicinity of 4.85 /zm and a second major reflectance peak at 5.20 /zm. A series of C = O absorptions are present from 5.7 to 6.1 /zm and at 6.23 /zm there is an absorption from aromatic skeletal vibrations. A spikelike reflectance peak is present at 6.41 /zm. Absorption is intense throughout the rest of the TIR, with a minor rise in reflectance occuring in the 9.7-10.5 /~m range. Xylan and B-glucan (Fig. 2) are members of a group of polysaccharides known as hemiceiluloses due to their similarity to cellulose in structure and function. Bglucan is a o-glucose polymer while xylan is a polymer of D-xylose units. Xylan is the most abundant hemicellulose in angiosperms, making up to 20-30% of the dry weight in woody tissues (Aspinall, 1980). The TIR reflectance spectra obtained for both compounds have a resemblance to cellulose. Arabinogalactan (Fig. 2) is a polysaccharide composed of linear D-galactan chains with sidechains of L-arabinofuranose residues. Arabinogalactans are most abundant in gymnosperms, where they may account for up to 30% of the dry weight in woody tissues. The TIR reflectance spectrum of arabinogalactan bears some similarity to that of cellulose, except for the abrupt rise in reflectance at 9.2 #m. Starch (Fig. 2) is polysaccharide formed of glucose units and is the principal food
C D. ELVIDGE
storage molecule for plants. The TIR reflectance spectrum of starch exhibits intense absorption from 2.8 to 3.5/zm, increased reflectance in the 4.2-5.5 ~tm range, a major absorption at 6.03 /zm, a reflectance peak at 6.40/xm, and intense absorption from 6.8 to 20.0/zm. Because of its strategic food value and the ease with which starch can be broken down in biologic processes, it is not a major component in most dry plant materials. Pectins (Fig. 2) are polymers formed from galacturonic acid and are found in cell wails and in the middle lamella between adjacent ceils. Pectins are most abundant in fruits. Spectra for two types of pectins are presented in Fig. 2: citrus and apple. The citrus pectin is characterized by intense absorption from 2.8 to 3.3 /zm, a broad increase in reflectance from 4.25 to 5.45 /xm, and intense absorption from 5.65 to 20.0 #m. The apple pectin exhibits intense absorption from 2.8 to 20.0/~m. Waxes provide a protective coating for leaves, stems, seeds, and other plant parts. Their primary function is to resist desiccation, but they may also serve as a useful barrier to certain pathogenic organisms. There is large chemical variation among the compounds known as waxes, and the composition of waxes varies widely among plant species. The TIR reflectance spectrum for carnauba wax is presented in Fig. 3. The spectrum is characterized by a large number of distinctive features and is quite unusual compared to the other compounds examined. The most abundant nitrogen bearing compound in green leaves is the protein D-ribulose 1-5-diphosphate carboxylase. This enzyme plays a critical role in the fixation of carbon in photosynthesis. It accounts for 30-50% of the nitrogen in green leaves. While this compound is not
273
THERMAL INFRARED REFLECTANCE OF DRY PLANTS
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CARNAUBA W A X
D-RIBULOSE 1-5-DIPHOSPHATE CARBOXYLASE__ 1
W
z W _J I,~ W
cc
HUMIC ACID 037 J_, L-~_%.
, ii
j
~
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TANNIC ACID
2.5
I 4.0
I 6.0
I 8.0
I 10.0
1 12.0
I 14.0
I 16.0
I 18.0
I 20.0
WAVELENGTH (pro) FIGURE 3. Reflectance spectra of carnauba wax, v-ribulose 1-5-diphoshate carboxylase, humic acid, and tannic acid. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0/tm is provided for each spectrum.
274
a component of dry plant materials, its TIR reflectance spectrmn (Fig. 3) is provided because of its possible importance in the analysis of spectral features in green leaves. Tannins are polyphenol compounds of varying size and complexity (Haslam, 1981). Along with lignin, tannins are ranked among the most persistent of all natural hydrocarbons (Crawford, 1981). They are most abundant in the cell walls and woody tissues of vascular plants. The TIR reflectance spectrum of tannic acid is shown in Fig. 3. Tannic acid has intense absorption in the 2.8-4.0 g m region, a reflectance plateau from 4.25 to 5.45 gm, and intense absorption from 5.8 to 20.0/~m. Humic acid (Fig. 3) is a dark-colored, partly colloidal material derived from the decay of plant materials. It is an important constituent of some softs and is also present in decaying plant litter. The TIR reflectance spectrum of humic acid is characterized by an intense absorption at 2.94 g m and a doublet with absorptions at 3.42 and 3.52 gm. There is a broad, rounded reflectance peak from 3.6 to 5.5 gm, and strong absorption from 5.7 to 20.0 ~tm.
Spectra of Plant Materials The spectra for materials from bigberry manzanita are shown in Fig. 4. The green leaf had very low reflectivity and no spectral features. Half of the weight of the green leaf was water. The other plant materials for this species were quite dry in comparison (Table 1). The brown leaf exhibits a spectrmn similar to that of tannic acid, with a single prominent reflectance plateau between 4.25 and 5.45 g m. The gray leaf spectrum bears some
C. D. ELVIDGE
resemblance to humic acid (absorptions at 3.41 and 3.52 gm) and also features a holocellulose reflectance peak at 6.47 gm. The pericarp is the fleshy covering over the seed. As fruit tissue, it likely has a high pectin content and has a spectrum similar to apple pectin. The red bark features a low reflectance plateau between 4.25 and 5.45 gm. The brown wood, gray wood, and gray seed spectra exhibit mixtures of spectral features from lignin and holocellulose. This spectral signature is quite common in dry plant materials and will be referred to as a lignocellulose spectrum. The spectra for materials from California buckwheat are shown in Fig. 5. The smooth upper surface of the green leaf exhibits prominent reflectance peaks at: 3.41, 3.50, 6.14, 7.52, 12.65, and 14.00 gm. The features at 6.14, 7.52, 12.65, and 14.00 # m are not observed on the tomenrose (hairy) underside of the same leaf, which instead displays a lignocellulose spectrum. The C - - H stretches at 3.41 and 3.50 appear as sharp peaks in the specular reflecting front surface of the green leaf and as an absorption doublet in the diffuse reflecting green leaf underside. The upper surface of the brown leaf retains some of the reflectance peaks of the green leaf upper surface: 3.41, 3.50, 6.14, and 7.52/~m. The loss of reflectance peaks at 12.65 and 14.00 gm suggests that the set of peaks present in the green leaf originate from several compounds and that certain of these compounds have decayed or have been leached out of the brown leaf. The tomentose lower surface of the brown leaf exhibits a lignocellulose spectrum. The gray leaf upper surface features none of the reflectance peaks present in the green leaf and brown leaf upper surface. This suggests that
275
]'I-IERMAL INFRARED REFLECTANCE OF DRY PLANTS
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BIGBERRY M A N Z A N I T A (P~rcto$taohvlos glauca)
_
0 43
RED BARK
W
z
PERICARP 0.25
W U_ W
i.
__
-
_
~,5,,
J-~
__
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-'-----
: ~_ ~ ' _ ~ T r : . . . . l * ,
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GREEN LEAF 0.25
It_
2.5
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4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
WAVELENGTH (IJm) FIGURE 4. Reflectance spectra o{ plant materials from bigberry manzanita. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0/ira is provided for each spectrum.
276
C. D. ELVIDGE
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CALIFORNIA BUCKWHEAT
~
Eriogonum~
~/t/~ /V
0.87 GRAY WOOD
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0,52
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oc::
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2.5
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1
8.0
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10.0 12.0 14.0 WAVELENGTH (urn)
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16.0
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18.0
1
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FIGURE 5. Reflectance spectra of plant materials from CalLfomia buckwheat. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0/~m is provided for each spectrnm.
277
FI-IERMALINFRAREDREFLECTANCEOF DRY PLANTS I
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BIG SAGEBRUSH (Artemisia tridentata)
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a
uJ C.) Z
BROWN FLOWERS "~
0.33
/
O UJ _J LL LU
rr0.72 i~
GRAY LEAF
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-1
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SENESCED LEAF
0.52
GREEN LEAF
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10.O 12.0 14.0 WAVELENGTH (urn)
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16.0
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18.0
20.0
FIGURE 6. Reflectancespectra of plant materials from big sagebrush. The spectra have been displaced vertically to avoidoverlap. The percent reflectanceat 10.0/~m is providedfor each spectrum.
278
the compounds responsible for the reflectance peaks in the green and brown leaf upper surfaces are entirely decayed or leached out of the gray leaf. The lower surface of the gray leaf had a spectrum nearly identical to the upper surface. The brown flower, brown wood, and gray wood all display examples of lignocellulose spectra. The gray bark also display lignocellulose spectral features, with a suggestion of the spectral features of humic acid. The spectra for materials from big sagebrush are displayed in Fig. 6. All of these plant materials exhibit lignoeellulose spectra. The green leaf of this species is canescent (covered with grayish-white fine hairs), which accounts for the strong lignin and cellulose material features in the green leaf. The senesced leaf was collected (dead) from a living plant canopy. At dry times of the year large numbers of leaves senesce, but may remain in the plant canopy for considerable periods of time before they drop. Spectra for plant materials from pinyon pine are presented in Fig. 7. The green needle exhibits low reflectance and no TIR spectral features. The pine sap is composed of terpenes and also has low reflectance and no TIR spectral features. The brown needle was collected from a living plant canopy. Its spectrum exhibits a lignocellulose reflectance dip at 4.7/zm, but otherwise the brown needle spectrum is more similar to that of tannic acid. The decayed black needle and the gray bark both display some lignoeellulose spectral features, but additional compounds also appear to be exhibiting influences on their spectra. The spectra for the brown cone, brown wood, and gray wood are more typical of relatively pure mixtures of lignin and holocellulose.
C D. ELVIDGE
The spectra for materials from giant wildrye are shown in Fig. 8. The green stem exhibits a minor reflectance peak at 9.3/~m. This reflectance peak is also present in the green leaf spectrum, though an additional reflectance peak at 9.59/zm is superimposed upon it. The 9.3 /.tin reflectance peak is most prominent in the brown leaf, where it superimposes the broad rise in reflectance from 10.2 to 13.4 /zm associated with holocellulose. The brown leaf also exhibits a distinct reflectance plateau from 4.25 to 5.35 /~m, similar to tannic acid. With further exposure to the elements, the brown leaves turn yellow and finally gray. Both the yellow and gray leaves display well-developed lignocellulose spectra, though the reflectance peak at 9.3 /zm persists. The reflectance peak at 9.3/zm may be due to the presence of silica. Certain grasses are known to accumulate silica in their leaf and stem tissues. The yellow stem and gray stem both have lignoeellulose spectra. The spectra of Mornaon tea plant materials are presented in Fig. 9. The leaves of Mormon tea are reduced to small scales, and photosynthesis is carried out by the stems. The green stems show low reflectance, with only minute reflectance features. Dry yellow stems collected from a living plant canopy exhibit a minor rise in reflectance between 4.25 and 5.45/lm. This rise in reflectance becomes pronounced in the spectrum of the gray stem. The brown wood, gray wood, and gray bark all exhibit well-developed lignocellulose spectra. The spectra measured for white peppermint materials are displayed in Fig. 10. The green leaf exhibits a series of minor reflectance features. The brown leaf and gray leaf spectra show some
279
I'I-IERMAL INFRARED REFLECTANCE OF DRY PLANTS
t
I I I PINYON PINE (Pinus monophylla)
t
I
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1.56
LU
z
I
1
BROWN WOOD
0.96
BROWN CONE
0.95
BLACK NEEDLE
0.84
BROWN NEEDLE
0.30
SAP
0.22
GREEN NEEDLE
(..) LIJ LI,. ILl 137
L
,,,.J
l-2.5
I 4.0
I 6.0
I 8.0
I I 10.0 12.0 WAVELENGTH (~m)
I 14.0
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20.0
FIGURE 7. Reflectance spectra.of plant materials from pinyon pine. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0/zm is provided for each spectrum.
(;. D. ELVIDGE
280
I I I GIANT WlLDRYE (Elymu8 condensatus)
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w (..) Z (...) 111 11, w n'-
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I 8.0
l 10.0
I 12.0
WAVELENGTH
I 14.0
1 16.0
-
I 18.0
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(pro)
FIGURE 8. Reflectance spectra of plant materials [rom giant wildrye. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0 #m is provided for each spectrum.
['HERMAL INFRARED REFLECTANCE OF DRY PLANTS
I MORMON
(Ephedra
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WAVELENGTH (pro) FIGURE 9. Reflectance spectra of plant materials from Mormon tea. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0/~m is provided [or each spectrum.
182
C. D. ELVIDGE
I I I WHITE PEPPERMINT (Eucalyptus pulchella)
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WAVELENGTH (pro) FIGURE 10. Reflectance spectra of plant materials from white peppermint. The spectra have been displaced vertically to avoid overlap. The percent reflectance at I0.0/lm is provided for each spectrum.
THERMAL INFRARED REFLECTANCE OF DRY PLANTS
lignoeellulose features, but appear to be modified by some additional compounds. The seed capsule spectrum exhibits a prominent reflectance plateau from 4.25 to 5.45/~m and intense absorption from 5.7 to 20.0 pm. The seed capsule had a distinct odor suggestive of terpenes, which may account in part for the observed spectrum. The brown wood, tan bark, and brown bark all display welldeveloped lignoceUulose spectra. The tan bark spectrum is unusual in that the rise in reflectance between 10.0 and 15.0/~m exceeds the reflectance rise in the 4.0-5.5 /xm region. Discussion
The spectra of dry plant materials reported here fill in a major gap in the remote sensing knowledge base regarding the TIR reflectance characteristics of vegetation. Spectral differences can be observed between species and between diverse plant materials from a single species. Spectral changes associated with plant decay have also been observed. Many of the reflectance spectra of dry plant materials appear to be mixtures of holocellulose and lignin reflectance features. The exact balance of lignin and holocellulose features varies from sample to sample, but appear repeatedly. Additional compounds may influence or even dominate the spectra of certain plant materials. These results indicate that there is a basic lignocellulose spectrum for dry plant materials, which can be modified by the presence of compounds such as tannins or terpenes. Lignin, cellulose, and hemicellulose occur as intimate mixtures, making up 80-98% of the dry weight of most terrestrial plant materials (Crawford, 1981). Therefore, it is not sur-
283
prising that these compounds should be so evident in the spectra of dry plant materials. The diffuse reflectance spectra (acquired with the mirror carriage rotated 15 ° ) of the compounds and dry plant materials were not reported here. The spectral features observed in the diffuse plus specular reflectance measurements were invariably repeated in the diffuse spectra. The primary constituents of dry plant materials are evidently diffuse reflectors. This is in contrast to the observation by Salisbury (1986) and Salisbury and Milton (1986; 1988) of purely specular reflectance for green leaves in 8-14 /~m region. The apparent magnitude of spectral features depends upon the standard against which they are measured. The diffuse reflecting gold-coated sandpaper has an average reflectance of 44% relative to aluminum mirror. If the samples are referenced to the gold-coated sandpaper there is a 2.27 times increase in spectral contrast. The directional hemispheric reflectance (diffuse and specular) of a sample can be used to predict emittance using Kirchoff's law: emissivity = 1 reflectance. The measurement of directional hemispheric reflectance requires an integrating sphere, an accessory which was not available. It must be emphasized that Kirchoff's law does not apply to biconical reflectance. Biconical reflectance measurements should not be used to predict the magnitude of emission features. However, biconical reflectance measurements made with the same instrument show a strong correspondence to field emission measurements made of the same materials (Bartholomew et al., 1988). In addition, Salisbury and Milton (1988) and
284
C, D. ELVIDGE
Salisbury (1986) found the same spectral cause the three-dimensional structure of features in both specular and directional plant canopies tend to act as blackbody hemispheric reflectance measurements of cavities due to internal scattering. green leaves from cherry, oak, maple, and beech trees. These results indicate the The author gratefully acknowledges general utility of specular plus diffuse Dr. Anne Kahle for providing access to biconical reflectance measurements for the FTIR instrument plus computing and indicating the location of TIR emission graphics facilities at the Jet Propulsion features. Laboratory. The manuscript benefitted The highest reflectance contrast for dry from reviews by Mary Jane Bartholomew, plant materials occurs in the 3-5 gm Frank Palluconi, and Jack Salisbury. atmospheric window. Dry plant materials exhibit intense absorption in the 3 /~m region and have their highest reflectance References in the 4.0-5.5 /~m range. Green leaves Aspinall, G. O. (1980), Chemistry of cell wall have low reflectance in this range due to polysaccharides, in The Biochemistry of intense leaf water absorption. At first Plants, A Comprehensive Treatise, Volume glance, multispectral data in the 3 - 5 gm 3, Carbohydrates': Structure and Function region would appear to be a useful range (J. Preiss, Ed.), Academic, New York, pp. for measuring vegetation characteristics. 473 -500. However, the 3 - 5 /~m window falls on Bartholomew, M. J., Kahle, A. B., and Hoover, the long wavelength edge of the solar G. (1988), Infrared spectroscopy (2.3 to 20 radiation curve and the short wavelength micrometers) for the geologic interpretaedge of the terrestrial emittance curve, tion of remotely sensed multispectral thermal infrared data, Int. 1. Remote Sens., which complicates the use of this atmoforthcoming. spheric window. Night time measurements may be required to make use of Blackwell, J., and Marchessault, R. H. (1971), the 3 - 5 / z m region using passive sensors. Investigations of the structure of cellulose and its derivatives, High Polym. 5:1-37. The reflectance features reported here are relatively small in magnitude (0-6%). Chua, M. G. S., and Wayman, M. (1979), The reflectance difference between plant Characterization of autohydrolysis aspen materials are generally even smaller. The (P. tremuloides) lignins. Part 3. Infrared and ultraviolet studies of extracted magnitude of these spectral features imautohydrolysis lignin, Can. 1. Chem. plies that TIR sensors on aircraft or 57:2603-2611. spacecraft will have to have high signal to noise ratios to distinguish spectral differ- Colvin, J. R. (1980), Biosynthesis of cellulose, in The Biochemistry of Plants, A Compreences in plant canopies. hensive Treatise, Volume 3, Carbohydrates: There remains a continuing need for Structure and Function (J. Preiss, Ed.), high spectral resolution TIR measureAcademic, New York, pp. 543-570. ments (both active and passive) of whole plant canopies. It is likely that whole Crawford, R. L. (1981), Lignin Biodegradation and Transformation, Wiley-Intersciplant canopy measurements would find ence, New York, pp. 1-5. lower reflectance (higher emittance) than the laboratory spectra of plant materials Gates, D. M. (1980), Biophysical Ecology, Springer-Verlag, New York, pp. 235-238. reported here and elsewhere. This is be-
THERMALINFRAREDREFLECTANCEOF DRYPLANTS
285
Nash, D. B. (1986), Mid-irdrared reflectance spectra (2.3-22 /tm) of sulfur, gold, KBr, MgO and halon, Atrpl. Opt. 25:2427-2433. Reginato, R. J., Jackson, R. D., and Pinter, P. J. (1985), Evapotranspiration calculated Haslam, E. (1981), Vegetable tannins, in The from remote multispectral and ground staBiochemistry o f Plants, A Comprehensive tion meteorological data, Remote Sens. EnTreatise, Volume 7, Secondary Plant Prodviron. 18:75-89. ucts (E. E. Conn, Ed.), Academic, New York, pp. 527-556. Rodriguez, N. F., and Suty, L. (1983), Studies on bagasse and sugar-can lignins. II. InHatfield, J. L. (1983), Remote sensing estimafrared absorption spectra, Pap. Celul. tors of potential and actual crop yield, Re38:14-22. mote Sens. Environ. 13:301-311. Hergert, H. L. (1971), Infrared spectra, in Salisbury, J. W. (1986), Preliminary measurements of leaf spectral reflectance in the Lignins: Occurrence, Formation, Structure 8-14 /~m region, Int. ]. Remote Sens. and Reactions (K. V. Sarkanen and C. H. 7:1879-1886. Ludwig, Eds.), Wiley-Interscience, New York, pp. 267-297. Salisbury, J. W., and Milton, N. M. (1986), Preliminary measurements of spectral sigKahle, A. B., and Rowan, L. C. (1980), Evalunatures of tropical and temperate plants in ation of multispeetral middle infrared airthe thermal infrared, in Proceedings of the craft images for lithologic mapping in the Fifth Thematic Conference on Remote East Tintic Mountains, Utah, Geology Sensing for Exploration Geology, Environ8:234-239. mental Research Institute of Michigan, Ann Kneizys, F. X., Settle, E. P., Gallery, W. O., Arbor, MI, Vol. 1, pp. 131-143. Chetwynd, J. H., Jr., Abreu, L. W., Selby, Salisbury, J. w., and Milton, N. M. (1988), J. E. A., Clough, S. A., and Fenn, R. W. Thermal infrared (2.5 to 13.5 /~m) direc(1983), Atmospheric transmission/raditional hemispherical reflectance of leaves, ance: Computer Code LOWTRAN-6, Reforthcoming. port AFGL-TR-83-0187, Air Force GeoSalisbury, J. w., walter, L. S., and Vergo, N. physics Laboratory, Bedford, MA. (1987), Mid-Infiared (2.1-25 ttm) Spectra Liang, C. Y. (1972), Infrared spectroscopy of Minerals, 1st ed., U.S. Geological Survey and physical properties of cellulose, in InOpen-File Report 87-263. strumental Analysis of Cotton Cellulose and Modified Cotton Cellulose (R. T. O'Con- Wong, C. L., and Blevin, W. R. (1967), Infrared reflectance of plant leaves, Aust. ]. nor, Ed.), Marcel Dekker, New York, pp. Biol. Sci. 20:501-508. 59-91.
Gates, D. M., and Tantrapom, W. (1952), The reflectivity of deciduous trees and herbaceous plants in the infrared to 25 microns, Science 115:613-616.
Lyon, R. J. P. (1963), Analysis of rocks and minerals by reflected infrared radiation, Econ. Geol. 58:274-284.
Received6 October1987;revised261uly 1988.
265
Thermal Infrared Reflectance of Dry Plant Materials: 2.5-20.0
CHRISTOPHER D. ELVIDGE Desert Research Institute and Agricultural Experiment Station, University of Nevada System, Reno, Nevada 89506-0220
Thermal infrared reflectance spectra were measured for suites of plant materials from seven species. All of the spectra have low overall reflectance (0.0-6.0%). Dry plant materials generally yield spectra characterized by lignin and holocellulose spectral features, denoted by reflectance increases in the 4.0-5.5/~m region, at 6.4 #m, and between 10.0 and 14.0 #m.
documentation have yet to be achieved for vegetation. Information regarding the thermal inRecent high spectral resolution refrared (TIR) spectral characteristics of flectance measurements of green leaves earth surface materials is important for in the TIR by Salisbury (1986) and understanding and modeling the ex- Salisbury and Milton (1986; 1988) rechange of radiant energy between the vealed a wide range of distinctive spectral biosphere, lithosphere, and atmosphere. features. While the number of species In addition, this spectral region contains measured remains small, the work of many fundamental absorptions associated Salisbury and Milton indicates that there with specific molecular bonds. The devel- is considerable potential in remote sensopment of advanced TIR sensors de- ing for discriminating vegetation characsigned to identify and measure the inten- teristics with either active or passive TIR sity of these spectral features will provide sensors having high spectral resolution. significant advance in our abilities to To date, TIR reflectance spectra have identify earth surface materials using re- been measured for a small number of mote sensing. species, and generally only green leaves The TIR spectral characteristics of all have been measured. Green leaves are major mineral species have been studied clearly of major importance in most plant intensively and catalogued in works such canopies. However, most plant canopies as Lyon (1963) and Salisbury et al. (1987). also contain dry plant materials such as The TIR emittance features for silicate dry leaves, dry reproductive structures, minerals in the 8-12 #m region are of bark, and wood. As components of the sufficient magnitude to permit litholo- plant canopy and associated litter, dry gic discriminations using the airborne plant materials contribute to the upThermal Infrared Multispectral Scanner welling TIR radiance in terrestrial envi(TIMS) (Kahle and Rowan, 1980). The ronments. The level of contribution from TIR absorption and transmission charac- the various components of plant canopies teristics of the atmosphere have also been may be expected to vary both spatially extensively detailed (e.g., Kneizys et al., and seasonally, providing detail in re1983). Similar levels of understanding and motely sensed data. Introduction
©Elsevier Science Publishing Co., Inc., 1988 655 Avenue of the Americas, New York, NY 10010
0034-4257/88/$3.50
C. D. ELVIDGE
266
This paper presents the results of a preliminary examination of the TIR reflectance features of dry plant materials from plant canopies and associated plant litters. The approach has been to measure the TIR reflectance spectra for a suite of plant canopy and litter components from individual species. Suites of spectra are presented for seven species. In addition, TIR reflectance spectra are presented for major compounds found in plant canopies and associated litter.
Previous Investigations A large number of broadband TIR emission studies of crops and other vegetation types can be found in the literature. Green vegetation is essentially a blackbody when broadbands are employed. As a result, broadband TIR (8-12 /zm) emission measurements are routinely used to survey radiant temperatures of plant canopies. Such measurements are used to estimate evapotranspiration (e.g., Reginato et al., 1985) or green biomass when combined with visible-near infrared data (Hatfield, 1983). There have been relatively few studies of the spectral characteristics of plant materials in the TIR. Early reflectance measurements of green leaves were reported in tabular form due to the coarse spectral resolution employed. Gates and Tantraporn (1952) report specular reflectance measurements for 27 species at 3.0, 5.0, 7.5, 10.0, and 15.0 /xm. Wong and Blevin (1967) measured diffuse plus specular reflectance of green leaves from 15 species with bands 0.1-0.3 ~tm wide centered at 2.8, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, and 14.0/zm. These early studies found that green leaves had low reflectance (0.0 to 10.0%)
in the TIR. The low reflectance of green leaves was attributed to the intense TIR absorption of liquid water present in the leaf tissue. Higher reflectance values were generally found on leaves having copious hair or thick waxy cuticles. Because of the broadbands used in making the measurements the spectral features of the leaves could not be resolved. Continuous narrow band TIR reflectance spectra of plant materials have only recently appeared in the literature. Gates (1980) provides TIR spectra of green leaves for three tree species over the range 2.5-25.0/~m. The three species were found to have unique spectral features and a reflectance range of 0.0-6.0%. Salisbury (1986) and Salisbury and Milton (1986) conducted high spectral resolution specular reflectance (biconical) measurements of green leaves in the 8-14 /lm region for 13 species. Salisbury and Milton (1988) report directional hemispheric reflectance spectra for green leaves from six tree species. The spectra of Salisbury and Milton provide the first well-documented set of TIR reflectance spectra of green leaves. Their results confirm the earlier discovery of species specific spectral features noted by Gates (1980). Because of the specular nature of the reflectance features observed in smooth (hairless) green leaves, Salisbury (1986) hypothesized that these features originated from compounds in the waxy cuticular layer. The species specific differences in the TIR would then be expressions of the variation in cuticle compositions between species.
Experimental Reflectance spectra were measured using an Analect Instruments Model 6200
THERMAL INFRARED REFLECTANCE OF DRY PLANTS
Fourier transform interferometer (FTIR) using a biconical reflectance attachment composed of two off-axis paraboloid mirrors positioned above a horizontal sample. Measurements of diffuse plus specular reflectance were made with the mirror carriage in the horizontal position. In addition, diffuse reflectance measurements were made with the carriage rotated 15 ° from horizontal. Except for the diffuse reflectance of water, only the spectra of diffuse plus specular reflectance are presented here. An aluminum-coated mirror was used as a standard for the diffuse plus specular reflectance measurements. Vapor deposited gold on a fine sandpaper (600 grit) was used as a standard for the diffuse reflectance measurements [as recommended by Nash (1986)]. The spectral resolution of the instrument was four wavenumbers. In terms of micrometers, spectral resolution ranges from 0.0025/~m at 2.5 /am to 0.159 ~tm at 20 #m. The diameter of the beam focus was 1 mm. Two hundred spectra were acquired and averaged from a single location on the sample material. Sets of 10 spectra were acquired at several points on the sample surface in order to select the representative point from which the 200 spectra were acquired. Suites of samples from seven plant species were collected and measured on the FTIR. The seven plant species are bigberry manzanita (ArctostaphyIos glauca), California buckwheat (Er/ogonum fasciculatum), big sagebrush ( Artemisia tridentata), pinyon pine (Pinus monophylla), giant wildrye ( Elymus condensatus), Mormon tea (Ephedra nevadensis), and white peppermint (Eucalyptus pulchella ). Green leaves were collected in the field, placed in plastic bags, and kept cool on ice until
267 TABLE 1 Weight Percent Water
Arctostaphylos glauca Green lea/ Wood Red bark Pericarp Gray lea/ Brown lea/
48.89 7.26 8.40 10.86 7.19 5.45
Eriogonum fasciculatum Green lea/ Gray bark Wood Brown flowers Gray leaves Brown leaves
60.58 5.81 7.08 6.59 9.14 6.71
Artemisia tridentata Green leaves Gray bark Wood Gray leaves Brown flowers Senesced leaves
55.63 6.44 5.78 6.56 7.67 8.33
Pinus monophylla Green needles Gray bark Wood Brown cone Black needles Brown needles Sap
53.71 7.03 7.26 7.59 8.71 6.42 2.09
Elgmus condensatus Green lea/ Green stem Gray stem Gray leaf Yellow stem Yellow leaf Brown lea/
72.77 78.69 6.52 5.10 6.27 5.22 6.12
Ephedra nevadensis Green stems Gray bark Wood Gray stems Yellow stems
45.16 7.26 7.10 7.94 5.80
Eucalyptus pulchella Green leaf Brown bark Tan bark Wood Seed capsules Gray lea/ Brown leaf
50.27 7.87 7.30 8.46 9.16 7.35 5.65
.9.68
(;. D. ELV1D(;I~ I
1
1
I
~f~/~
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I
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0.32
COTTON CELLULOSE
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o z
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LOBLOLLY PINE LIGNIN
_
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-
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b
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I
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.
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20.0
FIGURE 1. Reflectance spectra of cellulose, lignin, and water. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0/zm is provided for each spectrum. Both the diffuse and diffuse plus specular reflectance spectra of water are presented.
269
THERMAL INFRARED REFLECTANCE OF DRY PLANTS
the measurements were made. Dry plant materials were collected in the field and measured without additional drying or preparation. After being measured on the FTIR, both the green and dry plant materials were dried to constant weight at 60°C to determine their water contents, which are listed in Table 1. Powdered samples of a number of compounds known to be plant material constituents were available in purified form. The compounds measured were: spruce and cotton cellulose, loblolly pine lignin, sweetgum lignin, water, B-glucan, xylan, arabinogalactan, starch, citrus and apple pectin, carnauba wax, D-ribulose 1-5-diphosphate carboxylase, humic acid, and tannic acid.
Spectra of Plant Compounds
TABLE 2
Cellulose Absorption Features a
Wavelength (~m) 2.90 3.03 3.15 3.37 3.38 3.41 3.44 3.46 3.48 3.51 6.12 6.80 6.94 7.06 7.27 7.33 7.49 7.60 7.83 7.96 8.16 8.33 8.66 9.03
Water (Fig. 1) accounts for 40-80% of the fresh weight of green leaves. The diffiase reflectance of water is near zero throughout the TIR. However, the specular reflectance of water is characterized by reflectance peaks at 3.08 and 6.16/~m, with a broad rise in reflectance from 12 to 20 /~m. While water is a major constituent of green leaves, it is only a minor constituent of the dry plant materials found in plant canopies (Table 1). Cellulose (Fig. 1) is the most abundant organic compound in terrestrial ecosystems, forming one third to one half of the dry weight of most plants (Colvin, 1980). Cellulose is a polysaccharide, formed from the straight linkage of D-glucose units. It persists in natural environments due to its insolubility in water and organic solvents and its resistance to biologic decay. The spectra for cellulose derived from cotton and from spruce are presented in Fig. 1.
9.28 9.43 9.66 9.80 9.95 10.04 10.36 11.21 12.50 13.16 14.29 15.38
IDENTIFICATION OH stretching same same C - - H stretching same CH 2 asymmetric stretching C - - H stretching same same CH 2 symmetric stretching adsorbed water OH in-plane bending CH 2 symmetric bending C - - H bending same OH in-plane bending CH 2 wagging C - - H bending
OH in-plane bending asymmetric bridge C - - O - - C stretching asymmetric in-phase ring stretching skeletal vibrations involving C - - O stretching same same same same same same asymmetric out-of-phase stretching ring breathing CH 2 rocking OH out-of-phase bending same
aFrom Blackweil and Marchessault (1971) and IAang
(1972). The absorption features of cellulose identified by Blackwell and Marchessault (1971) and Liang (1972) are listed in Table 2. While the absorption features in Table 2 were derived from transmission spectra, they are still usehd guides in the
270
C. D. ELVIDGE
identification of the absorption features found in reflectance spectra. Cellulose has an intense absorption in the 2.9-3.2/zm region due to OH stretching. There is a minor reflectance peak at 3.28 gm. At 3.45 gm there is an intense absorption from C - - H stretching. Cellulose reflectance rises from 3.5 to a major reflectance peak at 4.29 gm. There are minor absorptions at 4.45, 4.66, and 4.85 ftm. The highest reflectance for the cellulose powders occurs at 5.31 gm. Following this there is an absorption at 6.08 gm plus a reflectance peak at 6.42 ftm. The spectral range from 6.9 to 10.1 gm is characterized by a series of overlapping absorptions. There is a broad rise in reflectance from 10.2 to 13.4 /zm, interrupted by a prominent absorption at 11.11 ftm. This final absorption corresponds to TABLE 3
Lignin Absorption Features"
WAVELENCTH (p,m) 2.92 3.42 3.48 3.55 5.76 5.81 6.02 6.23 6.62 6.80 6.85 6.99 7.30 7.52 7.87 8.13 8.77 8.85 9.22 9.66 10.31 11.80 12.27 13.00
the 11.21 asymmetric out-of-phase absorption listed on Table 3. The 13.4-20.0 gm region is characterized by intense absorption. Lignin (Fig. 1) is a complex polymer of phenylpropanoid units which encrusts and penetrates fibrils of cellulosic material. The quantity of lignin in plant materials ranges from 10 to 35% of the dry weight (Crawford, 1981). Lignin is one of the most persistent of all nab/rally occuring hydrocarbons. Two milled wood lignin samples were measured: lobloUy pine and sweetgum. The mechanical milling procedure is preferable over extraction techniques involving solvents, which tend to alter the spectrum of the lignin residue (Hergert, 1971). The lignin absorption features identified by Hergert (1971), Chua and Wayman (1979), and
IDENTIFICATION OH stretching C - - H stretching in methyl and methylene groups same same C = O stretching in unconjugated acids and esters C = O stretching in unconjugated ketone and carboxyl groups C = O stretching in para substituted aryl ketones aromatic skeletal vibrations same C - - H deformations from methyl and methylene groups salne
aromatic skeletal C - - H symmetric deformation syringyl ring breathing with C - - O stretching guaiacyl ring breathing with C - - O stretching syringyl and guaiacyl ring breathing with C - - O stretching aromatic C - - H in-plane deformation, guaiacyl type aromatic C - - H in-plane deformation, syringal type C - - O deformation in secondary alcohols and aliphatic ethers aromatic C - - H in-plane deformation, guaiacyl type and C - - O deformation in primary alcohols = C H out-of-plane deformations same SalTle saine
"From Hergert (1971), Chua mad Wayman (1979), and Rodriguez and Suty (1983).
FI-IERMAL INFRARED REFLECTANCE OF DRY PLANTS
i
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271
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POLYSACCHARIDES
m
0.1 1
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O. 1 9
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w _.1 u. iii ¢,r
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I 6.0
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I I I 0.0 12.0 WAVELENGTH (IJm)
I 14.0
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20.0
FIGURE 2. Reflectance spectra of polysacchaxides. The spectra have been displac~d vertically to avoid overlap. The percent reflectance at 10.0/~m is provided [or each spectrum.
272
Rodriquez and Suty (1983) are listed in Table 3. Lignin has a major absorption in the 2.9-3.0 ~m region, attributable to OH stretching. There is a sharp reflectance peak at 3.25 /~m. At 3.37 /zm there is an intense absorption associated with C - - H stretching. Lignin reflectance rises abruptly from 3.6 /zm to its reflectance peak at 4.25/zm. There is a dip in reflectance centered in the vicinity of 4.85 /zm and a second major reflectance peak at 5.20 /zm. A series of C = O absorptions are present from 5.7 to 6.1 /zm and at 6.23 /zm there is an absorption from aromatic skeletal vibrations. A spikelike reflectance peak is present at 6.41 /zm. Absorption is intense throughout the rest of the TIR, with a minor rise in reflectance occuring in the 9.7-10.5 /~m range. Xylan and B-glucan (Fig. 2) are members of a group of polysaccharides known as hemiceiluloses due to their similarity to cellulose in structure and function. Bglucan is a o-glucose polymer while xylan is a polymer of D-xylose units. Xylan is the most abundant hemicellulose in angiosperms, making up to 20-30% of the dry weight in woody tissues (Aspinall, 1980). The TIR reflectance spectra obtained for both compounds have a resemblance to cellulose. Arabinogalactan (Fig. 2) is a polysaccharide composed of linear D-galactan chains with sidechains of L-arabinofuranose residues. Arabinogalactans are most abundant in gymnosperms, where they may account for up to 30% of the dry weight in woody tissues. The TIR reflectance spectrum of arabinogalactan bears some similarity to that of cellulose, except for the abrupt rise in reflectance at 9.2 #m. Starch (Fig. 2) is polysaccharide formed of glucose units and is the principal food
C D. ELVIDGE
storage molecule for plants. The TIR reflectance spectrum of starch exhibits intense absorption from 2.8 to 3.5/zm, increased reflectance in the 4.2-5.5 ~tm range, a major absorption at 6.03 /zm, a reflectance peak at 6.40/xm, and intense absorption from 6.8 to 20.0/zm. Because of its strategic food value and the ease with which starch can be broken down in biologic processes, it is not a major component in most dry plant materials. Pectins (Fig. 2) are polymers formed from galacturonic acid and are found in cell wails and in the middle lamella between adjacent ceils. Pectins are most abundant in fruits. Spectra for two types of pectins are presented in Fig. 2: citrus and apple. The citrus pectin is characterized by intense absorption from 2.8 to 3.3 /zm, a broad increase in reflectance from 4.25 to 5.45 /xm, and intense absorption from 5.65 to 20.0 #m. The apple pectin exhibits intense absorption from 2.8 to 20.0/~m. Waxes provide a protective coating for leaves, stems, seeds, and other plant parts. Their primary function is to resist desiccation, but they may also serve as a useful barrier to certain pathogenic organisms. There is large chemical variation among the compounds known as waxes, and the composition of waxes varies widely among plant species. The TIR reflectance spectrum for carnauba wax is presented in Fig. 3. The spectrum is characterized by a large number of distinctive features and is quite unusual compared to the other compounds examined. The most abundant nitrogen bearing compound in green leaves is the protein D-ribulose 1-5-diphosphate carboxylase. This enzyme plays a critical role in the fixation of carbon in photosynthesis. It accounts for 30-50% of the nitrogen in green leaves. While this compound is not
273
THERMAL INFRARED REFLECTANCE OF DRY PLANTS
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I 8.0
I 10.0
1 12.0
I 14.0
I 16.0
I 18.0
I 20.0
WAVELENGTH (pro) FIGURE 3. Reflectance spectra of carnauba wax, v-ribulose 1-5-diphoshate carboxylase, humic acid, and tannic acid. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0/tm is provided for each spectrum.
274
a component of dry plant materials, its TIR reflectance spectrmn (Fig. 3) is provided because of its possible importance in the analysis of spectral features in green leaves. Tannins are polyphenol compounds of varying size and complexity (Haslam, 1981). Along with lignin, tannins are ranked among the most persistent of all natural hydrocarbons (Crawford, 1981). They are most abundant in the cell walls and woody tissues of vascular plants. The TIR reflectance spectrum of tannic acid is shown in Fig. 3. Tannic acid has intense absorption in the 2.8-4.0 g m region, a reflectance plateau from 4.25 to 5.45 gm, and intense absorption from 5.8 to 20.0/~m. Humic acid (Fig. 3) is a dark-colored, partly colloidal material derived from the decay of plant materials. It is an important constituent of some softs and is also present in decaying plant litter. The TIR reflectance spectrum of humic acid is characterized by an intense absorption at 2.94 g m and a doublet with absorptions at 3.42 and 3.52 gm. There is a broad, rounded reflectance peak from 3.6 to 5.5 gm, and strong absorption from 5.7 to 20.0 ~tm.
Spectra of Plant Materials The spectra for materials from bigberry manzanita are shown in Fig. 4. The green leaf had very low reflectivity and no spectral features. Half of the weight of the green leaf was water. The other plant materials for this species were quite dry in comparison (Table 1). The brown leaf exhibits a spectrmn similar to that of tannic acid, with a single prominent reflectance plateau between 4.25 and 5.45 g m. The gray leaf spectrum bears some
C. D. ELVIDGE
resemblance to humic acid (absorptions at 3.41 and 3.52 gm) and also features a holocellulose reflectance peak at 6.47 gm. The pericarp is the fleshy covering over the seed. As fruit tissue, it likely has a high pectin content and has a spectrum similar to apple pectin. The red bark features a low reflectance plateau between 4.25 and 5.45 gm. The brown wood, gray wood, and gray seed spectra exhibit mixtures of spectral features from lignin and holocellulose. This spectral signature is quite common in dry plant materials and will be referred to as a lignocellulose spectrum. The spectra for materials from California buckwheat are shown in Fig. 5. The smooth upper surface of the green leaf exhibits prominent reflectance peaks at: 3.41, 3.50, 6.14, 7.52, 12.65, and 14.00 gm. The features at 6.14, 7.52, 12.65, and 14.00 # m are not observed on the tomenrose (hairy) underside of the same leaf, which instead displays a lignocellulose spectrum. The C - - H stretches at 3.41 and 3.50 appear as sharp peaks in the specular reflecting front surface of the green leaf and as an absorption doublet in the diffuse reflecting green leaf underside. The upper surface of the brown leaf retains some of the reflectance peaks of the green leaf upper surface: 3.41, 3.50, 6.14, and 7.52/~m. The loss of reflectance peaks at 12.65 and 14.00 gm suggests that the set of peaks present in the green leaf originate from several compounds and that certain of these compounds have decayed or have been leached out of the brown leaf. The tomentose lower surface of the brown leaf exhibits a lignocellulose spectrum. The gray leaf upper surface features none of the reflectance peaks present in the green leaf and brown leaf upper surface. This suggests that
275
]'I-IERMAL INFRARED REFLECTANCE OF DRY PLANTS
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BIGBERRY M A N Z A N I T A (P~rcto$taohvlos glauca)
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6.0
8.0
10.0
12.0
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18.0
20.0
WAVELENGTH (IJm) FIGURE 4. Reflectance spectra o{ plant materials from bigberry manzanita. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0/ira is provided for each spectrum.
276
C. D. ELVIDGE
I
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CALIFORNIA BUCKWHEAT
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Eriogonum~
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1
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FIGURE 5. Reflectance spectra of plant materials from CalLfomia buckwheat. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0/~m is provided for each spectrnm.
277
FI-IERMALINFRAREDREFLECTANCEOF DRY PLANTS I
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BIG SAGEBRUSH (Artemisia tridentata)
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FIGURE 6. Reflectancespectra of plant materials from big sagebrush. The spectra have been displaced vertically to avoidoverlap. The percent reflectanceat 10.0/~m is providedfor each spectrum.
278
the compounds responsible for the reflectance peaks in the green and brown leaf upper surfaces are entirely decayed or leached out of the gray leaf. The lower surface of the gray leaf had a spectrum nearly identical to the upper surface. The brown flower, brown wood, and gray wood all display examples of lignocellulose spectra. The gray bark also display lignocellulose spectral features, with a suggestion of the spectral features of humic acid. The spectra for materials from big sagebrush are displayed in Fig. 6. All of these plant materials exhibit lignoeellulose spectra. The green leaf of this species is canescent (covered with grayish-white fine hairs), which accounts for the strong lignin and cellulose material features in the green leaf. The senesced leaf was collected (dead) from a living plant canopy. At dry times of the year large numbers of leaves senesce, but may remain in the plant canopy for considerable periods of time before they drop. Spectra for plant materials from pinyon pine are presented in Fig. 7. The green needle exhibits low reflectance and no TIR spectral features. The pine sap is composed of terpenes and also has low reflectance and no TIR spectral features. The brown needle was collected from a living plant canopy. Its spectrum exhibits a lignocellulose reflectance dip at 4.7/zm, but otherwise the brown needle spectrum is more similar to that of tannic acid. The decayed black needle and the gray bark both display some lignoeellulose spectral features, but additional compounds also appear to be exhibiting influences on their spectra. The spectra for the brown cone, brown wood, and gray wood are more typical of relatively pure mixtures of lignin and holocellulose.
C D. ELVIDGE
The spectra for materials from giant wildrye are shown in Fig. 8. The green stem exhibits a minor reflectance peak at 9.3/~m. This reflectance peak is also present in the green leaf spectrum, though an additional reflectance peak at 9.59/zm is superimposed upon it. The 9.3 /.tin reflectance peak is most prominent in the brown leaf, where it superimposes the broad rise in reflectance from 10.2 to 13.4 /zm associated with holocellulose. The brown leaf also exhibits a distinct reflectance plateau from 4.25 to 5.35 /~m, similar to tannic acid. With further exposure to the elements, the brown leaves turn yellow and finally gray. Both the yellow and gray leaves display well-developed lignocellulose spectra, though the reflectance peak at 9.3 /zm persists. The reflectance peak at 9.3/zm may be due to the presence of silica. Certain grasses are known to accumulate silica in their leaf and stem tissues. The yellow stem and gray stem both have lignoeellulose spectra. The spectra of Mornaon tea plant materials are presented in Fig. 9. The leaves of Mormon tea are reduced to small scales, and photosynthesis is carried out by the stems. The green stems show low reflectance, with only minute reflectance features. Dry yellow stems collected from a living plant canopy exhibit a minor rise in reflectance between 4.25 and 5.45/lm. This rise in reflectance becomes pronounced in the spectrum of the gray stem. The brown wood, gray wood, and gray bark all exhibit well-developed lignocellulose spectra. The spectra measured for white peppermint materials are displayed in Fig. 10. The green leaf exhibits a series of minor reflectance features. The brown leaf and gray leaf spectra show some
279
I'I-IERMAL INFRARED REFLECTANCE OF DRY PLANTS
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FIGURE 7. Reflectance spectra.of plant materials from pinyon pine. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0/zm is provided for each spectrum.
(;. D. ELVIDGE
280
I I I GIANT WlLDRYE (Elymu8 condensatus)
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l 10.0
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WAVELENGTH
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-
I 18.0
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(pro)
FIGURE 8. Reflectance spectra of plant materials [rom giant wildrye. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0 #m is provided for each spectrum.
['HERMAL INFRARED REFLECTANCE OF DRY PLANTS
I MORMON
(Ephedra
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TEA
nevadensis)
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_
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8.0
10.0
12.0
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16.0
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WAVELENGTH (pro) FIGURE 9. Reflectance spectra of plant materials from Mormon tea. The spectra have been displaced vertically to avoid overlap. The percent reflectance at 10.0/~m is provided [or each spectrum.
182
C. D. ELVIDGE
I I I WHITE PEPPERMINT (Eucalyptus pulchella)
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20.0
WAVELENGTH (pro) FIGURE 10. Reflectance spectra of plant materials from white peppermint. The spectra have been displaced vertically to avoid overlap. The percent reflectance at I0.0/lm is provided for each spectrum.
THERMAL INFRARED REFLECTANCE OF DRY PLANTS
lignoeellulose features, but appear to be modified by some additional compounds. The seed capsule spectrum exhibits a prominent reflectance plateau from 4.25 to 5.45/~m and intense absorption from 5.7 to 20.0 pm. The seed capsule had a distinct odor suggestive of terpenes, which may account in part for the observed spectrum. The brown wood, tan bark, and brown bark all display welldeveloped lignoceUulose spectra. The tan bark spectrum is unusual in that the rise in reflectance between 10.0 and 15.0/~m exceeds the reflectance rise in the 4.0-5.5 /xm region. Discussion
The spectra of dry plant materials reported here fill in a major gap in the remote sensing knowledge base regarding the TIR reflectance characteristics of vegetation. Spectral differences can be observed between species and between diverse plant materials from a single species. Spectral changes associated with plant decay have also been observed. Many of the reflectance spectra of dry plant materials appear to be mixtures of holocellulose and lignin reflectance features. The exact balance of lignin and holocellulose features varies from sample to sample, but appear repeatedly. Additional compounds may influence or even dominate the spectra of certain plant materials. These results indicate that there is a basic lignocellulose spectrum for dry plant materials, which can be modified by the presence of compounds such as tannins or terpenes. Lignin, cellulose, and hemicellulose occur as intimate mixtures, making up 80-98% of the dry weight of most terrestrial plant materials (Crawford, 1981). Therefore, it is not sur-
283
prising that these compounds should be so evident in the spectra of dry plant materials. The diffuse reflectance spectra (acquired with the mirror carriage rotated 15 ° ) of the compounds and dry plant materials were not reported here. The spectral features observed in the diffuse plus specular reflectance measurements were invariably repeated in the diffuse spectra. The primary constituents of dry plant materials are evidently diffuse reflectors. This is in contrast to the observation by Salisbury (1986) and Salisbury and Milton (1986; 1988) of purely specular reflectance for green leaves in 8-14 /~m region. The apparent magnitude of spectral features depends upon the standard against which they are measured. The diffuse reflecting gold-coated sandpaper has an average reflectance of 44% relative to aluminum mirror. If the samples are referenced to the gold-coated sandpaper there is a 2.27 times increase in spectral contrast. The directional hemispheric reflectance (diffuse and specular) of a sample can be used to predict emittance using Kirchoff's law: emissivity = 1 reflectance. The measurement of directional hemispheric reflectance requires an integrating sphere, an accessory which was not available. It must be emphasized that Kirchoff's law does not apply to biconical reflectance. Biconical reflectance measurements should not be used to predict the magnitude of emission features. However, biconical reflectance measurements made with the same instrument show a strong correspondence to field emission measurements made of the same materials (Bartholomew et al., 1988). In addition, Salisbury and Milton (1988) and
284
C, D. ELVIDGE
Salisbury (1986) found the same spectral cause the three-dimensional structure of features in both specular and directional plant canopies tend to act as blackbody hemispheric reflectance measurements of cavities due to internal scattering. green leaves from cherry, oak, maple, and beech trees. These results indicate the The author gratefully acknowledges general utility of specular plus diffuse Dr. Anne Kahle for providing access to biconical reflectance measurements for the FTIR instrument plus computing and indicating the location of TIR emission graphics facilities at the Jet Propulsion features. Laboratory. The manuscript benefitted The highest reflectance contrast for dry from reviews by Mary Jane Bartholomew, plant materials occurs in the 3-5 gm Frank Palluconi, and Jack Salisbury. atmospheric window. Dry plant materials exhibit intense absorption in the 3 /~m region and have their highest reflectance References in the 4.0-5.5 /~m range. Green leaves Aspinall, G. O. (1980), Chemistry of cell wall have low reflectance in this range due to polysaccharides, in The Biochemistry of intense leaf water absorption. At first Plants, A Comprehensive Treatise, Volume glance, multispectral data in the 3 - 5 gm 3, Carbohydrates': Structure and Function region would appear to be a useful range (J. Preiss, Ed.), Academic, New York, pp. for measuring vegetation characteristics. 473 -500. However, the 3 - 5 /~m window falls on Bartholomew, M. J., Kahle, A. B., and Hoover, the long wavelength edge of the solar G. (1988), Infrared spectroscopy (2.3 to 20 radiation curve and the short wavelength micrometers) for the geologic interpretaedge of the terrestrial emittance curve, tion of remotely sensed multispectral thermal infrared data, Int. 1. Remote Sens., which complicates the use of this atmoforthcoming. spheric window. Night time measurements may be required to make use of Blackwell, J., and Marchessault, R. H. (1971), the 3 - 5 / z m region using passive sensors. Investigations of the structure of cellulose and its derivatives, High Polym. 5:1-37. The reflectance features reported here are relatively small in magnitude (0-6%). Chua, M. G. S., and Wayman, M. (1979), The reflectance difference between plant Characterization of autohydrolysis aspen materials are generally even smaller. The (P. tremuloides) lignins. Part 3. Infrared and ultraviolet studies of extracted magnitude of these spectral features imautohydrolysis lignin, Can. 1. Chem. plies that TIR sensors on aircraft or 57:2603-2611. spacecraft will have to have high signal to noise ratios to distinguish spectral differ- Colvin, J. R. (1980), Biosynthesis of cellulose, in The Biochemistry of Plants, A Compreences in plant canopies. hensive Treatise, Volume 3, Carbohydrates: There remains a continuing need for Structure and Function (J. Preiss, Ed.), high spectral resolution TIR measureAcademic, New York, pp. 543-570. ments (both active and passive) of whole plant canopies. It is likely that whole Crawford, R. L. (1981), Lignin Biodegradation and Transformation, Wiley-Intersciplant canopy measurements would find ence, New York, pp. 1-5. lower reflectance (higher emittance) than the laboratory spectra of plant materials Gates, D. M. (1980), Biophysical Ecology, Springer-Verlag, New York, pp. 235-238. reported here and elsewhere. This is be-
THERMALINFRAREDREFLECTANCEOF DRYPLANTS
285
Nash, D. B. (1986), Mid-irdrared reflectance spectra (2.3-22 /tm) of sulfur, gold, KBr, MgO and halon, Atrpl. Opt. 25:2427-2433. Reginato, R. J., Jackson, R. D., and Pinter, P. J. (1985), Evapotranspiration calculated Haslam, E. (1981), Vegetable tannins, in The from remote multispectral and ground staBiochemistry o f Plants, A Comprehensive tion meteorological data, Remote Sens. EnTreatise, Volume 7, Secondary Plant Prodviron. 18:75-89. ucts (E. E. Conn, Ed.), Academic, New York, pp. 527-556. Rodriguez, N. F., and Suty, L. (1983), Studies on bagasse and sugar-can lignins. II. InHatfield, J. L. (1983), Remote sensing estimafrared absorption spectra, Pap. Celul. tors of potential and actual crop yield, Re38:14-22. mote Sens. Environ. 13:301-311. Hergert, H. L. (1971), Infrared spectra, in Salisbury, J. W. (1986), Preliminary measurements of leaf spectral reflectance in the Lignins: Occurrence, Formation, Structure 8-14 /~m region, Int. ]. Remote Sens. and Reactions (K. V. Sarkanen and C. H. 7:1879-1886. Ludwig, Eds.), Wiley-Interscience, New York, pp. 267-297. Salisbury, J. W., and Milton, N. M. (1986), Preliminary measurements of spectral sigKahle, A. B., and Rowan, L. C. (1980), Evalunatures of tropical and temperate plants in ation of multispeetral middle infrared airthe thermal infrared, in Proceedings of the craft images for lithologic mapping in the Fifth Thematic Conference on Remote East Tintic Mountains, Utah, Geology Sensing for Exploration Geology, Environ8:234-239. mental Research Institute of Michigan, Ann Kneizys, F. X., Settle, E. P., Gallery, W. O., Arbor, MI, Vol. 1, pp. 131-143. Chetwynd, J. H., Jr., Abreu, L. W., Selby, Salisbury, J. w., and Milton, N. M. (1988), J. E. A., Clough, S. A., and Fenn, R. W. Thermal infrared (2.5 to 13.5 /~m) direc(1983), Atmospheric transmission/raditional hemispherical reflectance of leaves, ance: Computer Code LOWTRAN-6, Reforthcoming. port AFGL-TR-83-0187, Air Force GeoSalisbury, J. w., walter, L. S., and Vergo, N. physics Laboratory, Bedford, MA. (1987), Mid-Infiared (2.1-25 ttm) Spectra Liang, C. Y. (1972), Infrared spectroscopy of Minerals, 1st ed., U.S. Geological Survey and physical properties of cellulose, in InOpen-File Report 87-263. strumental Analysis of Cotton Cellulose and Modified Cotton Cellulose (R. T. O'Con- Wong, C. L., and Blevin, W. R. (1967), Infrared reflectance of plant leaves, Aust. ]. nor, Ed.), Marcel Dekker, New York, pp. Biol. Sci. 20:501-508. 59-91.
Gates, D. M., and Tantrapom, W. (1952), The reflectivity of deciduous trees and herbaceous plants in the infrared to 25 microns, Science 115:613-616.
Lyon, R. J. P. (1963), Analysis of rocks and minerals by reflected infrared radiation, Econ. Geol. 58:274-284.
Received6 October1987;revised261uly 1988.