Plant development and maturity (as opposed to growth or increase 
in size) cause many changes in canopy geometry, moisture content, and 
pigmentation of leaves which are also manifested in the reflectance | 
characteristics of canopies. Figure 3 shows the spectra of spring | 
wheat at several different maturity stages. A plot of spectral re- ; 
sponses through the season in selected wavelength bands is shown in 
Figure 4. The temporal pattern of changes in spectral response has : 
been demonstrated by the LACIE project to be an important feature for 
identifying wheat from Landsat MSS data. | 
Plant maturity affects canopy reflectance throughout the growing ; 
season; however, the variation early in the growing season caused by | 
different planting dates was considerably reduced by the time of plant 
heading. Percent soil cover, leaf area index, and plant water content 1 
had high linear correlations with reflectance in most wavelength bands 
and reflectance was sensitive to changes in these variables throughout 
the growing season. On the other hand, reflectance was only sensitive 
to changes in fresh and dry biomass early in the season. The 0.76-0.90 
um wavelength band has a high correlation with each crop canopy variable 
early in the growing season indicating a sensitivity to small amounts 
of biomass. As the canopy begins to ripen there was a significant de- 
crease in the capability to estimate the amount of vegetation present. 
Leamer et al.(1978) reported a similar decrease in the correlation of 
spectral response with canopy variables after heading for winter wheat. 
Correlation of Canopy Variables with Reflectance 
  
The linear correlation of the five different crop canopy variables 
with the thematic mapper and Landsat MSS bands are summarized in Table l. 
The relationships of four of these with spectral reflectance are illus- 
trated in Figures 5 and 6. Percent soil cover, leaf area index, and 
plant water have a high correlation with spectral reflectance in each 
of the six thematic mapper bands (Figures 5 and 6). The relationship 
of reflectance to fresh and dry biomass is non-linear, resulting in 
lower correlation values (Figure 6). 
The middle infrared band, 2.08-2.35 um, has the highest correlation 
of the six individual thematic mapper bands with fresh biomass, dry 
biomass, and plant water, and is highly correlated with leaf area index 
and percent soil cover. These results illustrate the importance of 
making measurements in several regions of the spectrum, espe- 
cially in the middle infrared wavelength region which is not measured 
by the present Landsat multispectral scanners. 
Correlations of the four Landsat MSS bands with the same five crop 
canopy variables are also shown in Table 1. The correlation of canopy 
variables with each Landsat MSS band is less than with the corresponding 
thematic mapper band. The lower correlation is attributed to the width 
and location of the bands with respect to the spectral characteristics 
of vegetation.