WORKING GROUP 4 
OLSON 
229 
Fig. 3. Light reflectance at 600 millimicrons, 
available soil moisture in the upper 30 inches 
of soil, rainfall, and incoming solar radiation 
during 1961 for an Acer saccharinum plantation 
at Urbana, Illinois, U.S.A. 
Fig. 4. Light reflectance at 600 millimicrons, 
available soil moisture in the upper 30 inches 
of soil, rainfall, and incoming solar radiation 
during 1961 fora Platanus occidentalis planta 
tion at Urbana, Illinois, U.S.A. 
soil than the other plantations, and the low growth rate may be associated 
with soil factors. 
The selected curves in fig. 1 illustrate the nature and magnitude of the re 
flectance changes that occurred, but they do not illustrate the nature of the 
seasonal trends involved. Seasonal trends are most evident when reflectance is 
considered at a single wavelength. Graphs of this type are shown in figs. 3, 4, 5, 
and 6 for reflectance at 600 millimicrons. These four figures also include sea 
sonal records of incoming solar radiation, rainfall, and available soil moisture 
in the upper 30 inches of soil. Note that there is a strong relationship between 
available soil moisture and light reflectance at 600 millimicrons. This rela 
tionship is also evident at other wavelengths. 
The increasing reflectance shown by cottonwood during August and Septem 
ber appears to be a departure from the general trend. However, this increasing 
reflectance marks the beginning of the fall color change in this species. For 
all species, reflectance increases due to the fall color change seem to be much