WORKING GROUP 4 
OLSON 
231 
Fig. 7. Panchromatic/minus-blue aerial photographs of an area of mixed pine and broad 
leaved forest types in Illinois, U.S.A., illustrating superior tone contrast between forest types 
in May (left photo) compared to no tone contrast between forest types in early September 
(right photo). (U.S. Dept, of Agriculture photographs). 
light reflected from foliage of broadleaved trees. The variations in reflectance 
noted during different parts of the 1961 growing season were probably large 
enough to produce visible changes in photographic tone on panchromatic/ 
minus-blue photographs, as similar reflectance differences did produce tonal 
separations during 1960. Although reflectance differences associated with soil 
moisture were relatively large, they tend to occur in all species at approx 
imately the same time. For this reason moisture influenced changes in light 
reflectance from tree foliage cannot be depended on to produce aerial photo 
graphic tone differences between species, unless soil or root characteristics 
cause one species, or timber type, to experience moisture stress significantly 
sooner than others. 
During this study reflectance differences associated with the fall color change 
were greater than those associated with moisture conditions. Since different 
species develop reflectance differences associated with the fall color change 
at different dates during the growing season, the importance of such phenol- 
ogic differences between species [Sayn-Wittgenstein, 1961] is re-emphasized. 
Based on limited photographic trials with Kodak Super XX and Kodak 
Plus X films exposed through a minus-blue (Wratten No. 12) filter, the 
differences in light reflectance associated with the fall color change produce 
definite differences in photographic tone between species. Tone differences