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Schwaller et al. (1981) studied the effects of heavy metal stress on the leaf 
reflectance properties of sugar maple seedlings. The seedlings were collected 
in an anomalous copper zone in Michigan's Keweenaw Peninsula and grown in the 
lab for about 150 days during which time they were treated with varying 
amounts of Cu and Mn. Across the whole visible and near IR spectrum (475- 1650nm) 
Schwaller et al. observed increased reflectance for the Cu treated leaves ; but 
only at 660 and 700 nm do the Mn treated plants show increased reflectance. 
They did not observe any decrease in reflectance at any wavelength for any of 
the treated plants. Schwaller et al. note that near infrared leaf reflectance 
(1300-2500 nm) is controlled in large part by the amount of liquid water in 
the leaf (Tucker, 1980) ; and relate differences between their work and that 
of Horler et al. (1980 b) in the near infrared region to lab conditions under 
which the plants were grown. This is a very significant finding for practical 
exploration work. Over the large area of an exploration program, it is 
reasonable to expect that even a farily uniformly distributed species may be 
growing under very different soil.moisture conditions. If this results in 
varying amounts of water in the leaf or needle structure, then the near infrared 
spectrum may be responding to these conditions, not heavy metal stress. 
  
  
  
The clear indication of the laboratory and greenhouse experiments is that 
heavy metal stressed plants show increased reflected radiance in the visible 
portion of the spectrum. This results from chlorosis of the vegetation which 
is induced by a deficiency of chlorophyll pigment. (Elvidge and Lyon, 1982). 
The results in the near infrared region are variable ; and since they may be 
only secondarily related to heavy metal stress, they are less reliable. 
GROUND BASED FIELD STUDIES 
  
A number of reflectance measurements have been undertaken on natural vegetation 
growing in the field on various mineral deposits. Notable among the earlier 
studies are those of Yost and Wenderoth (1971), Howard et al. (1971), and Press 
(1974). These investigators studied red spruce, fir balsam, ponderosa pine, and 
cork oad growing in various kinds of .ore deposits including porphyry Cu-Mo, 
vein Cu, and vein Pb-Zn ; and they generally found higher reflectance values 
in the visible part of the spectrum for the trees growing on mineralized 
ground. The results in the near infrared part of the spectrum are variable with 
some workers reporting increased reflectance and others reporting decreased 
reflectance. 1 
  
Horler et al. (1980 b) undertook field studies of oak vegetation. at a Cu-As mine 
and found a positive correlation of Cu and As in the soil with reflectance at 
660 nm. They also noted that there was no correlation between leaf reflectance 
and Cu content of the leaves or between soil and leaf Cu content. Horler et al. 
speculate that the reflectance effects of heavy metal stress are included 
by interactions between Cu and the tree roots not Cu.in the leayes. A similar 
lack of correlation between the reflectance properties of lodgepole pine 
needles and metal content of the vegetation and soil was obseryed by this 
author in an experiment at Heddleston, Montana (Birnie and Hutton, 1976). 
  
While Horler et a1.(1980 b) feel that the change in reflectance properties of 
the leaves correlates best with total chlorophyll content, Howard et al. (1971) 
suggest that the increase in reflectance from mineralized vegetation is a. 
function of vegetation density (Ponderosa pine), not changes within the needles. 
This suggestion is based on the field measurements involving whole branches 
which showed increased responses over the lab measurements of the needles alone. 
This indicates one of the significant changes encountered when reflectance 
measurements are moved from the lab to the field. Depending on the area over 
which reflectance measurements are integrated, the reflectance properties of