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Remote sensing for resources development and environmental management
Damen, M. C. J.

Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
Relationship between soil and leaf metal content and Landsat MSS
and TM acquired canopy reflectance data
Institute for Image Processing and Computer Graphics, Research Centre Joanneum, Graz, Austria
ABSTRACT: The presence of high concentrations of copper, lead, and zinc in the soils under
lying a spruce tree stand is not mirrored in the metal content of the spruce needles, except
for zinc. A much stronger correlation exists between Landsat canopy reflectance data and soil
metal content than with spruce needle metal content. This implies that the toxic effects of
the heavy metals on the spruce trees are centered in the root systems rather than in the fo
liar parts of the trees.
Plants respond to the uptake of toxic
amounts of heavy metals into their system by
undergoing physiological and morphological
changes that are detectable using spaceborne
sensor systems. These metal-induced changes
are commonly expemplified by a reduction in
leaf pigment and water content and a decrease
in plant foliar biomass or density, which,
in turn, result in changes in the spectral
response of the stressed plants. Although the
relationship between soil and leaf metal con
tent and canopy reflectance can conceptually
appear straightforward, it is often much more
complex and less well-defined than it ini
tially appears. Such is the case with the
study described in this paper, which set out
to establish the connection between soil geo
chemistry, leaf biogeochemistry, and changes
in Landsat MSS and TM reflectance data from
a coniferous tree stand growing in soils con
taining high concentrations of copper, lead,
and zinc.
Most plant species restrict their metal
uptake by exclusion or internal regulatory
mechanisms operating at the plant roots-soil
interface and within the plant's metabolic
systems, provided that the available metal
concentration in the soil does not exceed a
certain critical threshold, which varies for
each species and set of environmental site
conditions. Above this threshold concentra
tion, the regulatory mechanisms breakdown
and are unable to prevent an unstricted up
take of metals by the plant. The uptake pro
cess operating in a plant involves the ab
sorption of elements by the plant root sys
tem, their translocation through the aerial
parts of the plant, and their subsequent de
position in the plant's leaves and woody
parts. Heavy metals, therefore, can accumu
late in all parts of the plant, although a
large fraction of the absorbed metals is pre
vented from reaching the aerial parts of a
plant by being precipitated in the tissue
cells of the root system. The roots of plants
generally contain greater amounts of metals
than the aerial parts, particularly at high
soil metal concentrations (Hawkes and Webb,
1962; Antonovics et al., 1971). Cannon (1960)
reported ratio values of heavy metals in
plant roots to aerial parts as high as 200:1.
Plants take up from the soil only those
elements that are freely and readily avail
able in water-soluble form. Element uptake
occurs mainly in response to a plant's nutri
tional requirements, and most plants attempt
to control the absorption and level of con
centration of the various elements within
their system, regardless of the amounts in
the soil. Plants exhibit a strong biological
absorption for zinc, moderate for copper, and
weak for lead (Perel'man, 1967), which mirror
the moderate, moderately severe, and severe
levels of toxicity, respectively, of these
metals to plants (Hawkes and Webb, 1962;
Bowen, 1966; Brooks, 1972).
The uptake pattern of copper and lead
differs markedly from that of zinc. The ab
sorption of zinc into a plant's system is
normally unrestrictive, which indicates that
an internal tolerance mechanism, rather than
an exclusion mechanism, operates in plants for
zinc. For copper and lead, their uptake by
plants is small compared to zinc, and is con
trolled by an exclusion mechanism situated in
the plant root system. Both copper and lead,
and even zinc, are largely accumulated in the
plant root tissues, which generally contain
greater concentrations of the metals than the
leaves and twigs.
The Landsat MSS and TM sensor systems
collect reflectance data in the visible to
shortwave infrared spectral region, which
can be used to assess the state of health of
vegetation. Although the four sensor band-
widths of the MSS system encompass a number
of plant-energy relationships useful for de-
terming the condition of a plant canopy, the
presence of more than one important plant-
radiation association within these bands de-
minishes their ability to detect changes in
the spectral properties of plants related to
their health.