However, in mountainous areas separation of vegetation types on radar imagery
(Gelnett, Dellwig, Bare, 1978) is less effective than separation on LANDSAT
imagery as shown in the minimal contrast between the deciduous woodland and
savanna in hills south of Fazao, Togo (Figure 4a, b).*
Particularly encouraging in the mapping of vegetation on radar is the
emergence of characteristic tone-texture signatures for several genera of
tropical trees; the earliest recognized were those of the mangrove in Darien
Province, Panama (MacDonald, 1969). The bright speckled return, apparently
the result of multiple reflections from the dense interlocking network of
moist vegetation as observed on K-band (.87 cm) imagery (Figure 5) is repro-
duced in brightness on X-band (3.0 cm) imagery (Figure 6) of mangroves in
Cameroon. It is hoped that as additional images are acquired in other areas
of similar environment, characteristic tone-texture radar signatures for
mangroves can be documented and the cause defined.
To date, the most unique return to be recorded from vegetation is from
a banana plantation in Guatemala (Figure 7a, b). The overall configuration
of the banana tree suggests that the high return results from multiple reflec-
tion of the energy from the large flat leaves in much the same manner that
energy would be returned from a corner reflector. Although in the field the
leaves tend to be shredded as a result of wind action, shredding within the
plantation should not be as extensive as it is along the outer limits of the
plantation. Nonetheless, even when shredded, sufficiently large flat areas
remain intact to provide the necessary reflective surfaces. A small area of
similar return has been observed on the image from Cameroon but has yet to
be field-checked.
To a considerable degree vegetation signatures will be season dependent
but will also be influenced by other factors. Dense stands of young oil palms
(Elaeis guiniensis) in Togo and Nigeria generate a dark smooth return (Figure
8a) as opposed to the bright speckled return (Figure 8b) from the coconut
trees (Cocos nucifera). Although superficially there appears to be little
difference in the configuration of the canopy of the two species (Figure 9a, b),
contrasts in leaf posture exist which would affect the nature of the return.
However, the contrast between the plantation configuration of the oil palms
and the more natural environment of the coconut trees should also be considered.
Until the influence of such a contrast can be understood, such signatures
cannot be utilized for definition of these three types in other environments.
Other factors may also influence the return from vegetation. In a large
rubber plantation in Cameroon (Figure 6) some tonal contrasts can be observed
along field boundaries within the plantation. Such contrasts may be related
to: (1) a variation in growth stage which is suggested by shadowing along
field boundaries due to differences in tree height, (2) differences in species,
or (3) differences in planting pattern. Contrasts in return when looking at
row crops from different directions have been noted (Batlivala and Ulaby, 1976;
Morain and Coiner, 1976) and might be expected from larger vegetation types
although indications are that this would not be the case at 3 cm (X-Band)
wavelength.
*In illustrations which show comparisons between radar and only one band of
LANDSAT imagery, the band of LANDSAT imagery which best displays the terrain
or vegetation characteristic illustrated has been used.
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