A Geobotanical Approach to the Tropical Rain Forest Environment of the Carajás Mineral Province (Amazon Region, Brazil), Based on Digital TM-Landsat and DEM Data
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IN PE- 9030 - PRE/ 1030 INT. J. REMOTE SENSING, 1994, VOL. 15, NO. 8, 1633-1648 A geobotanical approach to the tropical rain forest environment of the Carajás Mineral Province (Amazon Region, Brazil), based on digital TM-Landsat and DEM data W. R. PARADELLA National Institute for Space Research (INPE), 12201-970- São José dos Campos, S. Paulo, Brazil M. F. F. DA SILVA, N. DE A. ROSA Paraense Emílio Goeldi Museum, 66000-Belém, Pará, Brazil and C. A. KUSHIGBOR INTERA Technologies Ltd, Ottawa, Ontario, Canada, K1Z 8R9 ( Received 27 November 1992; in final form 15 July 1993) Abstract. Digital TM Landsat images integrated with elevation model variables were used to evaluate the potentiality of geobotany for lithological unit discrimi- nation in the tropical rain forest environment of the Carajás Mineral Province, Brazilian Amazon region. The data set was analysed through digital image processing techniques (enhancements and non-supervised classificatidn). The investigation has shown that the Up-Land rain forest vegetation in the area is mainly controlled by elevation and slope which refiect variations in the geology. Botanical verification has also indicated that the physiognomy (density and stratification), would be the most important vegetation attribute which infiuences the remote sensing responses. The research has provided useful information for the geological model of the area. Thus, by understanding the relationships among vegetation, terrain descriptors and geology, geobotanical remote sensing provides an additional tool for geological exploration in this kind of environment. 1. Introduction The extraction of geological information in rain forest environments based on optical remote sensing (RS) data is always problematic. Papers dealing with this theme are rare in the literature and are restricted either to spatial attribute analysis (Erb 1982) or to application related to specific site conditions where latentes cause changes in the biomass density which results in spectral contrasts (Taranik et al. 1978). The possibility of using vegetation spectral response to infer bedrock character- istics is an interesting alternative that must be investigated. However, the complexity of the environment and the assumed poor relationship between rock and vegetation, have been considered important factors that do not favour the spectral RS performance in such difficult terrain (Birnie 1982). 0143-1161/94 $10.00 () 1994 Taylor & Francis Ltd 1634 W. R. Paradella et al. The research described here was designed to address the kind of pertinent RS questions which are faced by geological exploration in tropical rain forest environ- ments, such as, whether it is possible to infer characteristics of the substratum through spectral responses detected from the rain forest vegetation canopies, or, which are the main variables involved in the process? The strategy pursued in this investigation takes into consideration the fact that one of the factors that controls the distribution of plant communities in the tropical rain forest environment is the topography (Ab' Sáber 1986). In addition, geology is a fundamental parameter that controls topography. Thus, the hypothetical link vegetation-rock should be sought through the analysis of the relationship between terrain attributes (elevation, slope, etc.) and vegetation parameters that control spectral response (flora, cover density, stratification, etc.). This issue could be properly clarified through a geobotanical model based on orbital RS and terrain descriptors obtained from Digital Elevation Model (DEM). 2. Concepts Geobotany deals with the relationship between vegetation and environmental factors. Since the mid-nineteenth century, it has been recognized that distinct plant associations exist on different geological substratum and this might be used to characterize the geology of the area concerned (Karpinsky 1841, in Brooks 1972). Thus, geobotany should not only be understood as a way of prospecting with the purpose of detecting induced mineral stress in vegetal species. Emphasis should also be put in terms of its usage as an alternative tool for geological mapping. For a heavily dense vegetated area, data collected from orbital optical sensors is related to a mixed assemblage of plants (i.e., to the plant communities). So, the influence of the structure and the composition of the communities in the response detected should be more emphasized than the spectral behaviour of individual leaves or isolated canopy species (Saraf and Cracknell 1989). More obvious spectral changes detected from plant communities in low spectral resolution data, (such as MSS and TM-Landsat), tend to be related to the physical property variations of soil/bedrock (moisture content/texture) rather than to the chemical variations (Milton and Mouat 1984). For dense vegetation cover environments, successful geobotanical applications based on orbital RS data have been restricted to the relatively homogeneous forest from the northern hemisphere (Green et al. 1985, Defeo et al. 1986, Bell et al. 1991, among others). Tropical rain forests however, present particular difficulties for geobotanical applications. First, the identification of species is not easy due to the scarcity of access for field observations, height of trees, their unsocial fashion distribution and distinct stratification. Secondly, due to nutrition and aeration reasons, the species are normally shallow-rooted, bear flowers and fruits at irregular intervals and shed them onto the ground where they are decomposed (Cole 1971, Jordan 1982, Pires and Prance 1985). Thus, species identification is sometimes almost impossible from ground levei. Finally, due to the high complexity of the environment, a greater number of species can be found in a few hectares of the Amazon rain forest as opposed to all the identified ones in the cold and temperate forests (Schubart 1986). At cold latitudes, a regional (background) geobotany approach has been developed to geological exploration (Bruce and Singhroy 1984, Bruce and Hornsby 1987). It is based on the hypothesis that the nature and the distribution of plant A geobotanical approach to lhe Amazon rain forest 1635 communities occur as a result of environmental conditions and therefore, variations in distribution from the established norm of an area may be indicative of changes in the geology. The concept has focused on community levei and has been tested through digital image processing of orbital RS data and elevation model variables (Hornsby et ai. 1988). Carajás, the most important Brazilian mineral province, is located on the easternmost border of the Amazon Region. The Province, with an area of 120 000 square kilometers is marked by mountainous terrains, deep chemical weathering which produces thick oxisols ('latosols) and few outcrops. Vegetation cover is typical of the Up-Land tropical rain forest communities with complex and multi- levei canopies and numerous species. Since this region can be considered an excellent test site, a long term remote sensing research programme has been implemented. The project encompasses aspects of remote sensing application such as multi-source data integration, visual methods of extraction of information, digital image processing and geobotanical model developments. This paper presents the final results of a five-year geobotanical investigation that was carried out by the authors in the Pojuca area, a metamorphic belt with various Cu-Zn deposits in the Carajás Province. 3. Study area A 230 square kilometre test area was selected for the investigation, centred on the Pojuca mine camp and located in the northern border of the Carajás Province (figure 1). The area has been receiving attention in regional exploratory programmes since 1974 when geochemical surveys indicated Cu-Zn anomalies in the Pojuca stream. The test-site is inserted in a wide regional sinistral shear zone (Itacaiimas shear belt) characterized by both ductile and brittle deformations aligned on a W-N-W trend (Araujo et ai. 1988). This zone is marked by tangential (low angle) strains that in Pojuca area are vertical due to Carajás transcurrent fault system. Costa and Hasui (1991) have studied regionally these mega-structures and proposed an origin for the metavolcanic and metasedimentary rocks from the area associated to transtension (tectonic basins), followed by granitoid intrusions and to transpression along the Carajás transcurrent system. The geology of the study area is incompletely mapped. Areas around the copper deposits have been mapped in detail (1 : 5000 and 1 : 10000), but much of the rest of the area has only been studied on a regional scale. The geologic maps shown in figures 2 and 3 were generalized from a detailed map published by the mining company Docegeo (Docegeo 1984) and from a recent regional mapping carried out by the Pará Federal University (Macambira et ai. 1990), respectively. According to these authors, the oldest Archean rocks in the area are known as the Xingu Complex, a sequence of tonalitic, granodioritic and granitic gneiss which occurs in the northeastern portion of the region and is related to rounded hilly low relief (altitudes from 150 up to 250 m). The Igarapé Pojuca Group is also Archean in age and occurs in tectonic discordance with surrounding Xingu rocks. It is a volcano-sedimentary sequence with a N7OW structural trending and characterized mainly by schists, banded iron formations and amphibolites. This Group is represented at the surface by a set of parallel ridges and valleys