Multitemporal monitoring of the forest cover in Côte d'Ivoire from the 1960s to the 2000s, using Landsat satellite images

摘要

The phenomenon of deforestation is a reality in Côte d'Ivoire, which dates back to the colonial period [1]. This study aims to determine the spatial and temporal evolution of dense rainforests using remote sensing data. Many previous studies have attempted to determine the area of forest cover in Côte d'Ivoire ([2], [3], [4], [5], [6] and [7]). Most of these studies were conducted from either aerial photographs or satellite images of low spatial resolutions such as NOAA / AVHRR [8]. However, through these studies, methods of estimates of the area of forest cover generally vary from one author to another, thus causing an inconsistency in the results [9]. Remote sensing allows a better assessment of the scope and scale of the problem of deforestation. Using multi-temporal data, one can better detect and analyze changes in the forest cover. By comparing images of previous years with recent scenes, it becomes possible to effectively measure the differences in the extent of deforestation and loss of rainforests in Côte d'Ivoire ([10] and [11]). In a first phase, the country's dense forest cover of the 1960s was determined from the digitization of existing vegetation map. In a second phase, the dense forest cover of the 1980s and 2000s were obtained by processing of Landsat TM and ETM+ satellite images. First the geometric correction of the satellite images was performed. Then, series of supervised classifications based on the maximum likelihood method were done to generate maps of land cover, with overall accuracies of 90%. From these land cover maps, the class of dense forests was extracted and converted into a map layer in vector format. By crossing dense forests covers of the 1980s and 2000 with those of the 1960s, the specific characterizations of those forests were derived (Figure 1, 2, 3). The results show a dense humid forest cover of about 8.4 million ha in 1960, with the presence of large blocks. The 1980s and 2000s have dense forest cover of 2.6 million ha and 1.35 million ha, respectively (Tables 1, 2, 3). These latter periods are characterized by high loss and fragmentation of rainforests in Côte d'Ivoire. Thus, since its independence, the country has lost more than 80% of its forest cover. The results of this study show that clearing associated with industrial agriculture mainly cocoa, coffee and rubber farming as well as demographic pressure and abusive exploitation of valuable wood species for export are the main causes of deforestation in Cote d'Ivoire. This study advocates the restoration of forests through the implementation of an awareness policy and reforestation one hand, and an efficient monitoring and protection of national protected areas and parks.