Aperçu sur la fragmentation de la forêt naturelle dans la Réserve Spéciale d’Ambohitantely et ses alentours entre 1949 et 2017, Hautes Terres Centrales
Keywords:
Perte de forêt, isolation, dynamique forestière, télédétection, photo-interprétationAbstract
Cette étude vise à caractériser la dynamique de la couverture forestière et la fragmentation de la forêt naturelle de la Réserve Spéciale d’Ambohitantely et les zones périphériques dans un rayon de 10 km de la limite de l’aire protégée entre 1949 et 2017. Au total, cinq images satellitaires pour les années 1989, 1995, 2002, 2010 et 2017, et 59 clichés de photographies aériennes prises en 1949 ont été utilisées. La télédétection et le système d’information géographique ont été utilisés pour la cartographie de l’occupation du sol pour les six périodes d’études, ainsi que pour l’analyse de la dynamique de la couverture forestière et l’estimation de la perte de surface forestière. Six métriques disponibles sur le logiciel FRAGSTATS ont été sélectionnées pour l’analyse de la fragmentation à l’échelle du paysage à savoir, le nombre de parcelles (NP), la densité de parcelles (PD), la variabilité de la taille des parcelles (AREA_SD), l’indice de la dimension fractale (FRAC_MN), l’indice de contiguïté (CONTIG_MN) et l’indice d’agrégation (AI). Après une classification supervisée, les classes d’occupation du sol ont été reclassées en forêt ou non-forêt. La dynamique de la couverture forestière dans la zone étudiée a montré qu’une vaste zone forestière a été convertie en zone non forestière. L’estimation de la perte de forêt indique que le taux annuel dans la réserve varie, et la plus importante estimée à 586,4 ha soit 4,05% par an a été enregistrée entre 1995 et 2002, et la plus faible est de 473,4 ha soit 0,41% par an, entre 1949 et 1989. Les résultats ont montré la diminution du nombre de fragments ainsi que la densité des fragments depuis 1989 à 2017, ce qui indique la disparition de fragments forestiers. En parallèle, la réduction de l’indice de la dimension fractale et de la variabilité de la taille des parcelles révèlent la simplification de la forme des fragments et la faible diversification de la superficie des différents fragments. L’augmentation de l’indice d’agrégation contre la diminution de l’indice de contiguïté confirme l’isolement des fragments.
Abstract
This study aims to characterize the dynamics of forest cover and fragmentation of the natural forest of the Ambohitantely Special Reserve between 1949 and 2017 and within a radius of 10 km of the boundary limit. Five different periods of satellite images were employed, specifically the years 1989, 1995, 2002, 2010, and 2017, as well as aerial photographs taken in 1949. Remote sensing and geographic information systems were used for land cover mapping for the six study periods, as well as for analyzing forest cover dynamics and estimating forest cover loss. Using the software FRAGSTATS, six different metrics were selected for the analysis of forest fragmentation at the landscape level: number of patches (NP), patch density (PD), patch size standard deviation (AREA_SD), mean patch fractal dimension (FRAC_MN), contiguity index (CONTIG_MN), and aggregation index (AI). Following a supervised classification, land cover classes were reclassified as forest or non-forest. The dynamics of forest cover at the site and over the study period indicated that considerable zones of forest were transformed to non-forested areas. The estimate of forest loss indicates that the annual rate in the reserve varies, and the largest estimated at 586.4 ha or 4.05% per year was recorded between 1995 and 2002, and the lowest is 473.4 ha or 0.41% between 1949 and 1989. The results indicate a decrease in the number of fragments as well as the density of fragments from 1989 to 2017 associated with the disappearance of forest. In parallel, the reduction of the mean patch fractal dimension and variability of the patch size denotes the simplification of the fragments’ shapes and the slight diversification of the areas of the different fragments. An increase in the aggregation index as compared to a decrease in the contiguity index confirms the isolation of the fragments.
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