Analysing deforestation by remote sensing coupled with structural equation models: Example of the cloud forest of mount Oku (Cameroon) [Analyse de la dynamique de déforestation par télé détection couplée aux modèles d'équations structurales: Exemple de la forêt néphéliphile du mont Oku (Cameroun)]

摘要

Habitat destruction and land use change are major causes of biodiversity erosion on Earth, especially in tropical regions. Monitoring studies or even surveys of species and community diversity in changing landscapes or disturbed areas require an assessment of the land cover changes over time. Here, we present a method combining remote sensing and structural equation models (SEM) to describe and analyse landscape dynamics. We focus on the mount Oku as an example, which is a mountain area located in north-west Cameroon. This site hosts the largest remaining tract of Central African cloud forest, a biodiversity hotspot threatened by contemporary land-use changes. Our aim is to characterize land cover changes over the last three decades and to quantify and elucidate the causes of deforestation and forest fragmentation. For this purpose, we integrate several Landsat satellite images taken between 1978 and 2007 in a Geographic Information System (GIS), and compare changes in land-use types. We assess forest fragmentation over this period by comparing the number, the area and the perimeter of forest fragments, and derive a forest fragmentation index. Finally, we quantify the respective effects of natural (altitude, slope) and human (human density, distance to villages) factors on deforestation using SEM. We evidence two periods in the dynamics of the cloud forest cover: a period of intense deforestation (1978-2001), followed by a period of re-afforestation (2001-2007). The forest cover lost 62.1% of its area (12,060 ha) between 1978 and 2001, corresponding to an averaged deforestation rate of 579 ha.yr~(-1). Deforested areas were mainly converted into crop lands (+75.6% between 1978 and 1988), under the pressure of an increasing human density, which was multiplied by 20 between 1921 and 2005 and doubled during the last 18 years. After 2001, the forest cover stabilized with the appearance of numerous small fragments of secondary forest, reflecting the success of the local biodiversity protection programs. Despite this recent progression of the forest cover, the proportion of ancient forest has continuously decreased from 1978 to 2007, indicating that deforestation is still ongoing. In 2007, the forest cover was a mosaic composed of 66% of recent secondary forests (i.e. forests appeared after 1978) and only 34% of ancient forests (i.e. present before 1978). Between 1978 and 2007, the number of forest fragments increased from 2627 to 5183, their average area decreased from 7.4 to 1.8 ha, and perimeter from 912 to 446 m, and the forest fragmentation index increased by 285.7%. Deforestation started from lower altitudes (<2300 m, before 1988) and progressed towards higher altitudes (2100-2900 m) between 1988 and 2001. SEM showed that altitude and slope had a significant negative effect on human density, explaining why the deforestation has been low on steep slopes and/or at higher altitudes. Our study reveals that the last fragments of the primary cloud forest of mount Oku could be definitively lost in the next decades. As tropical primary forests are irreplaceable ecosystems that host numerous endemic species, we recommend (1) to urgently protect the last remnants of ancient forest for their biological value; clear-cuts, grazing in forest interior and fires should be excluded from these areas, and (2) to extend secondary forests around ancient forest fragments so that they can act as a protective buffer and be managed as community forests and used for hunting and logging activities.