# Detection of Forest Tree Losses in Cote d'Ivoire Using Drone Aerial Images
## 1 Introduction  
Tropical forests are the focus of important global issues related to the preservation of biodiversity, climate change, and sustainable development [1]. In West Africa, the conversion of forest areas into farmland over the 2000-2010 period has been estimated at 19% by the Food and Agriculture Organization (FAO) and represents a loss of about 870,000 ha per year [2]. In Cote d'Ivoire, more than 40% of the forests disappeared within 25 years (between 1990 and 2015), with forest cover decreasing from 7.8 million ha in 1990 to 5.1 million ha in 2000 and then to 3.4 million ha in 2015, representing about 11% of the national territory [3]. To face this challenge, Cote d'Ivoire has been involved in the international mechanism for reducing emissions from deforestation and forest degradation, sustainable management of natural resources, enhancement of forest carbon stocks, and conservation of forests (REDD+) since June 2011. A new strategy for forest preservation,rehabilitation, and expansion was adopted at the beginning of 2019, a few months before the adoption of a new forestry policy in July of the same year [4]. Essentially focused on classified forests, this policy first aims to reduce deforestation in classified forests by upgrading the remaining well-preserved classified forests to protected area status. Second, it aims to \"regularize\" the situation of highly degraded classified forests by introducing a new status for classified agroforests [5], which is still being widely deliberated at the time of writing. This policy is to be based in particular on the deployment of an operational land and land clearing monitoring system. However, the monitoring methods are not adapted to the Ivorian context, which is marked by shaded cultivation, high cloud cover, and new clearing methods that are difficult to detect. Monitoring studies of tropical surface conditions have tremendously diversified in recent years with the development of new sensors and the establishment of new methods [6; 7; 8; 9; 10; 11; 12]. In Cote d'Ivoire, monitoring land use and land cover dynamics by remote sensing faces many challenges. Land clearing starts in a very discrete way in small areas and progressively extends to the entire classified forest or protected area [13; 14]. In this context, the traditional use of Landsat-type satellite images [14] is justified by the high availability of image archives (over 30 years) but may be limited for annual monitoring due to their low acquisition frequency and spatial resolution (30 m). The constellation of Sentinel-2 satellites, in operation since 2015 (Sentinel-2A) and 2017 (Sentinel-2B), provides free images at 10, 20, and 60 m resolutions. This represents a strong interest for West African institutions that still do not have sufficient financial means for the acquisition of images. The use of these sensors makes it possible to more clearly distinguish land use and land cover categories and makes images without clouds attainable. Indeed, the intertropical zone, particularly Cote d'Ivoire, is subject to heavy cloud cover that limits the application of optical remote sensing [11]. Following the lead of [15], several studies have shown the potential of Sentinel-2 sensors to significantly contribute to land monitoring. In Cote d'Ivoire, these images were used to assess the dynamics of land cover and land use change on a regional scale covering the current study area [12]. The results revealed an annual deforestation rate of 0.35% in the Bossematie classified forest. The present study seeks to assess intra-annual tree loss in this forest. These two studies are therefore complementary from the point of view of exploiting the potential use of Sentinel-2 images and aerial images acquired by drone for forest management. The reduced endurance of UAVs is a limiting factor for forestry uses. The use of drones is thus a tool for analysis at the local scale only [16].