## 1 Introduction  
The period 2015-2016 has been determined as an El Nino year [1]. A severe \"drought\" and an associated unprecedented outbreak of fires have been observed in the north of the Republic of Congo from January to April 2016. First observations report [2] that the fires took place in specific open forest formations, the Marantaceae forests. While the forests of the Congo Basin are second only to the Amazon Basin in extent, unlike the latter, these forests have, in general, been spared important deforestation due to large scale exploitation driven by commercial food production and farming [3; 4; 5]. However, selective logging activities, where specific species of trees are extracted from the forest, are widespread in most of the basin [6]. Of concern, to both the national forest service and to the local management, is the occurrence of fires in active logging concessions, threatening current commercial extraction of round-wood, future regeneration of stock and biodiversity. Fire is a very common event on the African continent, however, the Congo Basin rain forests are usually too humid to burn [7]. In southeast Asia and in the Amazon Basin, tropical rainforests fires are observed more frequently, as a consequence of current land-use practices [8]. Under normal conditions, fire remains rare in tropical rainforests and, consequently, most plants and animals are poorly adapted [9]. Pan-tropical forest fires are often more severe during El Nino events [8]. In 1997-1998, fires associated with an exceptional drought caused by El Nino devastated large areas of tropical rain forests in Indonesia [10]. [PERSON] et al., 2012 [11] observed a relationship between monthly rainfall deficit and fires in Borneo during El Nino events for the years 1980-2000. In Amazonia, [PERSON] et al., 1999 [12] highlight an increased vulnerability of the Brazilian Amazonian forests to fire during drought events caused by El Nino. In addition, the hypothesis of a positive feedback between recent logging activities and fire is made for Amazonian [13] and Indonesian forests [10]. In contrast, very little is known about increased vulnerability of Central African tropical forests to fire due to El Nino related droughs and/or logging activities. In the last decades, remote sensing has been widely used to monitor active fires and burnt areas at the global scale. Sensors like Advanced Very High Resolution Radiometer (AVHRR), VEGETATION (VGT), Moderate Resolution Imaging Spectroradiometer (MODIS) and MEdium Resolution Imaging Spectrometer (MERIS) with spatial resolutions between 300 m and 1 km are generally used [14; 15]. Their daily revisit cycle is useful to capture active fire signatures or burn scars, however, the relatively coarse spatial resolution causes underestimations in the case of small burnt area extent [16]. Comparisons between different burnt area products generally show high variation in results [16]. Coarse spatial resolution satellite data also present methodological challenges in terms of assigning the correct vegetation class to the burnt area, which is important either for estimating emissions or for evaluating the ecological impact of fires [17]. For tropical rainforests specifically, more knowledge is needed about which vegetation formation is burning in order to understand why the fire is occurring at specific locations and times. Fusion between coarser and medium to high spatial resolution (5-30 m) satellite data have been used e.g. by [PERSON] et al., [18] to overcome the problem of reduced temporal resolution of higher resolution satellite such as Landsat, ASTER or RapidEye. In tropical evergreen forests, an important challenge is the persistent cloud cover, requiring frequent acquisitions of optical data. Synthetic Aperture Radar (SAR) sensors can be used to overcome this problem because of their ability to penetrate clouds and haze [8; 19]. High spatial resolution SAR data have been used together with coarse optical data in a few studies [19; 20].