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Three decades of satellite observations provide unprecedented information about forest change trajectories in the humid tropics., Accurate characterization of tropical moist forest changes is needed to support conservation policies and to quantify their contribution to global carbon fluxes more effectively. We document, at pantropical scale, the extent and changes (degradation, deforestation, and recovery) of these forests over the past three decades. We estimate that 17% of tropical moist forests have disappeared since 1990 with a remaining area of 1071 million hectares in 2019, from which 10% are degraded. Our study underlines the importance of the degradation process in these ecosystems, in particular, as a precursor of deforestation, and in the recent increase in tropical moist forest disturbances (natural and anthropogenic degradation or deforestation). Without a reduction of the present disturbance rates, undisturbed forests will disappear entirely in large tropical humid regions by 2050. Our study suggests that reinforcing actions are needed to prevent the initial degradation that leads to forest clearance in 45% of the cases.

International audience; Woody vegetation dynamics in the Sahel have been debated since the great droughts of the 1970s–1980s. Here, we combined high-resolution satellite and aerial imagery, field inventory, and historical botanical records to study woody vegetation trends over the years 1965, 1980, 2008, and 2018 in the Ferlo, the Sahelian sylvo-pastoral zone of Senegal. While tree density has decreased from 1965 (14.8 trees ha−1) to 1980 (13.4 trees ha−1) and 2008 (11.9 trees ha−1), tree density has stabilized in 2018 (12.2 trees ha−1). The relatively moderate decrease in tree density over 50 years characterized by extensive human pressure and droughts, as well as the rather stable woody cover following the drought years after the 1980s, do not support narratives of widespread desertification in this region. However, we observed a shift in the composition of species. While the drought-resistant tree Balanites aegyptiaca showed a stable abundance, Acacia tortilis showed strong increases and other species like Sclerocarya birrea and Combretum glutinosum decreased. In addition, recent field surveys show that the ratio between shrubs and trees has increased towards more shrubs. The observed loss of species diversity combined with the increase of drought-resistant species is in line with current observations for savanna ecosystems in the context of an increased aridity.

International audience; To allow for the production of timber while preserving conservation values, forestry regulations in the Congo Basin have made Forest Management Plans (FMP) mandatory in logging concessions. This paper uses original high-resolution maps of forest-cover changes and official records on the activities of logging concessions to analyze the impact of FMP on deforestation in this region. We apply quasi-experimental and difference-indifference approaches to evaluate the change in deforestation in concessions managed under an approved FMP. We find that between 2000 and 2010, deforestation was 74% lower in concessions with an FMP compared to others. Building on a theory of change, further analyses revealed that this decrease in deforestation takes time to occur and is highest around communities located in and nearby logging concessions, and in areas close to previous deforestation. These findings suggest that FMP help avoid deforestation by allowing logging companies to rotate cycles of timber extraction, thereby avoiding the overexploitation of areas that were previously logged, and by the better regulation of access to concessions by closing former logging roads to limit illegal activities such as shifting agriculture, hunting and the illegal harvest of timber or fuel-wood.

International audience; In a context of fast land changes due to human activity, the old 'Malthus vs. Boserup' debate about human pressure on natural resources is more than ever a prevalent subject. We illustrate this debate with an example based on the Tuy Province in West Burkina Faso, which has known an important development these last fifteen years, as mainly observable in the major regression of its natural vegetation. The objective of this article is to question the possible future scenarios of this region, by using a spatial model to understand past land change mechanisms and prospect plausible future ways of development. In particular, the spatial model describes the vegetation clearance processes identified during field campaigns, which we used to draw prospective scenarios and assess their possible effect on natural vegetation evolution. The processing of remote sensing images helped us reveal that one quarter of the total study area was cleared during the last fourteen years. Surveys carried out in the field enabled the identification of the three main processes responsible for these changes: farm size expansion, creation of new farms due to family nuclearization and migrant settlement. The model was then built to reproduce these three processes, and was validated by comparison with the land use classification of remote sensing images. Our model was also used to explore past clearance mechanisms: we found that 90% of the clearance was shared equally between farm size expansion and nuclearization processes, the settlement of migrants being responsible for less than 10% of the clearance. Model outputs also showed a shift in the clearance schemes compared to ancient practices: land characteristics are no longer considered when land is cleared and now clearance also occurs in neighbouring villages with available lands. These results suggest that the region has finally evolved similarly to a Malthusian rationale, even if the past dynamics resulted from a complex combination of factors. Finally, we analysed several prospective scenarios to assess the impacts of i) different population evolutions (normal demographic growth, emigrations and demographic regulation), ii) the implementation of protected areas in each village, iii) an intensification of farming systems. Two possible solutions for reducing natural vegetation clearance in the region are discussed: Emigration to other regions or a demographic regulation accompanied with an intensification of agricultural systems. However, the question remains whether such changes can be accomplished rapidly enough to abate the pressing natural vegetation decrease threat and to maintain an acceptable livelihood in the region.

International audience; A wide range of environmental and societal issues such as food security policy implementation requires accurate information on biomass productivity and its underlying drivers at both regional and local scales. While many studies in West Africa are conducted with coarse resolution earth observation data, few have tried to relate vegetation trends to explanatory factors, as is generally done in land use and land cover change (LULCC) studies at finer scales. In this study, we proposed to make a bridge between vegetation trend analysis and LULCC studies to improve the understanding of the various factors that influence the biomass production changes observed in satellite time series (using integrated Normalized Difference Vegetation Index [NDVI] as a proxy). The study was conducted in two steps. In the first step, we analyzed MODIS NDVI linear trends together with TRMM growing season rainfall over the Sahel region from 2000-2015. A classification scheme was proposed that enables better specification of the relative role of the main drivers of biomass production dynamics. We found that 16% of the Sahel is re-greening-but found strong evidence that rainfall is not the only important driver of biomass increase. Moreover, a decrease found in 5% of the Sahel can be chiefly attributed to factors other than rainfall (88%). In the second step, we focused on the "Degré Carré de Niamey" site in Niger. Here, the observed biomass trends were analyzed in relation to land cover changes and a set of potential drivers of LULCC using the Random Forest algorithm. We observed negative trends (29% of the Niger site area) mainly in tiger bush areas located on lateritic plateaus, which are particularly prone to pressures from overgrazing and overlogging. The significant role of accessibility factors in biomass production trends was also highlighted. Our methodological framework may be used to highlight changing areas and their major drivers to identify target areas for more detailed studies. Finer-scale assessments of the long-term vulnerability of populations can then be made to substantiate food security management policies.; De nombreux problèmes environnementaux et sociétaux, tels que la mise en œuvre de politiques de sécurité alimentaire, nécessitent des informations précises sur la productivité de la biomasse et ses facteurs sous-jacents aux niveaux régional et local. Alors que de la plupart des études en Afrique de l’Ouest reposent sur des données d’observation de la Terre à faible résolution spatiale, peu d’entre elles ont tenté de relier les tendances de la production végétale à des facteurs explicatifs, comme c’est généralement le cas dans les études sur le changement d'utilisation et d'occupation des sols (LULCC) à des échelles plus fines. Dans cette étude, nous avons proposé d'établir un pont entre l'analyse des tendances de la végétation et les études LULCC afin d'améliorer la compréhension des divers facteurs qui influencent les changements de production de biomasse observés dans les séries chronologiques satellites (en utilisant l'indice de végétation par différence normalisée [NDVI] intégré). L'étude a été réalisée en deux étapes. Dans un premier temps, nous avons analysé les tendances linéaires de MODIS NDVI ainsi que les précipitations TRMM, sur la région du Sahel, pendant la saison des pluies, de 2000 à 2015. Un système de classification a été proposé, qui permet de mieux préciser le rôle relatif des principaux facteurs de la dynamique de la production de biomasse. Nous avons constaté que 16% du Sahel est en train de reverdir, mais nous avons trouvé des indications solides que les précipitations ne sont pas le seul facteur important d'augmentation de la biomasse. De plus, une diminution constatée dans 5% du Sahel peut être principalement attribuée à des facteurs autres que les précipitations (88%). Dans un deuxième temps, nous nous sommes concentrés sur le site "Degré Carré de Niamey" au Niger. Ici, les tendances de la biomasse observées ont été analysées en relation avec les changements de l'occupation des sols et un ensemble de facteurs potentiels de LULCC, avec l'algorithme Random Forest. Nous avons observé des tendances négatives (29% de la superficie du site du Niger) principalement dans les zones de brousses tigrées situées sur des plateaux latéritiques, particulièrement sujettes aux pressions dues au surpâturage et à la surexploitation. Le rôle important des facteurs d’accessibilité dans les tendances de la production de biomasse a également été mis en évidence. Notre cadre méthodologique peut être utilisé pour mettre en évidence les zones en évolution et leurs principaux moteurs afin d'identifier les zones cibles pour des études plus détaillées. Des évaluations plus précises de la vulnérabilité à long terme des populations peuvent ensuite être effectuées pour justifier les politiques de gestion de la sécurité alimentaire.

SummaryWhile around 20% of the Amazonian forest has been cleared for pastures and agriculture, one fourth of the remaining forest is dedicated to wood production [1]. Most of these production forests have been or will be selectively harvested for commercial timber, but recent studies show that even soon after logging, harvested stands retain much of their tree-biomass carbon and biodiversity [2,3]. Comparing species richness of various animal taxa among logged and unlogged forests across the tropics, Burivalova et al. [4] found that despite some variability among taxa, biodiversity loss was generally explained by logging intensity (the number of trees extracted). Here, we use a network of 79 permanent sample plots (376 ha total) located at 10 sites across the Amazon Basin [5] to assess the main drivers of time-to-recovery of post-logging tree carbon (Table S1). Recovery time is of direct relevance to policies governing management practices (i.e., allowable volumes cut and cutting cycle lengths), and indirectly to forest-based climate change mitigation interventions.

International audience; The Indochinese section of the Mekong Basin has been subjected to major environmental disturbances over the last half century. The Vietnam War is invoked as a central explanation for the extensive deforestation in specific areas while conflictinduced exoduses caused the abandonment of cultivated lands, followed by forest regeneration. Although the socio-economical consequences of these episodes have been analysed, their hydrological impacts remain unknown. This paper investigates hydrological changes in two catchments of the lower Mekong Basin that were either heavily bombed (in southern Laos) or depopulated (in northern Laos). This analysis is based on the widely and independently recognized fact that vegetation, via evapotranspiration, is a central driver of basin water yield. The analysis of the most complete Vietnam War air mission database and of available hydro-meteorological data over the period 1960–2004 reveals a sharp runoff increase in the southern catchment when bombing climaxed in the early 1970s while no hydrological change is observed in the northern catchment over the same period. From 1995 onwards, the northern and southern catchment's runoff productions are significantly lower and higher than in the pre-war conditions, respectively. Although causalities could not be ascertained because of data limitations, these short- and long-term hydrological shifts were found to be consistent, in terms of occurrence, spatial distribution and magnitude, with the expected changes in the vegetation cover, either denser in the north (in response to abandonment of cultivated lands) or sparser in the south (as a result of bomb-induced deforestation and soil degradations).