Caractérisation quantitative des paysages forestiers tropicaux : application à l'analyse de la déforestation par l'agriculture-sur-brûl is et annexes

Ce travail s'inscrit dans le cadre du projet "Alternatives-to-Slash-an d-Burn" (ASB). L'étude porte sur sept sites de la zone tropicale humide sujets à des processus de déforestation. L'objectif principal est de caractériser, comparer et suivre ces différents sites par une analyse quantitative des motifs de paysage. Les sites ont alors été comparés et différenciés quantitativement à l'aide des indices sélectionnés. Une étude de la variabilité interne des images a également été réalisée et a permis d'identifier plusieurs types de motifs de paysage à l'intérieur de chaque image. Une analyse de la dynamique du paysage a ensuite été menée sur un des sites. Elle nous a permis de suivre l'évolution spatiale du début du processus de déforestation à nos jours et de relier cette évolution au processus de déforestation. Nous avons choisi des indices qui reflètent la fragmentation de la forêt. Nous nous sommes enfin replacés dans une perspective thématique pour définir les sites à travers les caractéristiques du front de déforestation à l'oeuvre. Nous sommes parvenus à relier les motifs de paysages -mesurés par des indices spatiaux- aux facteurs physiques (proximité des villes, accès au marché, pénétration du milieu) mais peu aux autres facteurs de déforestation (politiques, socio-économiques) dont la composante spatiale est peu marquée à l'échelle spatiale étudiée

Tropical Forest Cover Change in the 1990s and Options for Future Monitoring

Despite the importance of the world¿s humid tropical forests, our knowledge concerning their rates of change remains limited. Two recent programmes (FAO 2000 Forest Resources Assessment and TREES II), exploiting the global imaging capabilities of Earth observing satellites, have recently been completed to provide information on the dynamics of tropical forest cover. The results from these independent studies show a high degree of conformity and provide a good understanding of trends at the pan-tropical level. In 1990 there were some 1150 million ha of tropical rain forest with the area of the humid tropics deforested annually estimated at 5.8 million ha (approximately twice the size of Belgium). A further 2.3million ha of humid forest is apparently degraded annually through fragmentation, logging and/or fires. In the sub-humid and dry tropics, annual deforestation of tropical moist deciduous and tropical dry forests comes to 2.2 and 0.7million ha, respectively. Southeast Asia is the region where forests are under the highest pressure with an annual change rate of K0.8 to K0.9%. The annual area deforested in Latin America is large, but the relative rate (K0.4 toK0.5%) is lower, owing to the vast area covered by the remaining Amazonian forests. The humid forests of Africa are being converted at a similar rate to those of Latin America (K0.4 to K0.5% per year). During this period, secondary forests have also been established, through re-growth on abandoned land and forest plantations, but with different ecological, biophysical and economic characteristics compared with primary forests. These trends are significant in all regions, but the extent of new forest cover has proven difficult to establish. These results, as well as the lack of more detailed knowledge, clearly demonstrate the need to improve sound scientific evidence to support policy. The two projects provide useful guidance for future monitoring efforts in the context ofmultilateral environmental agreements and of international aid, trade and development partnerships. Methodologically, the use of high-resolution remote sensing in representative samples has been shown to be cost-effective. Close collaboration between tropical institutions and inter-governmental organizations proved to be a fruitful arrangement in the different projects. To properly assist decision-making, monitoring and assessments should primarily be addressed at the national level, which also corresponds to the ratification level of the multilateral environmental agreements. The Forest Resources Assessment 2000 deforestation statistics from countries are consistent with the satellite-based estimates in Asia and America, but are significantly different in Africa, highlighting the particular need for long-term capacity-building activities in this continent.JRC.H.3-Global environement monitorin

Aboveground forest biomass varies across continents, ecological zones and successional stages: refined IPCC default values for tropical and subtropical forests

For monitoring and reporting forest carbon stocks and fluxes, many countries in the tropics and subtropics rely on default values of forest aboveground biomass (AGB) from the Intergovernmental Panel on Climate Change (IPCC) guidelines for National Greenhouse Gas (GHG) Inventories. Default IPCC forest AGB values originated from 2006, and are relatively crude estimates of average values per continent and ecological zone. The 2006 default values were based on limited plot data available at the time, methods for their derivation were not fully clear, and no distinction between successional stages was made. As part of the 2019 Refinement to the 2006 IPCC Guidelines for GHG Inventories, we updated the default AGB values for tropical and subtropical forests based on AGB data from >25 000 plots in natural forests and a global AGB map where no plot data were available. We calculated refined AGB default values per continent, ecological zone, and successional stage, and provided a measure of uncertainty. AGB in tropical and subtropical forests varies by an order of magnitude across continents, ecological zones, and successional stage. Our refined default values generally reflect the climatic gradients in the tropics, with more AGB in wetter areas. AGB is generally higher in old-growth than in secondary forests, and higher in older secondary (regrowth >20 years old and degraded/logged forests) than in young secondary forests (<= 20 years old). While refined default values for tropical old-growth forest are largely similar to the previous 2006 default values, the new default values are 4.0-7.7-fold lower for young secondary forests. Thus, the refined values will strongly alter estimated carbon stocks and fluxes, and emphasize the critical importance of old-growth forest conservation. We provide a reproducible approach to facilitate future refinements and encourage targeted efforts to establish permanent plots in areas with data gaps