Contribution of ‘human induced fires’ to forest and savanna land conversion dynamics in the Luki Biosphere Reserve landscape, western Democratic Republic of Congo

peer reviewedHuman-induced fire is one of the most important determinants of forest cover and change in tropical and subtropical regions of the world. Yet its impact on forest cover and forest cover change remains unclear, as fires in Africa generally do not spread over very large area. This is particularly the case in the Democratic Republic of Congo (DRC), a region of the world that is still poorly investigated. Here, we propose to study the effect of humaninduced fire on land use and land cover change in a protected area of the DRC, i.e. the Luki Biosphere Reserve (LBR). We investigate tree cover changes in and around the reserve between 2002 and 2019 using Landsat 7 ETM+, Landsat 8 OLI/TIRS and MODIS MCD12Q1 images and quantify human induced fires using MODIS MCD64A1 images. The study combines land use and land cover (LULC) change detection analysis of four images, two acquired in 2002 and two acquired in 2019, with multi-temporal assessment of annual burnt area acquired between 2002 and 2019 from MODIS MCD64A1 to assess the role of fire in LULC changes and the sensitivity of different LULC types to fire. The results show a dynamic conversion of primary forest to secondary forest over about 16% of the area, the evolution of savanna to secondary forest over 9.6% (Landsat image) and the replacement of secondary forest by savanna over 8.1% (MODIS image) of the total area of Luki Reserve. Of the total area undergoing land use change, 34.1% (Landsat image) and 35.7% (MODIS image) were caused by fire, which however did not cause a significant LULC change. For the LULC types that experienced fire events, the least stable type was KEYWORD

Effet des plantations d’Acacia sp sur les macronutriments primaires des sols sableux d’Ibi village au plateau des Bateke (Kinshasa, République Démocratique du Congo)

Résumé L’agroforesterie avec Acacia auriculiformis et Acacia mangium au plateau des Bateke, est l’une des techniques recommandées pour ce terroir à vocation agricole, aux sols sableux et pauvres. L’évaluation des effets de ces légumineuses ligneuses sur les macronutriments primaires du sol reste parmi les questions pendantes autour de cette pratique. Des échantillons de sols ont été prélevés à Ibi village jusqu’à 60 cm de profondeur, subdivisés en deux tranches de 30 cm chacune, sous des parcelles d’Acacia auriculiformis et Acacia mangium de 5 ans, une savane et une forêt naturelle d’au moins 10 ans. Afin de déterminer les effets des parcelles d’Acacia sp par comparaison aux écosystèmes naturels du milieu, les macronutriments primaires (azote, phosphore et potassium) et le pH ont été dosés. L’analyse univariée et l’analyse discriminante ont été utilisées pour des fins statistiques. Les résultats de l’étude ne montrent pas de différences significatives des sols sous les deux espèces d’Acacia. Ils font cependant ressortir le fait que les sols sous ces légumineuses présentent des différences significatives avec la savane, pour les quantités de phosphore qui sont augmentées ; les teneurs sont passées à 48.23 ± 13.38 ppm contre 39.26 ±11.80 ppm dans les 30 premiers cm du sol ; et, avec la forêt pour l’azote et le potassium, ces derniers, sont présents en quantités plus élevée sous la forêt. Ces résultats indiquent ainsi qu’après 5 ans de plantation d’Acacia sp, les sols présentent des indices d’amélioration faisant penser à une évolution vers les conditions sous forêt. Mots clés : sol sableux, Acacia sp, macronutriments primaires, Plateau des Bateke, RD Congo Abstract Agroforestry with Acacia auriculiformis and Acacia mangium species, is one of the recommended fertility improvement techniques for poor-sandy soils of the plateau de Bateke. Assessing the effects of these woody legumes on soil’s primary macronutrients, remains one of the most importants questions that need to be solved around this practice. In order to assess the impact of Acacia plots fallow compared to natural forest and savanna, soil samples up to 60 cm deep, divided into two tranches of 30 cm were collected at Ibi village under 5 years old Acacia auriculiformis and Acacia mangium plantations, and 10 years old natural savanna and forest. Laboratory analyses where conducted to measure pH and primary macronutrients (nitrogen, phosphorus and potassium). univariate analysis of variance and discriminant analysis were used for statistics. The study showed no significant differences between soils under the two Acacia varieties; whereas significant differences for phosphorus were found between Savanna and the Acacia plots, as P content were shown to increase from 39.26 ±11.80 ppm to 48.23 ± 13.38 ppm at the 30 cm first depth; and with the forest where, nitrogen and potassium quantities were found to be higher under forest than under acacia. These results indicate that after 5 years, soil under Acacia auriculiformis and Acacia mangium plots shown progress from savanna to conditions under natural forest. Keywords : Sandy soil, Acacia sp, primary macronutrients, Bateke plateau, DR Cong

Detecting vulnerability of humid tropical forests to multiple stressors

Humid tropical forests play a dominant role in the functioning of Earth but are under increasing threat from changes in land use and climate. How forest vulnerability varies across space and time and what level of stress forests can tolerate before facing a tipping point are poorly understood. Here, we develop a tropical forest vulnerability index (TFVI) to detect and evaluate the vulnerability of global tropical forests to threats across space and time. We show that climate change together with land-use change have slowed the recovery rate of forest carbon cycling. Temporal autocorrelation, as an indicator of this slow recovery, increases substantially for above-ground biomass, gross primary production, and evapotranspiration when climate stress reaches a critical level. Forests in the Americas exhibit extensive vulnerability to these stressors, while in Africa, forests show relative resilience to climate, and in Asia reveal more vulnerability to land use and fragmentation. TFVI can systematically track the response of tropical forests to multiple stressors and provide early-warning signals for regions undergoing critical transitions