Fire-derived phosphorus fertilization of African tropical forests

Central African tropical forests face increasing anthropogenic pressures, particularly in the form of deforestation and land-use conversion to agriculture. The long-term effects of this transformation of pristine forests to fallow-based agroecosystems and secondary forests on biogeochemical cycles that drive forest functioning are poorly understood. Here, we show that biomass burning on the African continent results in high phosphorus (P) deposition on an equatorial forest via fire-derived atmospheric emissions. Furthermore, we show that deposition loads increase with forest regrowth age, likely due to increasing canopy complexity, ranging from 0.4 kg P ha−1 yr−1 on agricultural fields to 3.1 kg P ha−1 yr−1 on old secondary forests. In forest systems, canopy wash-off of dry P deposition increases with rainfall amount, highlighting how tropical forest canopies act as dynamic reservoirs for enhanced addition of this essential plant nutrient. Overall, the observed P deposition load at the study site is substantial and demonstrates the importance of canopy trapping as a pathway for nutrient input into forest ecosystems.

Establishment and functioning of the savanna marshes of the Lopé National Park in Gabon since the termination of the African humid period and the arrival of humans 2500 years ago

Holocene paleoecological studies in tropical Africa are rare because most lakes either dried out at the termination of the African Humid Period or have since filled up. However, tropical sedge marshes can be an alternative to perform long-term ecological studies. The Lopé National Park (LNP) in Gabon is a mosaic of forest and savanna enclosed in the equatorial forest, where open areas facilitated the development of peat marshes accumulating several-meter-thick sediment. In order to reconstruct the historical dynamic in these marshes through a local and regional point of view, we compared sedimentological, continuous X-ray fluorescence, and stable isotopic analyses on sediment cores from six herbaceous marshes in the LNP. A reliable chronological frame was based on 50 14C dates, over the last 2500 years in most sites, and reaching 9000 years in one marsh. We show that the origin of these marshes is a major hydrological change, 3450 and 2300 years ago, that affected the entire region, almost concomitantly with the diffusion of Iron Age population. The sedimentation within marshes is homogenous with low intra-site variability. In contrast, high inter-sites variability evidences that the functioning of the marsh itself exerts a much more significant influence than in lakes. However, a regional event is recorded between 1400 and 800 years ago, concurrently with an archeological trace hiatus throughout the forest hinterland of West Central Africa.

Living under a Fluctuating Climate and a Drying Congo Basin

Humid conditions and equatorial forest in the Congo Basin have allowed for the maintenance of significant biodiversity and carbon stock. The ecological services and products of this forest are of high importance, particularly for smallholders living in forest landscapes and watersheds. Unfortunately, in addition to deforestation and forest degradation, climate change/variability are impacting this region, including both forests and populations. We developed three case studies based on field observations in Cameroon, the Central African Republic, and the Democratic Republic of Congo, as well as information from the literature. Our key findings are: (1) the forest-related water cycle of the Congo Basin is not stable, and is gradually changing; (2) climate change is impacting the water cycle of the basin; and, (3) the slow modification of the water cycle is affecting livelihoods in the Congo Basin. Developmental and environmental actions in the Congo Basin need to properly consider the slight modification of this water cycle in watersheds that affect products and services from the forest.