RESUMO
Quantitative and qualitative loss of tropical forests prompted international policy agendas to slow down forest loss through reducing emissions from deforestation and forest degradation (REDD)+, ensuring carbon offset payments to developing countries. So far, many African countries lack reliable forest carbon data and monitoring systems as required by REDD+. In this study, we estimate the carbon stocks of a naturally forested landscape unaffected by direct human impact. We used data collected from 34 plots randomly distributed across the Mount Birougou National Park (690 km2) in southern Gabon. We used tree-level data on species, diameter, height, species-specific wood density and carbon fraction as well as site-level data on dead wood, soil and litter carbon to calculate carbon content in aboveground, belowground, dead wood, soil and litter as 146, 28, 14, 186 and 7 Mg ha-1, respectively. Results may serve as a benchmark to assess ecosystem carbon loss/gain for the Massif du Chaillu in Gabon and the Republic of Congo, provide field data for remote sensing and also may contribute to establish national monitoring systems. RESUME: Les pertes qualitatives et quantitatives de forêt tropicale ont poussé les calendriers politiques internationaux à ralentir la perte de forêts au moyen des mécanismes REDD+, qui garantissent le paiement compensatoire des émissions de carbone aux pays en développement. Jusqu'à présent, de nombreux pays africains ne disposent pas encore de données fiables sur le carbone forestier, pas plus que de systèmes de suivi exigés par les REDD+. Dans cet article, nous estimons les stocks de carbone d'un paysage de forêt naturelle non affecté par des impacts humains directs. Nous avons utilisé les données provenant de 34 parcelles réparties au hasard dans le Parc National du Mont Birougou (690 km²), dans le sud du Gabon. Nous avons utilisé trois niveaux de données pour les espèces, le diamètre, la hauteur et la densité spécifique du bois par espèce, et la fraction de carbone ainsi que des données au niveau du site sur le carbone du bois mort, du sol et de la litière pour calculer le contenu en carbone au-dessus du sol, en dessous, dans le bois mort, le sol et la litière, à savoir, respectivement, 146, 28, 14, 186 et 7 mg ha-1. Ces résultats peuvent servir de données de référence pour évaluer la perte ou le gain de carbone de l'écosystème pour le Massif du Chaillu, au Gabon, et en République du Congo, constituer des données de terrain pour la détection à distance et aussi contribuer à établir des systèmes de suivi au niveau national.
RESUMO
We extended the applicability of the ecosystem model BIOME-BGC to floodplain ecosystems to study effects of hydrological changes on Quercus robur L. stands. The extended model assesses floodplain peculiarities, i.e., seasonal flooding and water infiltration from the groundwater table. Our interest was the tradeoff between (a). maintaining regional applicability with respect to available model input information, (b). incorporating the necessary mechanistic detail and (c). keeping the computational effort at an acceptable level. An evaluation based on observed transpiration, timber volume, soil carbon and soil nitrogen content showed that the extended model produced unbiased results. We also investigated the impact of hydrological changes on our oak stands as a result of the completion of an artificial canal network in 1971, which has stopped regular springtime flooding. A comparison of the 11 years before versus the 11 years after 1971 demonstrated that the hydrological changes affected mainly the annual variation across years in leaf area index (LAI) and soil carbon and nitrogen sequestration, leading to stagnation of carbon and nitrogen stocks, but to an increase in the variance across years. However, carbon sequestration to timber was unaffected and exhibited no significant change in cross-year variation. Finally, we investigated how drawdown of the water table, a general problem in the region, affects modeled ecosystem behavior. We found a further amplification of cross-year LAI fluctuations, but the variance in soil carbon and nitrogen stocks decreased. Volume increment was unaffected, suggesting a stabilization of the ecosystem two decades after implementation of water management measures.