Carbon stocks and dynamics in Sub-Saharian Africa
Author(s)
Henry, Matieu
Date Issued
June 7, 2010
Type
Doctoral Thesis
Abstract
Climate change has become the most important global environmental crisis and its consideration has revealed the emergence of a new political regime that has been idealized under the hypothetical “carbon city”. The Carbon City corresponds to a political community of people who can act on the carbon cycle and enjoy its productions and services. Forests play a key role into this new city because they significantly contribute to the carbon cycle and ecosystem stability and they represent an object to meet the consensus among the different groups of interests. Physically represented by the United Nations Convention on Climate Change (UNFCC), the climate community tends to establish an equitable repartition of the efforts to mitigate climate changes and adapt to future modifications of our environment. A fair distribution of the efforts requires the quantification of the contribution made by each country through scientific expertise. However, sub-Saharan African countries are mostly in margin of the process mostly because of the lack of knowledge about the quantitative contribution these countries can make to the global efforts. Quantification of carbon stocks has become crucial for sub-Saharan African countries to increase their participation to the climate discussions arena and develop actions to mitigate an environmental crisis they are the most vulnerable to. Better understanding the carbon cycle also allows a better comprehension of our interaction between the global climate and our activities. The aim of this thesis is to better understand the dynamics of carbon stocks in sub-Saharan Africa. The objectives were (1) to understand the most important factors influencing the estimation of carbon stocks and carbon stock changes, (2) to inventory the state of knowledge at continental scale and (3) to fulfill some of the gaps while identifying the necessary further researches. Three field experiments where implemented in Ghana to measure the variability of biomass within plots, between plots and between wet evergreen, moist evergreen and moist semi-deciduous forests. The wet evergreen forest was selected to analyze the variation of biomass within and between trees based on an original method to build tree allometric equation. The three forests were logged and the wet evergreen forest was selected to measure the impact of logging activities on carbon stocks. In parallel, an intensive data collection in various libraries of the world has allowed the development of the first tree volume and biomass allometric database and the first biomass and carbon stock database at continental scale. The data were organized and reviewed to allow the comparison between equations and carbon stocks data.
About two third of the continental carbon stock is found in the soil. Using different soil databases and soil maps may result in variation of the soil organic carbon estimates of about 30%. The choice of the tree allometric equation to estimate the tree biomass can induce variation of aboveground biomass of about 40%. Using already available biomass data, the aboveground carbon stocks ranges 10-105 Pg C. Using the data collected in this study allowed increased the number of data and its harmonization. It follows a major reduction in the variability of carbon stocks that is between 58 and 76 Pg C. Measurements in humid tropical forests in Ghana revealed variations of biomass at different scales, from trees to different types of forests. Aboveground biomass ranges 348-364 ha-1 between ecological zones, 218-485 Mg ha-1 between plots and 0 - 4681Mg ha-1 within plots of 1 ha. Carbon stocks are on average 355 Mg ha-1 with 49, 39, 9, 1, 0.9, 0.8, and 0.2% are in the aerial part of trees, soil organic carbon, roots, litter, dead wood, lianas and palms. The biomass of a tree was mainly influenced by the diameter of the trunk, the crown diameter and the wood density in a tropical rainforest of Ghana. However, various factors influence the ecosystem biomass variability. At plant scale, the tree biomass is influenced by the tree species, the plant functional type and the growth strategies. At ecosystem scale, the biomass is influenced by the effect of topography, plant distribution, slope, soils, history of perturbations and forest management. At biome scale, the biomass is influenced by climate, ecological zone, age, structure, and management.
When considering the impact of anthropic activities on carbon stocks, deforestation and forest exploitation emissions ranged 0.06-0.5 and 0.03 – 0.08PgC Yr-1 respectively. While deforestation had already been studied in previous research, I have focused on forest degradation caused by selective logging. In Ghanaian rainforests, selective logging impact 12 Mg C ha-1. Based on the relation between harvested volume and the impact of forest
carbon stocks, I have estimated that 88 Tg C yr-1 are being emitted at continental scale which means selective logging would contribute to 20– 25% of the continental C balance. However, the potential for decreasing GHG emissions from selective logging has been limited by the accuracy and the cost of field measurements. Forest restoration has been identified as the highest potential activity for emission reduction and C sequestration in Ghana. While this activity is not recognized by the climate convention, enhancement of forest C stock can sequester significant amount of C. It was estimated that intensification of already existing cocoa farms was the most feasible option to sequester important amount of C in Ghana while increasing the farming income by about 19%. However, carbon sequestration and financial compensation are among many other aspects to better consider when implementing developing activities. Land and forest tenure issues have to be particularly addressed in Ghana to allow effective implementation. The mis-consideration of some social entities would lead to massive forest destructions that already happened in the past.
Sub-Saharan Africa faces important gaps related to the understanding of the contribution of the African ecosystems to the C cycle. Only one percent of the necessary tree allometric equations is currently available. The impact of degradation and deforestation on the role of the decomposition of wood and carbon dynamics in soil are particularly poorly understood. The dynamics of forest regeneration after perturbations such as selective logging are poorly known. While C stocks are assumed to recover after a 40 years period, the structure of the forest and the biodiversity are still strongly affected. The use of remote sensing is often presented as a panacea and the solution to monitor natural ressources such as forest biomass. However, I have pointed out that the estimation of biomass using available satellite imageries of Ghana is not an accurate option while the use of high resolution imageries are too costly for countries that have low human and technical capacities to achieve their forest inventories using these techniques. This study highlights the issue of the language used to describe the environment. The global land descriptions are often not in relation with the language used in the field to describe the vegetation in term of structure, floristic composition, position in the landscape and management. The identification of the degraded forest is strongly limited by the poor definitions and the poor previous consideration and analysis. While several attempts to harmonize the vegetation descriptors have been developed on the basis of classification systems, however, they have been facing the complexity of wording, defining and translating our perception of environment. At last, very few socio-economic studies have considered the impact of carbon services to local farmer livelihood. However, they are considered as part of the main process of deforestation. There is an urgent need to identify the potential for improving farmer livelihood while sequestering carbon stocks and reducing emissions from deforestation and forest degradation. This would be achieved by an increasing consideration of agricultural activities in developing contries by the climate change convention. Still, the agricultural sector will be considered if a carbon quantification system can be implemented for farming.
Additional information
Dottorato di ricerca in Ecologia forestale
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