PhD Dissertation:

Cedric GOUSSANOU 2018. Estimation and monitoring of carbon fluxes in tropical forest ecosystems in Benin, West Africa. Ecole Doctorale Pluridisciplinaire, « Espaces, Cultures Et Developpement », Université d’Abomey-Calavi, BENIN, 129p. 

Promotor: Prof. Dr Ir. Brice SINSIN.

Abstract: The quantification of the contribution of tropical forests to global carbon stocks and climate change effects mitigation requires availability of data and tools in aboveground biomass and soil. This study intend (i) to make available volume and biomass models for species-specific and site-specific in a semi-deciduous tropical forest in West Africa, (ii) to estimate forest carbon stock, (iii) to monitor carbon flux, and (iv) to generate soil organic carbon (SOC) reference data for the benefit of REDD+ initiatives. Fieldworks were conducted in Lama forest reserve across three land-use types namely undisturbed forest, degraded forest and fallow. Non-destructive sampling approach was carried out on 501 sample trees to analyse stem volume and biomass. SOC was derived from direct measurements of organic matter (OM) content in soil. Six hundred and seventy-five soil samples were collected along 30 cm black cotton soil depth. The samples were analysed for bulk densities and for soil OM using loss-on-ignition method. Litterfall production and carbon fluxes were studied over two monitoring years at monthly intervals based on collect in 225 squares litter traps spread out in the whole forest and by land-use types.

From the modelling of volume and biomass as functions of diameter at breast height (Dbh) and stem height, species-specific models have better predictive capabilities than generic models. These allometric equations were applied to estimate carbon stocks of three land-use types. Carbon stock of the undisturbed forest was higher than disturbed forest. Carbon stock was positively correlated to basal area and negatively related to tree density. The study shown that large trees constitute an important component to include in the sampling approach to achieve accurate carbon quantification in forestry. OM and C contents and SOC were higher in the upper soil layer and decreased with depth. The low variation of these soil factors within each land-use type and their fairly homogeneous spatial distribution across land-use types confirmed that soils in degraded forest and fallow reached equilibrium, considering undisturbed forest as reference. Total litterfall production over the studied period (2 years) was estimated at 1.06 tdm ha^-1. Seasonality and land-use types have significant effects on litter years fall variation. The litterfall trends were not uniform on the whole forest and presented similarities across land-use types. Leaf litter was the major contributor to total litterfall than wood litter and reflected the semi-deciduous trait of the forest. Therefore, littefall was more related to plants phenology rather than environmental variables. The fire disturbance that occurred during the study period suggested a longer monitoring period to establish the temporal pattern of litterfall.

The present study demonstrated the hypothesis that species-specific models are preferred to generic models, and concluded that further research should be oriented towards development of specific models to cover the full range of dominant tree species of African forests. The study explained the application of biomass models and ground truth data to estimate reference carbon stock of forests. Returns of nutrients to soil through litterfall affected soil organic carbon dynamics and required a specific attention to understand carbon balance of forest ecosystems.

Keywords: allometrics models, biomass, soil organic carbon, litterfall, tropical forest ecosystems