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CARBOAFRICA - flux tower of Ankasa Conservation Area

The flux tower of Ankasa Conservation Area is a station set for monitoring CO2 fluxes in the only wet evergreen protected area in Ghana. The structure and its associated research were developed under the scope of the CARBOAFRICA project, and was the first of its type in African tropical forest. Previous studies have been carried out in the Amazon and Asia.

A 65 m tall steel tower was equipped with sensing devices for measuring different parameters:

  • CO2 fluxes at the top of the tower, which were measured with an eddy covariance system, composed of a sonic anemometer and a CO2/H2O analyser. A software tool (EccoCatch) acquired and stored data (wind speed, CO2/H2O concentrations and temperature) at 30-minute intervals.
  • CO2, air temperature, humidity and other relevant physical parameters of forest ecosystems such as solar radiation and irradiative properties of the forest were measured along a vertical profile. CO2 concentrations were determined at 6 levels, from 0.2 to 40m through a gas analyser coupled to a logger.

Complementary, a complete soil characterization was carried out with 40 samples collected across the whole area, and existing plant species were identified and characterised in two 1000 x 10 m transects. These transects were oriented leaving their branches oriented to North, South, West and East.

The tower started working in 2008, and is still continuing to carry out measurements. Preliminary assays showed a daily cycle on atmospheric CO2 fluxes, with a sink activity with regard to the forest. This trend was similar to the one observed in the Amazon, but night-time storages and CO2 gradients should be exactly determined in order to correctly evaluate carbon exchanges with the atmosphere. In addition, soil was demonstrated to be a major supplier of the forest total carbon content as it stores a big amount of organic carbon (higher than in some soils from the Amazon and Central Africa). So, not only aboveground biomass but also organic carbon must be considered for obtaining reliable values of Soil Organic Carbon in tropical soils. Biodiversity monitoring showed a wide range of vegetal species (39 families, 115 genera and 175 tree species). A preliminary analysis showed that biodiversity increased, following the gradient of moisture.

Preliminary research activities carried out in this tropical rainforest highlighted the need to better characterize tropical forest ecosystems in order to obtain more precise data on carbon fluxes. A combination of in situ fluxes and inventories is needed to constrain the carbon budget of the tropical forests of Africa. Also, the soil carbon dynamics is poorly investigated and may bring new insights on possible carbon storage in the soil. Nowadays, data obtained from this flux tower can be obtained from the FLUXNET network.

 

Source:
“Ankasa flux tower: a new research facility for the study of the carbon cycle in a primary tropical forest in Africa”. Africa and the Carbon Cycle-Proceedings of the Open Science Conference on “Africa and Carbon Cycle: the CarboAfrica project”. Accra (Ghana) 25-27 November 2008.