Response of CO2 fluxes and productivity to water availability in two contrasting ecosystems in northern Benin (West Africa)

Abstract

& Key message CO 2 fluxes were measured during 18 months in a forest and a savannah in northern Benin. Higher values of carbon fluxes were found during the wet season at each site. A strong dependency of carbon fluxes on water relations was found in two contrasting sites. The forest sequestered 640 ± 50 and the savannah 190 ± 40 g C m −2 year −1 . & Context In West Africa, the main mechanisms or factors governing the dynamics of ecosystems, especially the dynamics of the carbon fluxes and productivity, still remain less known. This study reports the carbon fluxes over two contrasting ecosystems, notably a protected forest (lat 9.79°N, long 1.72°E, alt 414 m) and a cultivated savannah (lat 9.74°N, long 1.60°E, alt 449 m) in northern Benin. The two sites were among those equipped by the AMMA-CATCH observatory and Ouémé 2025 project. & Aims Flux data were analyzed at the daily and seasonal scales in order to understand their controlling variables. We discussed the patterns of CO 2 fluxes and the characteristics of the two ecosystems. The study also focused on the different water usage strategies developed by the two ecosystems since the alternation between dry and wet seasons highly influenced the seasonal dynamics. Finally, the annual carbon sequestration was estimated together with its uncertainty. & Methods The carbon fluxes were measured during 18 months (July 2008-December 2009) by an eddy-covariance system over two contrasting sites in northern Benin. Fluxes data were computed following the standard procedure. The responses of CO 2 fluxes to the principal climatic and edaphic factors, and the canopy conductance were studied. & Results A clear CO 2 fluxes response to main environmental factors was observed, however with difference according to the seasons and vegetation types. The ecosystem respiration showed the highest values during the wet season and a progressive decrease from wet to dry periods. Also, the carbon uptake values were high during the wet period, but Handling Editor: Erwin DreyerContribution of the co-authors Expédit Evariste Ago conducted mainly the data analysis and prepared the manuscript with contributions from all co-authors. The field work with the eddy-covariance technique was carried out by Jean-Martial Cohard and Sylvie Galle who processed the raw data of fluxes. Marc Aubinet and Euloge Kossi Agbossou assisted in the data analyses. All authors read and approved the final manuscript which was submitted. low during the dry period. However, the CO 2 fluxes for the protected forest were always higher than that for the cultivated savannah within each defined period. This was due to the seasonal changes not only in phenology and physiology but also to the acclimation to environmental conditions, especially to the soil water availability. The water use efficiency was influenced by VPD during the day conditions for two ecosystems. However, the VPD response curve of water usage was relatively constant for the protected forest during the transitional and wet seasons. In contrary, for t...

Dynamics of water vapor and energy exchanges above two contrasting Sudanian climate ecosystems in Northern Benin (West Africa)

“…These high transpiration rates are due to vegetation activity which even leads to leaf renewal during this dry season in the region [ Seghieri et al ., ]. It is also associated with high physiological activity as shown in the sap flow analysis [ Awessou et al ., ] as well as in the CO 2 fluxes [ Ago et al ., , , ] in the studied area. This phenomenon was also observed farther north above a savanna in Ghana by Quansah et al .…”

“…The second site is located in a woodland, the most widespread vegetation type in tropical Africa [White, 1983], near Bellefoungou village (latitude 9.791°N, longitude 1.718°E, 445 m asl) (hereafter Bellefoungou (BE)). Vegetation of this woodland consists in an open stand of trees with crown heights of around 15 m (Figure 1b), covering 60%-80% of the total soil area [Ago et al, 2016] [Seghieri et al, 2012]. This species composition is reasonably homogeneous around the BE flux tower and contributes to the observed footprint (about 60,000 m 2 ) whatever the season [Mamadou, 2014].…”

Variations in diurnal and seasonal net ecosystem carbon dioxide exchange in a semiarid sandy grassland ecosystem in China's Horqin Sandy Land

“…Arid and semiarid ecosystems cover 30 % to 40 % of the global terrestrial surface (Poulter et al, 2014). The extent and distribution of these areas are increasing in response to factors such as climate change, changes in wildfire frequency and intensity, and changes in land use (Asner et al, 2003;Hastings et al, 2010). These ecosystems are important because they account for 30 % to 35 % of terrestrial net primary productivity (Gao et al, 2012;Liu et al, 2016a) and approximately 15 % of the global soil organic carbon pool (Lal, 2004;Liu et al, 2016a).…”

“…Compared with the more constant sink that is typically measured in mesic ecosystems, dryland ecosystems showed a wide range of carbon sink or source functions for diverse vegetation types (Biederman et al, 2017). Moreover, water availability (e.g., precipitation, evapotranspiration, soil moisture) plays a dominant role in regulating ecosystem carbon fluxes and their responses to climatic change in dryland ecosystems (Niu et al, 2008;Biederman et al, 2016;Ago et al, 2016). However, to our knowledge, there has been no report on the intra-annual and inter-annual variation in ecosystem-scale carbon fluxes in China's Horqin Sandy Land, an important dryland ecosystem in northern China.…”

“…Sandy land includes multiple overlapping ecotones, including transition zones between areas with different population pressures and between semi-humid and semiarid areas, and occurs in typical agro-pastoral ecotones. The ecological environment is fragile and extremely sensitive to climate change and human activities (Bagan et al, 2010;Zhao et al, 2015). The region's sandy grassland grows on aeolian sandy soils or areas with sandy soils as the substrate and is typical of the grassland vegetation that develops in sandy land (Munkhdalai et al, 2007).…”