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- Tagesson, Torbern, et al.
(författare)
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Dynamics in carbon exchange fluxes for a grazed semi-arid savanna ecosystem in West Africa
- 2015
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Ingår i: Agriculture, Ecosystems & Environment. - : Elsevier BV. - 1873-2305 .- 0167-8809. ; 205, s. 15-24
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Tidskriftsartikel (refereegranskat)abstract
- The main aim of this paper is to study land-atmosphere exchange of carbon dioxide (CO2) for semi-arid savanna ecosystems of the Sahel region and its response to climatic and environmental change. A subsidiary aim is to study and quantify the seasonal dynamics in light use efficiency (epsilon) being a key variable in scaling carbon fluxes from ground observations using earth observation data. The net ecosystem exchange of carbon dioxide (NEE) 2010-2013 was measured using the eddy covariance technique at a grazed semi-arid savanna site in Senegal, West Africa. Night-time NEE was not related to temperature, confirming that care should be taken before applying temperature response curves for hot dry semi-arid regions when partitioning NEE into gross primary productivity (GPP) and ecosystem respiration (R-eco). Partitioning was instead done using light response curves. The values of epsilon ranged between 0.02 g carbon (C) MJ(-1) for the dry season and 2.27 g C MJ(-1) for the peak of the rainy season, and its seasonal dynamics was governed by vegetation phenology, photosynthetically active radiation, soil moisture and vapor pressure deficit (VPD). The CO2 exchange fluxes were very high in comparison to other semi-arid savanna sites; half-hourly GPP and R-eco peaked at -43 mu mol CO2 m(-2) s(-1) and 20 mu mol CO2 m(-2) s(-1), and daily GPP and R-eco peaked at -15 g C m(-2) and 12 g C m(-2), respectively. Possible explanations for the high CO2 fluxes are a high fraction of C4 species, alleviated water stress conditions, and a strong grazing pressure that results in compensatory growth and fertilization effects. We also conclude that vegetation phenology, soil moisture, radiation, VPD and temperature were major components in determining the seasonal dynamics of CO2 fluxes. Despite the height of the peak of the growing season CO2 fluxes, the annual C budget (average NEE: -271 g C m(-2)) were similar to that in other semi-arid ecosystems because the short rainy season resulted in a short growing season. Global circulation models project a decrease in rainfall, an increase in temperature and a shorter growing season for the western Sahel region, and the productivity and the sink function of this semi-arid ecosystem may thus be lower in the future. (C) 2015 Elsevier B.V. All rights reserved.
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| 3. |
- Tagesson, Torbern, et al.
(författare)
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Very high CO2 exchange fluxes at the peak of the rainy season in a West African grazed semi-arid savanna ecosystem
- 2016
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Ingår i: Geografisk Tidsskrift. - : Informa UK Limited. - 0016-7223. ; 116:2, s. 93-109
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Tidskriftsartikel (refereegranskat)abstract
- Africa is a sink of carbon, but there are large gaps in our knowledge regarding the CO2 exchange fluxes for many African ecosystems. Here, we analyse multi-annual eddy covariance data of CO2 exchange fluxes for a grazed Sahelian semi-arid savanna ecosystem in Senegal, West Africa. The aim of the study is to investigate the high CO2 exchange fluxes measured at the peak of the rainy season at the Dahra field site: gross primary productivity and ecosystem respiration peaked at values up to −48 μmol CO2 m−2 s−1 and 20 μmol CO2 m−2 s−1, respectively. Possible explanations for such high fluxes include a combination of moderately dense herbaceous C4 ground vegetation, high soil nutrient availability and a grazing pressure increasing the fluxes. Even though the peak net CO2 uptake was high, the annual budget of −229 ± 7 ± 49 g C m−2 y−1 (±random errors ± systematic errors) is comparable to that of other semi-arid savanna sites due the short length of the rainy season. An inter-comparison between the open-path and a closed-path infrared sensor indicated no systematic errors related to the instrumentation. An uncertainty analysis of long-term NEE budgets indicated that corrections for air density fluctuations were the largest error source (11.3% out of 24.3% uncertainty). Soil organic carbon data indicated a substantial increase in the soil organic carbon pool for the uppermost.20 m. These findings have large implications for the perception of the carbon sink/source of Sahelian ecosystems and its response to climate change.
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