# More C uptake during the dry season? The case of a semi-arid agro-silvo-pastoral ecosystem dominated by Faidherbia albida, a tree with reverse phenology (Senegal)

Olivier Roupsard, Frédéric Dô, Alain Rocheteau, Christophe Jourdan, Didier Orange, Laure Tall, Sidy Sow, Waly Faye, Djim M.L. Diongue, Khalisse Diouf, et al.

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Olivier Roupsard, Frédéric Dô, Alain Rocheteau, Christophe Jourdan, Didier Orange, et al. More C uptake during the dry season? The case of a semi-arid agro-silvo-pastoral ecosystem dominated by Faidherbia albida, a tree with reverse phenology (Senegal). EGU General Assembly 2020, EGU, May 2020, Vienne, Austria. 1 p., 10.5194/egusphere-egu2020-11203. https://hal-05174881v1

HAL Id: hal-05174881

Submitted on 21 Jul 2025

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# More C uptake during the dry season? The case of a semi-arid agro-silvo-pastoral ecosystem dominated by Faidherbia albida, a tree with reverse phenology (Senegal)

Olivier Roupsard<fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Do Frederic<fnref n="2" />, Rocheteau Alain<fnref n="2" />, Jourdan Christophe<fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Orange Didier<fnref n="2" />, Tall Laure<fnref n="3" />,<fnref n="4" />, Sow Sidy<fnref n="3" />,<fnref n="5" />, Faye Waly, Diongue Djimm M.L.<fnref n="7" />, Diouf Khalisse, Agbohessou Yélognissé<fnref n="3" />,<fnref n="5" />, Diatta Seydou<fnref n="3" />,<fnref n="5" />, Faye Serigne<fnref n="7" />, Sarr Mame Sokhna<fnref n="5" />, Sanogo Diaminatou<fnref n="5" />, le Maire Guerric<fnref n="2" />,<fnref n="9" />, Vezy Remi<fnref n="10" />,<fnref n="11" />, Seghieri Josiane<fnref n="2" />, and Cournac Laurent<fnref n="2" />

Agro-silvo-pastoralism is a highly representative Land Use in Africa, often presented as a strategical option for ecological intensification of cropping systems towards food security and sovereignty.

We set up a new long-term observatory ("Faidherbia-Flux") to monitor and model microclimate, energy and C balance in Niakhar (central Senegal, rainfall ~ 500 mm), dominated by the multipurpose tree Faidherbia albida (12.5 m high; 7 tree ha⁻<fnref n="1" />; 5% canopy cover). Faidherbia is an attractive agroforestry tree species in order to partition fluxes, given that it is on leaf during the dry season (October-June) and defoliated during the wet season, just when crops take over. Pearl-millet and groundnut crops were conducted during the wet season, following annual rotation in a complex mixed mosaic of ca. 1 ha fields.

Early 2018, we installed an eddy-covariance (EC) tower above the whole mosaic (EC1: 20 m high). A second EC system was displayed above the crop (EC2: 4.5 m if pearl-millet, 2.5 m if groundnut) in order to partition ecosystem EC fluxes between tree layer and crop+soil layers. Sap-flow was monitored from April 2019 onwards in 5 faidherbia trees (37 sensors).

The ecosystem displayed moderate but significant daily CO₂ and H₂O fluxes during the dry season, when faidherbia (low canopy cover) was in leaf and the soil was evaporating. At the onset of the rainy season, the soil bursted a large amount of CO₂. Just after the growth of pearl-millet in 2018, CO₂ uptake by photosynthesis increased dramatically. However, this was largely compensated by

<footnote n="1">¹CIRAD, UMR Eco&Sols, Dakar, Senegal</footnote>

<footnote n="2">²Eco&Sols, Univ Montpellier, CIRAD, INRAE, IRD, Montpellier SupAgro, Montpellier, France</footnote>

<footnote n="3">³LMI IESOL, Dakar, Senegal</footnote>

<footnote n="4">⁴ISRA LNRPV, Dakar, Senegal</footnote>

<footnote n="5">⁵ISRA CNRF, Dakar, Senegal</footnote>

<footnote n="7">⁷UCAD, Département de Géologie, Dakar, Sénégal</footnote>

<footnote n="9">⁹CIRAD, UMR Eco&Sols, Montpellier, France</footnote>

<footnote n="10">¹⁰CIRAD, UMR AMAP, Montpellier, France</footnote>

<footnote n="11">¹¹AMAP, Univ Montpellier, CIRAD, CNRS, INRAE, IRD, Montpellier, France</footnote>
high ecosystem respiration. Surprisingly in 2019, although the crop was turned to groundnut, the fluxes behaved pretty much the same as with pearl millet in 2018: comparing annual balances between 2018 and 2019 we obtained [454, 513] for rainfall (P: mm yr⁻¹), [3500, 3486] for potential evapotranspiration (ETo: mm yr⁻¹), [0.13, 0.15] for P/ETo, [470, 497] for actual evapotranspiration (E: mm yr⁻¹), [2809, 2785] for net radiation (Rn: MJ m⁻² yr⁻¹), [1686, 1645] for sensible heat flux (H: MJ m⁻² yr⁻¹), [-3.2, -2.8] for net ecosystem exchange of C (NEE: tC ha⁻¹ yr⁻¹), [-11.8, -11.1] for gross primary productivity (GPP: tC ha⁻¹ yr⁻¹) and [8.6, 8.3] for ecosystem respiration (Re: tC ha⁻¹ yr⁻¹). The energy balance (Rn-H-LE) was nearly nil indicating that the EC system behaved reasonably. E was very close to P, indicating that little or no water would recharge the deep soil layers.

Now comparing the dry (2/3 of the year) and wet (1/3) seasons: surprisingly, NEE was more effective during the dry season [-3.9, -1.7]. This was the result of Re being much lower on a daily basis as well as cumulated over the entire seasons [57, 84], whereas GPP was similar [-10.8, -12.1].

We found a good match between E measured above the whole ecosystem (EC1), and the sum of tree transpiration (T, measured by sapflow) + E measured just above crops + soil (EC2) throughout the wet and dry seasons.

The "Faidherbia-Flux" observatory is registered in FLUXNET as SN-Nkr and is widely open for collaboration.