The African continent is facing one of the driest periods in the past three decades as well as continued deforestation. These disturbances from both human pressure and climate change threaten vegetation carbon stocks and highlight the need for improved capabilities of monitoring large-scale aboveground carbon stock dynamics. What is the role of the different vegetation types (shrubland, woody savannah, rainfall forest)? What is their sensitivity to droughts?
A new and unique tool produced by French scientific teams
Scientists from INRA and their colleagues1 have produced a new data set of the vegetation index referred to as L-VOD (L-band vegetation optical depth), retrieved from space-borne observations of the SMOS satellite over 2010-2016. The data have been used to quantify annual aboveground biomass-carbon changes in sub-Saharan Africa stocks over the time period.
The L-VOD index computed from low frequency microwave observations allows sensing the whole canopy layer, while remote sensing observations made so far (including VOD indexes from high frequency microwave observations) were limited to sensing the top of the canopy, especially over relatively dense forests.
In this study, the L-VOD index brings a temporal dimension to global, but static, maps of the aboveground biomass.
An new tool to carry out an innovative monitoring of the dynamics of aboveground vegetation carbon stocks
This new method allows to monitor the seasonal dynamics of vegetation carbon losses and gains and also to link them with the impact of climate.
The results demonstrate the key interest of L-VOD as a complementary data source for the quantification and monitoring of carbon stocks for national reports and largescale efforts in the framework of international initiatives such as the United Nations Framework Convention on Climate Change (UNFCCC) and the Intergovernmental Panel on Climate Change (IPCC).
The L-VOD index is particularly relevant to study relatively dense vegetation ecosystems where the signal measured from other remotely-sensed system saturates. It is also very relevant for semi-arid regions with little inventory data.
First applications of the new L-VOD tool to monitor carbon changes in sub-Saharan Africa
The study demonstrates there is an overall negative net carbon budget for sub-Saharan Africa (−0.10 PgC yr−1) over 2010-2016 and that the majority of the net losses occurred in drylands. In the latter regions, the gross loss per year represents approximately 5% of the dryland total carbon stocks in Africa. Overall, most of the detected decreases in carbon stocks in drylands were related to abnormally low soil moisture and rainfall conditions. The analyses showed there is a high inter-annual variability in the vegetation carbon stocks with gains during very wet years in (2011) and losses during in 2015 and early 2016, following a severe El Niño episode.
This study questions the general understanding that drylands may serve as carbon sinks on the long term. Indeed, the authors have found that dry years have partly reversed this trend and that drylands (especially in southern Africa) turned from being a carbon sink into a source over 2010-2016, demonstrating that climate controls short-term variations in carbon stocks at large scales.
So, in the long term we need to reassess whether woody vegetation in African savannahs will continue to be a carbon sink.
Changes in aboveground vegetation carbon stocks in sub-Saharan Africa over 2010–2016. Regions with significant negative (carbon source) or positive (carbon sink) carbon changes are shown, respectively, in red or green.
© M. Brandt – Université de Copenhague
[1] Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark
[1] List of French teams involved:
Unité « Interaction Sol Plante Atmosphère » (Inra, Bordeaux Sciences Agro), centre Inra Nouvelle-Aquitaine
Laboratoire Evolution et Diversité Biologique (CNRS – IRD – UT3 Paul Sabatier - ENFA)
Laboratoire des sciences du climat et de l’environnement (CEA-CNRS-UVSQ)
Centre d’études spatiales de la biosphère (Cnes-CNRS-IRD-UT3 Paul Sabatier)