Research results

A new tool to monitor aboveground vegetation carbon stocks: first application to the African continent

Our new study uses unprecedented data sources to measure vegetation carbon stock dynamics at continental scale. The study demonstrates that over the African continent, the net carbon balance is negative for 2010-2016, and that most of the carbon losses occurred in dryland savannahs. The results were published in the journal Nature Ecology and Evolution.

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.

Continental scale monitoring of vegetation carbon dynamics requires satellite based techniques, however, conventional satellites are limited to sensing the upper canopy layer. Consequently, the monitoring of vegetation dynamics is limited to the top green parts of the canopies which are not directly linked to aboveground biomass carbon.

Our French colleagues around Jean Pierre Wigneron (CEA, CNES, CNRS, INRA) have produced a new data set retrieved from space-borne observations of the SMOS satellite starting in 2010. The data set is based on low frequency passive microwave emissions, which are insensitive to cloud cover and green vegetation and thus able to quantify aboveground biomass carbon of the entire vegetation layer, including stems and branches, even when the vegetation is dense.

Our group had the chance to be the first group testing these new data, with groundbreaking results. For the first time, scientists were able to monitor large scale carbon stock dynamics at annual scale. The groups expect this tool to be a key in future monitoring of carbon losses and gains for national reports and large-scale efforts, such as the United Nations Framework Convention on Climate Change (UNFCCC) and the Intergovernmental Panel on Climate Change (IPCC).

A first application to the African continent showed highly dynamic carbon stocks, and especially dryland savannahs showed surprisingly high gross losses which were caused by recent drought years. The study concludes that the new tool is close to be operational and highlights the importance of drylands in the global carbon balance.

Screenshot from 2018-03-27 15-03-42.png
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 Copenhagen

Article at Nature Ecology and Evolution:

Brandt M, Wigneron J-P, Chave J, Tagesson T, Penuelas J, Ciais P, Rasmussen K, Tian F, Mbow C, Al-Yaari A, Rodriguez-Fernandez N, Schurgers G, Zhang W, Chang J, Kerr Y, Verger A, Tucker C, Mialon A, Rasmussen LV, Fan L, Fensholt R. 2018. Satellite passive microwaves reveal recent climate-induced carbon losses in African drylands. Nature Ecology & Evolution 1. DOI: 10.1038/s41559-018-0530-6

Further reading:

Mongabay: new remote sensing technique used to determine carbon losses in sub-saharan africa

Article at Carbon Brief

SMOS

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Research results

Remote sensing of vegetation in drylands: Evaluating vegetation optical depth (VOD) using NDVI and in situ data over Sahel

Tian, F.; Brandt, M.; Liu, Y. Y.; Verger, A.; Tagesson, T.; Diouf, A. A.; Rasmussen, K.; Mbow, C.; Wang, Y.; Fensholt, R. Remote sensing of vegetation dynamics in drylands: Evaluating vegetation optical depth (VOD) using AVHRR NDVI and in situ green biomass data over West African Sahel. Remote Sensing of Environment 2016, 177, 265–276.

 

  •  A Long-term VOD dataset is evaluated against NDVI and in situ biomass observations.
  • Both VOD and NDVI reflect the spatio-temporal patterns of biomass in West Sahel.
  • VOD captures variations of woody plant foliage biomass better than NDVI.
  • VOD and NDVI seasonal metrics differ for optimal long-term monitoring of biomass.

Monitoring long-term biomass dynamics in drylands is of great importance for many environmental applications including land degradation and global carbon cycle modeling. Biomass has extensively been estimated based on the normalized difference vegetation index (NDVI) as a measure of the vegetation greenness. The vegetation optical depth (VOD) derived from satellite passive microwave observations is mainly sensitive to the water content in total aboveground vegetation layer. VOD therefore provides a complementary data source to NDVI for monitoring biomass dynamics in drylands, yet further evaluations based on ground measurements are needed for an improved understanding of the potential advantages.

In this study, we assess the capability of a long-term VOD dataset (1992–2011) to capture the temporal and spatial variability of in situ measured green biomass (herbaceous mass and woody plant foliage mass) in the semi-arid Senegalese Sahel.

Results show that the magnitude and peak time of VOD are sensitive to the woody plant foliage whereas NDVI seasonality is primarily governed by the green herbaceous vegetation stratum in the study area. Moreover, VOD is found to be more robust against typical NDVI drawbacks of saturation effect and dependence on plant structure (herbaceous and woody compositions) across the study area when used as a proxy for vegetation productivity. Finally, both VOD and NDVI well reflect the spatial and inter-annual dynamics of the in situ green biomass data; however, the seasonal metrics showing the highest degree of explained variance differ between the two data sources. While the observations in October (period of in situ data collection) perform best for VOD (r2 = 0.88), the small growing season integral (sensitive to recurrent vegetation) have the highest correlations for NDVI (r2 = 0.90).

Overall, in spite of the coarse resolution of 25 km, the study shows that VOD is an efficient proxy for estimating biomass of the entire vegetation stratum in the semi-arid Sahel and likely also in other dryland areas.

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Research results

AGU 2015

We had three presentations at this years AGU fall meeting in San Francisco. Find the posters and presentations as PDFs here (the copyright is with the authors):