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Biophysical Changes in the Sahel: Ground and Satellite Based Evidence Across Scales and Disciplines

Periodic Reporting for period 1 - BICSA (Biophysical Changes in the Sahel: Ground and Satellite Based Evidence Across Scales and Disciplines)

Reporting period: 2015-05-01 to 2017-04-30

Human and climate induced desertification/degradation of arable lands has been of major concern for livelihoods and food security in drylands during recent decades. In this context, the semi-arid Sahel region in West Africa has been subject to intensive international research including several controversial and mutually contradicting studies. Recent Earth Observation (EO) studies show a positive trend in rainfall and vegetation greenness over the last decades (known as the re-greening of the Sahel). This has been interpreted as an increase in biomass and contradicts prevailing narratives of a vicious cycle of widespread degradation caused by human overuse and climate change. Apparent oppositions in definitions and scientific outcome result from a lack of detailed data and little investment in studying long-term ground-based observations, making many conclusions on greening/degradation premature and speculative.

This project aimed at applying new satellite data with a high level of detail combined with coarse satellite data covering several decades, as well as field observations to shed light on the environmental conditions and changes in Sahel. The project will improve the scientific understanding of linkages between ground and satellite based data across various spatial and temporal scales, the integration of the datasets and their applicability. New monitoring methods of biophysical variables address challenges in land management and food security in response to climatic changes.

The major objectives were to:
1. Evaluate the ability of long-term satellite vegetation data to reflect long time biomass dynamics in the Sahel.
2. Assess 3 decades of changes, dynamics and spatial heterogeneity of biophysical parameters (tree-, herb production and their species) by means of ground measurements.
3. Assess the interrelationship between ground measured biophysical parameters and EO data, model and extrapolate across spatial and temporal scales.
4. Transfer of knowledge: Translate observed changes in ecosystem services and effects on livelihoods.
The applied satellite data includes medium spatial resolution normalized difference vegetation index (NDVI) and Fraction of absorbed Photosynthetic Active Radiation (FAPAR) from MODIS (500 m; 2000-2015), PROBA-V (100 m, 2014-2016), SPOT-VGT (1 km; 1998-2010), GIMMS-3g (8 km; 1982-2015), as well as passive microwave vegetation optical depth (VOD) (25 km; 1992-2011), and satellite based rainfall estimates ARC2, CHIRPS (both 1981-2015). Very high spatial resolution (~0.5 m) data included imagery from WorldView-2, QuickBird-2 and GeoEye-1 commercial satellites. Field observations on woody cover, woody species, herbaceous and foliage biomass were available over three decades from field sites in Senegal, Mali and Niger.

First, long-term VOD and GIMMS-3g data were compared with field observations from Senegal (1987 to 2011) with highly satisfying results, especially for the VOD data that had not been validated with time series of field observations before. The results are published in

Tian et al., (2016) doi:10.1016/j.rse.2016.02.056

The applied methods for estimating biophysical parameters followed the scheme of linking variables measured in the field with satellite data to extrapolate field observations in space and time. Here this project made use of the high temporal frequency of NDVI and FAPAR time series which describe the seasonal characteristics of the phenology of the plants, which is very different between woody and herbaceous species. Whereas most woody vegetation keeps green leaves over the entire year, herbaceous grow only during the wet season (July-September) and wilt towards the start of the dry season. This fact was used to estimate the fraction of woody and herbaceous cover separately, and build models to estimate the biomass production per year. Results are woody cover and biomass maps with an unprecedented accuracy, published in:

Brandt et al., (2016) doi:10.1016/j.rse.2015.10.036

Diouf et al. (2015). doi:10.3390/rs70709122

Diouf et al. (2016). doi:10.3390/rs8080668

Then we applied above mentioned techniques to map changes over time, and found a very heterogeneous pattern of woody cover changes, but a generally positive trend for Sahel. Taken together, the results were an unprecedented synthesis of woody cover dynamics in Sahel and pointed to land use and human population density as important drivers, however only partially offsetting a general post-drought increase. Results are published in:

Brandt, et al. (2016) doi:10.1016/j.rse.2016.05.027

Tian et al., (2016). doi:10.1111/gcb.13464

We used then very high spatial resolution satellite data to illustrate changes woody vegetation, as well as to train PROBA-V data and derive a woody cover map at a high level of detail, which allows to draw conclusions on the relationship between human management and woody cover. We found large scale increases and die offs in woody vegetation. Moreover, we found that in semi-arid Sahel, the woody cover in farmland is greater than in neighboring savannas. Results are published in:

Brandt et al., (2017). doi:10.20944/preprints201703.0173.v1

Brandt et al., (2017). doi:10.3390/rs9010039

Finally, we applied data sets on land use and human population in relation to NDVI and woody cover changes. We found that in Sahel, a decreasing greenness (NDVI) trend is coupled with an intensification of cropland, i.e. less fallowed and more cropped land. Moreover, we found that for entire Africa, woody vegetation in sparsely populated drylands shows a considerable increase over the 1992-2011 period, but woody vegetation decreased in densely populated areas, being a sign for deforestation. Although the decreases appear in areas that provide numerous ecological services (e.g. biodiversity, carbon storage), the general increase in dryland areas challenges the widely held view of an ongoing reduction of all woody vegetation in Africa. Results are published in:

Tong et al., (2017) doi:10.1016/j.rse.2017.01.030

Brandt et al., (2017). doi:10.1038/s41559-017-0081

The methods developed in this project were are transfered to other study areas (China, Syria):

Tong et al., (2016). doi:10.3390/rs8050357

Tong et al., (2017). doi:10.1016/j.jag.2016.09.013

Eklund et al. (2017) doi:10.1088/1748-9326/aa673a
Progress beyond the state of the art:
(1) we significantly advanced our knowledge on environmental change in the Sahel,
(2) we developed completely new methodologies to estimate woody cover (dynamics) in drylands,
(3) we advanced existing operational methods for estimating fodder biomass in Sahel,
(4) we developed a method to estimate green and non-green biomass in drylands separately,
(5) we established causality between human population growth and woody cover changes in sub-Saharan Africa,
(6) we found that farmlands in Sahel feature a greater woody cover than savannas; a finding that impacts on management and stakeholders,
(7) we explored a new data set, vegetation optical depth, that will likely be a major scientific tool in the future
(8) we transferred the knowledge to Syria and China, study areas of high interest in terms of human-environment studies.