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The Map Is not the Territory: How Satellite Remote Sensing and Ground Evidence Have Re-shaped the Image of Sahelian Desertification

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The End of Desertification?

Part of the book series: Springer Earth System Sciences ((SPRINGEREARTH))

Abstract

Satellite remote sensing, in particular the analysis of coarse resolution time series of vegetation indices, has played an important role in challenging earlier assumptions of widespread desertification in the Sahel. Findings of such analyses show a greening trend in much of the region since the early 1980s, which seems to suggest a positive development. On the other hand, a growing number of field studies of vegetation dynamics across the Sahel offer a more fine-scaled and nuanced picture of changes. Of particular interest with respect to degradation and rehabilitation is the woody component of the vegetation cover, which is less affected by short-term fluctuations in precipitation than the herbaceous component. We synthesized findings from published field studies on changes in the abundance and diversity of woody vegetation across the Sahel and spatially compared them with the remotely sensed greenness trends. Many field sites reported a decline in the abundance of woody vegetation since before the great droughts, in particular of large trees. In addition, the woody vegetation shifted from a diverse species composition towards fewer and more drought tolerant species in the majority of sites. However, some success stories of agroforestry management stood out as well, where formerly degraded farmlands were rehabilitated and in some cases have reached even higher tree densities than in the 1960s. The discrepancy between satellite-observed greening trends and changes in woody vegetation on the ground—in both directions—emphasizes the need of integrating multiple perspectives and scales in the interpretation of greening trends with respect to desertification.

“The Map is not the Territory” is an aphorism that goes back to the Polish-American scientist Alfred Korzybski and emphasizes that a representation of reality (=map) must not be confused with reality itself (=territory). Thus, maps and graphical data convey images that risk developing a life of their own, with the map preceding and even becoming the territory.

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Notes

  1. 1.

    Although the term “ground truth” is widely used in the remote sensing community to describe field observations that help interpret remote sensing imagery, it is disliked by many remote sensing scientists, because it might imply that satellite data are erroneous in some way and because reference data can come from sources not necessarily involving ground investigations. Despite its shortcomings, we use this term here for lack of a better and equally short alternative.

  2. 2.

    Since the NOAA AVHRR NDVI time series from 1981 to present combines observations from two different sensors flown on a series of fourteen satellites, inter-sensor calibration and bias correction for orbital drifts are prerequisites for the creation of long term stable NDVI time series (Pinzon and Tucker 2014). Much of the time series development has been done by the Global Inventory Modeling and Mapping Studies (GIMMS) group. As calibration and correction algorithms have evolved over time, each generation of the GIMMS NDVI time series shows slight differences from the previous one. One of the authors of this chapter compared trends of the most recent 3rd generation GIMMS NDVI with trends of the previous 2nd generation and found considerable differences in the spatial patterns of those trends for West Africa over an identical time period. Such differences, which are explained by data preprocessing alone, call into question the interpretability of NDVI trends from the NOAA AVHRR time series with respect to land degradation and desertification.

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Authors and Affiliations

  1. The University of Arizona, 1955 E. Sixth St., Tucson, AZ, 85719, USA

    Stefanie M. Herrmann

  2. Biocentre Klein Flottbek and Botanical Garden, Research Unit “Biodiversity Evolution and Ecology of Plants”, University of Hamburg, Ohnhorststr. 18, 22609, Hamburg, Germany

    Tene Kwetche Sop

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  1. Stefanie M. Herrmann
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  1. Great Wolford, Odessa Centre, Shipstpon-on-Stour, United Kingdom

    Roy Behnke

  2. Drylands Research, Milborne Port, United Kingdom

    Michael Mortimore

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Herrmann, S.M., Sop, T.K. (2016). The Map Is not the Territory: How Satellite Remote Sensing and Ground Evidence Have Re-shaped the Image of Sahelian Desertification. In: Behnke, R., Mortimore, M. (eds) The End of Desertification? . Springer Earth System Sciences. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-16014-1_5

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