International Journal of Applied Earth Observation and Geoinformation
Woody vegetation and land cover changes in the Sahel of Mali (1967–2011)
Introduction
The Sahel has been acclaimed as one of the “hot spots” of global environmental change in the last decades. As the 20th century progressed, settlements spread over the Sahel and most forests were cleared for agricultural purposes and an ever growing demand for wood (Brandt et al., 2014a). The degradation of the environmental conditions was accelerated by prolonged droughts in the region during the 1970s and 1980s and an overall decrease in annual precipitation (e.g. L’Hote et al., 2002). Scientists claimed deforestation to be the main causative factor for these climatic changes (Charney et al., 1975). However, several studies have shown that sea surface temperatures largely control Sahelian rainfall fluctuations (e.g. Giannini et al., 2008). Recently, investigators demonstrated again that land cover changes can have an accelerating effect on rainfall variations (e.g. Kucharski et al., 2012, Paeth et al., 2009). These studies put changes in land cover into the focus again and justify the need for detailed investigations on the actual extent of environmental change.
After the droughts in the 1970s and 1980s, the observed loss of woody vegetation cover was often considered as irreversible desertification and large parts of the Sahel were designated as degraded land (e.g. Kandji et al., 2006, Oldeman et al., 1990, Lamprey, 1988). However, almost no evidence of widespread degradation was found (e.g. Niemeijer and Mazzucato, 2002, Tiffen and Mortimore, 2002) and recent findings based on coarse-scaled analyses of satellite time series and ground data show an increase of vegetation greenness over most parts of the Sahel since the mid-1980s (e.g. Dardel et al., 2014, Brandt et al., 2014b, Herrmann et al., 2005, Olsson et al., 2005). However, due to a lack of historical data, it remains largely unclear if this is a return to pre-drought conditions or a transformation of land cover to a new equilibrium state.
High resolution imagery offers the possibility to detect single trees and large shrubs as objects. This has the major advantage that canopy cover can be directly mapped without the need to interpret mixed pixels by linear models (e.g. Herrmann et al., 2013, Larsson, 1993). This is an important factor, as the Sahelian vegetation largely depends on rainfall (Hickler et al., 2005) causing huge inter-annual variations in mixed pixels and making conventional change detection methods unreliable. This problem was often solved by trend analysis of time series (e.g. Brandt et al., 2014b, Anyamba and Tucker, 2005). However, these datasets begin in the 1980s and do not provide any information on the situation prior to the severe Sahel droughts. Beside aerial photography, Corona imagery from the 1960s is a source that offers unique pre-drought information on the Sahel. Moreover, it documents a time of beginning human expansion and clearance of natural bushland. So far, many studies use a qualitative approach, applying case studies and/or visual inspection to reconstruct the pre-drought Sahel with aerial photos and Corona imagery (e.g. Herrmann et al., 2013, Brandt et al., 2014a, Tappan et al., 2004, Gonzalez, 2001). Land- and tree-cover changes have also been mapped (e.g. San Emeterio and Mering, 2012, Ruelland et al., 2010, Tappan and McGahuey, 2007, Elmqvist, 2004, Tappan et al., 2000) using a variety of methods (see Ruelland et al., 2011). The studies revealed that most of the former bushland has been transformed to agricultural land and a significant reduction of tree density has been observed with a spreading of barren land and considerable impoverishment of woody species (Brandt et al., 2014a, Herrmann and Tappan, 2013, Gonzalez et al., 2012, Ruelland et al., 2010, Tappan et al., 2004, Elmqvist, 2004). These changes have significant effects on the ecosystem and people's daily lives. The dependence of the local population on the products from trees such as fire and construction wood, medicine and religious purposes (Maydell, 1990) is a factor of practical importance adding significance to regional-scaled environmental studies.
In greening and desertification debates, generalizations are commonly used, attempting to simplify a reality which is far more complex. We dismiss these paradigms and show the complexity and spatial variations on a local scale. High resolution panchromatic Corona imagery of 1967 and multispectral RapidEye imagery of 2011 form the basis of this study, which includes parts of the Dogon Plateau and the Séno Plain in the Sahelian zone of Mali. The two major aims are:
- 1.
To investigate and quantify land cover changes over 44 years including aspects of degradation and human expansion.
- 2.
To analyze changes in woody cover between 1967 and 2011 and find explanations for these.
Section snippets
Study area
The study area is located in the Mopti Region in Mali. It is approximately 3600 km2 large, featuring the towns of Sevaré in the north-west, and Bandiagara and Bankass in the east (see Fig. 1). Generally, the study area can be divided in the Dogon Plateau (75%) and the Seno Plain (25%) with the steep Bandiagara escarpment dividing the rocky plateau in the north from the sandy plains to the south. The plateau is inhabited by Dogon farmers and is characterized by a complex and rough morphology with
Land cover change
The land cover change map in Fig. 4 displays changes that occurred between 1967 and 2011 for all overlapping areas of the two datasets (see also Table 1). Large areas of the Seno Plain have been converted from dense woody vegetation to sparse woody vegetation. This reflects major land use changes in the Seno Plain during the twentieth century due to population increase and the spreading of settlements. During this time the land required for agriculture increased significantly so that large
Conclusion
The research presented in this study offers detailed insight on the state of the environmental change in the West African Sahel at a regional/local scale. Quantitative information on the woody vegetation cover and its change over 44 years is provided. Remote sensing, making use of high resolution Corona and RapidEye images of the years 1967 and 2011, supported by ground-truthing, is shown to be a useful tool for quantifying and comparing the land and woody vegetation cover over several decades
Acknowledgements
This research is part of the BMBF (German Ministry of Education and Research) funded project micle (grant number 01UV1007A/B) which aims to explain linkages between migration, climate and environment (www.micle-project.net). Many thanks go to Clemens Romankiewicz, who did the interviews with the local population and thus contributed greatly to this study. Furthermore, we would like to thank our African colleagues from the IER (Institute d’Economie Rurale) and the Eaux et Forêt for valuable
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