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Universität Bayreuth (2014)

Environmental Change Based on Earth Observation and Field Data – A Local Study in the Sahel Zone of Mali and Senegal

Brandt, Martin

Titre : Environmental Change Based on Earth Observation and Field Data – A Local Study in the Sahel Zone of Mali and Senegal

Auteur : Brandt, Martin

Université de soutenance : Universität Bayreuth

Grade : Doctor of Philosophy (PhD) 2014

In the past 50 years the Sahel region has experienced significant environmental changes. Droughts, human expansion and a general decline in annual rainfall have led to theories of widespread and irreversible degradation. Recently, this paradigm has been largely replaced by a greening Sahel phenomenon, triggered by increasing rainfall, and observed in satellite based vegetation data. The purpose of this study is to assess local long term vegetation trends in the Sahel of Mali and Senegal by combining satellite datasets and field data. This thesis is designed to improve the knowledge base of the processes responsible for satellite derived trends and thus to shed more light on the degradation and re-greening debates of the Sahel zone. A variety of earth observation products at different spatial and temporal resolutions were acquired and processed for two study areas around Bandiagara (Sahel of Mali) and Linguere (Sahel of Senegal). Intensive ground-truthing using interdisciplinary methods validates and explains vegetation changes observed in satellite data. High-resolution Corona (1965) and RapidEye (2011) imagery show woody vegetation and land cover change at tree level. Results reveal a significant reduction of natural vegetation, an increase of trees in cultivated areas and a general increase of cultivated land. Moreover, encroachment of degraded land and a moderate reduction in tree cover can be observed in both study areas. Climate Research Unit (CRU) climate data show a significant and rapid increase in average annual temperature since the 1960s. Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Climatology Center (GPCC) rainfall data reveal that annual rainfall was 15% lower in Linguere (Sahel of Senegal) and 13% in Bandiagara (Sahel of Mali) for the period 1970—2010 compared to 1930-1970. However, both study areas have seen a significant increase in rainfall over the period 1982—2010 (34% in Linguere and 54% in Bandiagara), signifying a possible end of the prolonged dry period. Coarse scale time series were studied from 1982-2010 using Normalized Difference Vegetation Index (NDVI) from Long Term Data Record (LTDR) as well as Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) from Geoland Version 1 (GEOV1) (both 5 km pixel resolution) and GIMMS3g (8 km pixel resolution). The datasets agree that in both study areas significant greening trends can be observed over the studied time period but significant spatial discrepancies are observed at local scale. Annual rainfall increased since the 2000s, explaining more than 50% of the observed variations. The positive greenness trends can be confirmed by time series of ground measured biomass observations (since 1987), showing a strong increment in woody biomass since the 1980s. However, the higher resolution Moderate Resolution Imaging Spectroradiometer (MODIS) (250 m) and SPOT-Vegetation (VGT) (1 km) data identify a heterogeneous pattern of spatial variability and the presence of active land degradation at a local level, accounting for approximately 5-10% of each study areas. Reasons for land degradation are both climatic and anthropogenic : (1) drought events, less rain and a higher temperature, and (2) an increased demand for cultivated areas and wood, especially in dry periods. However, beside the presence of degradation, greening areas prevail and cannot solely be explained by rainfall. Although reasons are often site-specific, several factors are true for both study areas : (1) agroforestry on farmer’s fields, (2) strict protection laws, (3) large scale reforestation, and (4) a widespread dispersion of robust species, which are resilient to anthropogenic and climatic stress. In spite of the overall positive vegetation trends, a massive species impoverishment was disclosed by interviews with village elders and long term species monitoring. Apart from few dispersing species Balanites aegyptiaca, Acacia raddiana), most other woody species have seen a decline and local extinction. Woody vegetation changes strongly depend on soil properties, which control human impact, drought resilience and vulnerability to erosion. Therefore, a heterogeneous pattern of environmental changes can be observed at a local level and neither the degradation nor the greening paradigm can be generalized.

Mots clés : Sahel ; Greening ; Degradation ; Environmental Change ; Earth Observation ; Senegal ; Mali


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Page publiée le 5 mars 2015, mise à jour le 30 décembre 2018