Ground- and satellite-based evidence of the biophysical mechanisms behind the greening Sahel

Making use of 27 years of ground measurements, we were able to find evidence of the role of trees and grass on the greening of the Senegalese Sahel. This was made possible by a close collaboration with our colleagues from the CSE, the Centre de Suivi Ecologique in Dakar. Moreover, woody species abundance data provided by Gray Tappan from 1983 shows changes in biodiversity over 30 years. We thus provide ground based evidences against the conventional view of irreversible degradation in the Sahel.

Keywords:

  • biodiversity;
  • biomass monitoring;
  • degradation;
  • greening;
  • Sahel;
  • vegetation change

Abstract

After a dry period with prolonged droughts in the 1970s and 1980s, recent scientific outcome suggests that the decades of abnormally dry conditions in the Sahel have been reversed by positive anomalies in rainfall. Various remote sensing studies observed a positive trend in vegetation greenness over the last decades which is known as the re-greening of the Sahel. However, little investment has been made in including long-term ground-based data collections to evaluate and better understand the biophysical mechanisms behind these findings. Thus, deductions on a possible increment in biomass remain speculative. Our aim is to bridge these gaps and give specifics on the biophysical background factors of the re-greening Sahel. Therefore, a trend analysis was applied on long time series (1987–2013) of satellite-based vegetation and rainfall data, as well as on ground-observations of leaf biomass of woody species, herb biomass, and woody species abundance in different ecosystems located in the Sahel zone of Senegal. We found that the positive trend observed in satellite vegetation time series (+36%) is caused by an increment of in situ measured biomass (+34%), which is highly controlled by precipitation (+40%). Whereas herb biomass shows large inter-annual fluctuations rather than a clear trend, leaf biomass of woody species has doubled within 27 years (+103%). This increase in woody biomass did not reflect on biodiversity with 11 of 16 woody species declining in abundance over the period. We conclude that the observed greening in the Senegalese Sahel is primarily related to an increasing tree cover that caused satellite-driven vegetation indices to increase with rainfall reversal.

Brandt, M., Mbow, C., Diouf, A.A., Verger, A., Samimi, C. & R. Fensholt (2015) Ground and satellite based evidence of the biophysical mechanisms behind the greening Sahel. Global Change Biology.

Woody vegetation and land cover changes in the Sahel of Mali (1967–2011)

Another very interesting publication using object based methods to detect single trees on very high resolution imagery is online.

Raphael Spiekermann, Martin Brandt, Cyrus Samimi, Woody vegetation and land cover changes inthe Sahel of Mali (1967–2011), International Journal of Applied Earth Observation and Geoinformation, Volume 34, February 2015, Pages 113-121.

It can be downloaded for free until late October using this link: http://authors.elsevier.com/a/1PeJA14ynR~DWs 

Highlights:

  • Woody cover, species and land cover change over 44 years are analyzed.
  • Object-based classifications are applied with high resolution images of 1967 and 2011.
  • Climate and especially human impact have caused extensive changes.
  • Changes are not always negative and a variety of spatial variations are shown.

Abstract:

In the past 50 years, the Sahel has experienced significant tree- and land cover changes accelerated by human expansion and prolonged droughts during the 1970s and 1980s. This study uses remote sensing techniques, supplemented by ground-truth data to compare pre-drought woody vegetation and land cover with the situation in 2011. High resolution panchromatic Corona imagery of 1967 and multi-spectral RapidEye imagery of 2011 form the basis of this regional scaled study, which is focused on the Dogon Plateau and the Seno Plain in the Sahel zone of Mali. Object-based feature extraction and classifications are used to analyze the datasets and map land cover and woody vegetation changes over 44 years. Interviews add information about changes in species compositions. Results show a significant increase of cultivated land, a reduction of dense natural vegetation as well as an increase of trees on farmer’s fields. Mean woody cover decreased in the plains (−4%) but is stable on the plateau (+1%) although stark spatial discrepancies exist. Species decline and encroachment of degraded land are observed. However, the direction of change is not always negative and a variety of spatial variations are shown. Although the impact of climate is obvious, we demonstrate that anthropogenic activities have been the main drivers of change.

Environmental change in time series – An interdisciplinary study in the Sahel of Mali and Senegal

Our next article is now online in the Journal of Arid Environments:

It is available here: http://www.sciencedirect.com/science/article/pii/S0140196314000536

Abstract: Climatic changes and human activities have caused major environmental change in the Sahel. Remote sensing studies detect various vegetation trends; however, explanations are rarely studied in detail. We present a methodology using time series, high-resolution imagery and fieldwork to validate trend analyses for two regions in the Sahel of Mali and Senegal. Both study areas show significant greening trends from 1982 to 2010. Reasons can be very site-specific, but several factors are valid for both research areas: (1) farmer-managed agro-forestry, (2) planting programs and protection laws, (3) widespread dispersion of robust species, which replace the former diverse woody vegetation and simulate a greening which conceals a shift in biodiversity and (4) an increase of annual rainfall. However, the situation is still far from the pre-drought conditions, which are reconstructed by Corona imagery (1965) and interviews with the local population. Rather a transformation is observed: a decrease in natural vegetation, tree density and diversity. Reasons are climatic and anthropogenic: (1) drought events, less rain and higher temperatures, (2) increased demand for cropping areas and wood, especially in times of droughts. Our example validates that climatic factors are important drivers of change, but much of today’s environment and vegetation composition is controlled by humans.

Local Vegetation Trends in the Sahel of Mali and Senegal Using Long Time Series FAPAR Satellite Products and Field Measurement (1982–2010)

We finally published an article dealing with local vegetation trends in the Sahel and data quality of long term time series (GEOV1 and GIMMS3g). It is published in the open access journal “Remote Sensing” and can be downloaded for free:

http://www.mdpi.com/2072-4292/6/3/2408

Brandt, Martin; Verger, Aleixandre; Diouf, Abdoul A.; Baret, Frédéric; Samimi, Cyrus. 2014. “Local Vegetation Trends in the Sahel of Mali and Senegal Using Long Time Series FAPAR Satellite Products and Field Measurement (1982–2010).” Remote Sens. 6, no. 3: 2408-2434.

Abstract: Local vegetation trends in the Sahel of Mali and Senegal from Geoland Version 1 (GEOV1) (5 km) and the third generation Global Inventory Modeling and Mapping Studies (GIMMS3g) (8 km) Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) time series are studied over 29 years. For validation and interpretation of observed greenness trends, two methods are applied: (1) a qualitative approach using in-depth knowledge of the study areas and (2) a quantitative approach by time series of biomass observations and rainfall data. Significant greening trends from 1982 to 2010 are consistently observed in both GEOV1 and GIMMS3g FAPAR datasets. Annual rainfall increased significantly during the observed time period, explaining large parts of FAPAR variations at a regional scale. Locally, GEOV1 data reveals a heterogeneous pattern of vegetation change, which is confirmed by long-term ground data and site visits. The spatial variability in the observed vegetation trends in the Sahel area are mainly caused by varying tree- and land-cover, which are controlled by human impact, soil and drought resilience. A large proportion of the positive trends are caused by the increment in leaf biomass of woody species that has almost doubled since the 1980s due to a tree cover regeneration after a dry-period. This confirms the re-greening of the Sahel, however, degradation is also present and sometimes obscured by greening. GEOV1 as compared to GIMMS3g made it possible to better characterize the spatial pattern of trends and identify the degraded areas in the study region.

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