New publications!


Our fantastic team had some new publications within the past half year, they are all worth to have a look:

This one deals with the question if agricultural intensification in Sahel causes an increase or decrease in NDVI trends. Surprisingly, we find a negative NDVI trend coupled with an increase in cropped areas which means that fallowed fields have a substantially higher NDVI than cropped fields.

Open access! Here we use great data sources to document dynamics in woody vegetation in central Senegal. Field data from 2000 to 2015, fantastic aerial photos from 1994, repeat photography from 1994 and 2015, satellite imagery at 50 cm resolution from 2005-2015, and finally MODIS time series. We find a high spatial and temporal dynamic, encroachment, die off, etc. It’s a very a colourfully illustrated study which will make you feel like travelling to Senegal..

A great story as well: We document how conservation projects in Southern China are able to impact on vegetation trends and propose an index which allows to put the invested money (for conservation projects) in relation with vegetation trends to be able to determine the project effectiveness.

A very clever way to combine optical and passive microwave satellite data: We assume that optical satellite data senses the green part of the vegetation and the passive microwaves the green plus non-green parts. So we combine both to estimate the non green vegetation (i.e. the wood) and look at global trends from 2000 to 2012 which allows us to map gradual gains and losses in woody cover.

Recent woody vegetation trends in Sahel

Our new paper looks at recent dynamics in woody vegetation in Sahel and finds some interesting patterns which are mainly controlled by human population density.

Martin Brandt, Pierre Hiernaux, Kjeld Rasmussen, Cheikh Mbow, Laurent Kergoat, Torbern Tagesson, Yahaya Ibrahim, Abdoulaye Wele, Compton J. Tucker, Rasmus Fensholt. Assessing woody vegetation trends in Sahelian drylands using MODIS based seasonal metrics. Remote Sensing of Environment, 2016, 183, 215-225.

  • Woody cover trends are estimated for Sahel based on MODIS dry season metrics.
  • Interannual fluctuations in foliage density are attenuated to monitor woody plant trends.
  • Increases (decreases) are seen in areas of low (high) human population.
  • Recent decreases only partially offset a general post-drought increase in Sahelian woody cover.

Woody plants play a major role for the resilience of drylands and in peoples’ livelihoods. However, due to their scattered distribution, quantifying and monitoring woody cover over space and time is challenging. We develop a phenology driven model and train/validate MODIS (MCD43A4, 500 m) derived metrics with 178 ground observations from Niger, Senegal and Mali to estimate woody cover trends from 2000 to 2014 over the entire Sahel at 500 m scale.

Over the 15 year period we observed an average increase of 1.7 (± 5.0) woody cover (%) with large spatial differences: No clear change can be observed in densely populated areas (0.2 ± 4.2), whereas a positive change is seen in sparsely populated areas (2.1 ± 5.2). Woody cover is generally stable in cropland areas (0.9 ± 4.6), reflecting the protective management of parkland trees by the farmers. Positive changes are observed in savannas (2.5 ± 5.4) and woodland areas (3.9 ± 7.3).

The major pattern of woody cover change reveals strong increases in the sparsely populated Sahel zones of eastern Senegal, western Mali and central Chad, but a decreasing trend is observed in the densely populated western parts of Senegal, northern Nigeria, Sudan and southwestern Niger. This decrease is often local and limited to woodlands, being an indication of ongoing expansion of cultivated areas and selective logging.

We show that an overall positive trend is found in areas of low anthropogenic pressure demonstrating the potential of these ecosystems to provide services such as carbon storage, if not over-utilized. Taken together, our results provide an unprecedented synthesis of woody cover dynamics in the Sahel, and point to land use and human population density as important drivers, however only partially and locally offsetting a general post-drought increase.


Fodder Biomass Monitoring in Sahelian Rangelands Using Phenological Metrics from FAPAR Time Series

From: Diouf, A.A., Brandt, M., Verger, A., Jarroudi, M.E., Djaby, B., Fensholt, R., Ndione, J.A., Tychon, B., 2015. Fodder Biomass Monitoring in Sahelian Rangelands Using Phenological Metrics from FAPAR Time Series. Remote Sensing 7, 9122–9148. doi:10.3390/rs70709122

Livestock farming constitutes the most widespread human activity and the dominant land use in rangeland ecosystems. At a global scale, it contributes 40% of the agricultural gross domestic product, and provides income for more than 1.3 billion people and nourishment for at least 800 million food-insecure people. In particular for the West African Sahel, livestock constitutes the first renewable resource and is mainly characterized by an extensive use of pastures in rangelands.


Since 1987 the Centre de Suivi Ecologique (CSE) operationally estimates the total annual biomass in Senegal in order to monitor the fodder availability of the national pastoral rangelands. Field data is collected along 1 km transects at 24 sites at the end of the wet season. Here, herbaceous and woody leaf biomass is measured and summed to the total available fodder biomass. This is done each year since 1987.

cse2 cse1


Using a linear regression with satellite images, the field data is projected to whole Senegal and gives stakeholders an estimation on the quantity and distribution of fodder biomass. Between 1987 and 1999, this method was implemented using the seasonal integrated NDVI (i.e., seasonal weighted average) from the Advanced Very High Resolution Radiometer (AVHRR) of the National Oceanic and Atmospheric Administration (NOAA) satellites acquired in Local Area Coverage (LAC) format at the CSE receiving station in Dakar. Since 2000, the 1-km SPOT-VEGETATION NDVI have been used.

In this context, we developed a new operational system for monitoring total fodder biomass, including both herbaceous and woody leaf biomass. The proposed method is based on multiple linear regression models using phenological variables derived from the seasonal dynamics of the FAPAR SPOT-VEGETATION time series and ground measurements of total biomass production collected in different Sahelian ecosystems in Senegal over 15 years.


A model with three variables – large seasonal integral (LINTG), length of growing season and end of season decreasing rate – performed best (MAE = 605 kg DM/ha; R² = 0.68) across Sahelian ecosystems in Senegal (data for the period 1999-2013). A model with annual maximum (PEAK) and start date of season showed similar performances (MAE = 625 kg DM/ha; R² = 0.64), allowing a timely estimation of forage availability. The subdivision of the study area using metrics related to ecosystem properties increased overall accuracy (MAE = 489.21 kg DM/ha; R² = 0.77). LINTG was the main explanatory variable for woody rangelands, whereas for areas dominated by herbaceous vegetation it was the PEAK metric. The proposed approach outperformed the established single-predictor model (MAE = 818 kg DM/ha and R² = 0.51) and should improve the operational monitoring of forage resources in Sahelian rangelands.


In the future, such early warning models should enable stakeholders to take decisions as early as September (current year as biomass shortage) with regard to livestock by triggering protocols designed for livestock management (e.g., Opération de Sauvegarde du Bétail ) in Senegal.

see the full document here: MDPI

Text and Figures: A.A. Diouf; Fotos: M. Brandt

What Four Decades of Earth Observation Tell us about Land Degradation in the Sahel

From: Mbow, C.; Brandt, M.; Ouedraogo, I.; de Leeuw, J.; Marshall, M. What Four Decades of Earth Observation Tell Us about Land Degradation in the Sahel? Remote Sens. 2015, 7, 4048-4067.

Land degradation mechanisms are related to two main categories, one related to climate change and one associated with local human impact, mostly land use change such as expansion of cultivation, agricultural intensification, overgrazing and overuse of woody vegetation. Land degradation characteristics, triggers and human influence are manifold and interrelated. Some of the indicators can be monitored using Earth Observation techniques (underlined in red):


During the last four decades, the Sahel was affected by below-normal precipitation with two severe drought periods in 1972–73 and in 1983–84. Because of this negative climate trend, many studies prioritized the Sahel “crisis” in terms of productivity loss and land degradation. These negative perceptions have been opposed with recent findings of improved greenness mostly in relation to recent improvement in rainfall.


The assessment of land degradation and quantifying its effects on land productivity have been both a scientific and political challenge. After four decades of Earth Observation applications, little agreement has been gained on the magnitude and direction of land degradation in the Sahel. The number of Earth Observation datasets and methods, biophysical and social drivers and the complexity of interactions make it difficult to apply aggregated Earth Observation indices for these non-linear processes. Hence, while many studies stress that the Sahel is greening, others indicate no trend or browning. The different generations of satellite sensors, the granularity of studies, the study period, the applied indices and the assumptions and/or computational methods impact these trends.



While there is a clearly positive trend in biomass production at Sahel scale, a loss in biodiversity and locally encroaching barren land are observed at the same time. Multi-scale Earth Observation analyses show that neither the desertification nor the greening paradigms can be generalized, as both attempt to simplify a very complex reality. Heterogeneity is an issue of scale, and very coarse-scaled vegetation trend analyses reveal a greening Sahel. However, locally-scaled studies are not uniform, observing greening and degradation at the same time.

We suggest several improvements: (1) harmonize time-series data, (2) promote knowledge networks, (3) improve data-access, (4) fill data gaps, (5) agree on scales and assumptions, (6) set up a denser network of long-term fields-surveys and (7) consider local perceptions and social dynamics, as local people’s perception of land degradation/improvements often disagree with Earth Observation analyses.

Thus, to allow multiple perspectives and avoid erroneous interpretations caused by data quality/scale issues/generalizations, we recommend combining multiple data sources at multiple scales. Furthermore, we underline the relevance of field data and experience, and results achieved by remote sensing techniques should not be interpreted without contextual knowledge.

Download the full article here: Paper at MDPI

see also:

Knauer, K., Gessner, U., Dech, S., Kuenzer, C., 2014. Remote sensing of vegetation dynamics in West Africa. International Journal of Remote Sensing 35, 6357–6396. doi:10.1080/01431161.2014.954062

New project started: BICSA – Biophysical Changes in the Sahel

To continue my research about the Sahel area, I received a Marie Curie Individual Fellowships (Call: H2020-MSCA-IF-2014) for the coming 2 years. The new project which started on May the 1st is named BICSA – Biophysical Changes in the Sahel: Ground and Satellite Based Evidence Across Scales and Disciplines. This is the abstract of the successful application:

Human and climate induced desertification has been a major issue for livelihoods and food security in drylands. In this context, the Sahel has been subject to various controversial studies. Earth Observation (EO) studies show a positive trend in vegetation greenness over the last decades, which has been interpreted as an increase in biomass and contradicts prevailing narratives of widespread degradation. However, new scientific outcome suggests a massive loss in biodiversity, which again contradicts the beneficial effects of the greening theory. These apparent oppositions result from little investment that has been made in studying long-term ground data. Thus, the overall purpose of this project is to assess the opposing trends of biomass increase and species decline in the Sahel. By combining a range of long-term in-situ field data records (1980s-today) with EO time series and Very High Resolution (VHR) satellite imagery, an improved understanding on the role of trees, herbs and species on the greening Sahel will be achieved. Trends will be translated in ecosystem services and beneficial effects on livelihoods. Knowing the underlying biophysical mechanisms of the Sahel greening will resolve contradictions regarding the greening/desertification paradigms and thus be basis for future studies. Furthermore, the scientific understanding of linkages between ground and satellite data and their applicability across scales will be improved. New monitoring methods of biophysical variables address challenges in land management and food security. To achieve this, I will be trained in cutting edge skills (EO time series; object based mapping; field monitoring of vegetation productivity/biodiversity; socializing pixels; ecological services). Finally, this project will encourage a North-South collaboration in common scientific interest that is relevant for development and environmental research.

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.


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


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: 


  • 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.


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.

Climate, Environment and Vegetation in the West African Sahel

Here’s a press release summarizing parts of my PhD. It was released by the University of Bayreuth ( and IDW ( in German (thanks to Pierre Gosselin for translating). As I got many emails on this, we want to make clear at this point that we do not aim to question climate change with this article! It shall give background information on the greening of the Sahel/desertification debates, and not on climatic changes/variability.

New research works show: Not global climate change alone, but rather foremost the local actions of people impact the face of their environment

Are the earth’s deserts continuously expanding? Or is green vegetation now spreading into regions that were once barren deserts? The West African section of the Sahel zone located at the southernmost edge of the Sahara, which extends from the Atlantic to the Red Sea, has been the source of reason for a wide variety of prognoses over the recent years. Extreme periods of drought during the 1970s and 1980s were considered as indices of growing desert regions across the globe. “Desertification” was the buzzword. However, over the last two decades a rise in precipitation has been observed across the West African Sahel. As a result there has been talk about the blanket perception that “the desert is greening”.

With this controversy as the backdrop, an international research team led by geographer Martin Brandt of the University of Bayreuth examined the vegetation development in the West African Sahel more closely. High and coarse resolution satellite data as well as wide range of measurement results from the last decades enabled conclusions to be drawn on climate and vegetation trends and field research brought regional and local particularities to light. Here some determinations were made: There is no uniform development in the West African Sahel. Not only the climate but also especially various forms of land-use – farming, forestry management or village development – are mostly responsible for the way the landscape there appears, and which resources it offers the people.

In the journal “remote sensing” researchers from Bayreuth (Germany), France, Spain and the Senegal report on their results. “The activity of man on location, for example the sustainable cultivation of selected green plants or the reforestation of forests, can impact the face of the landscape considerably,” says Martin Brandt. “Such initiatives and measures by the local population are far less dependent on large-scale climatic trends than what was earlier assumed. For this reason environmental and climate research should not be one-sidedly guided by blanket buzzwords such as ‘desertification’ or ‘greening Sahel’.”

Regional differences due to land and forest management – case studies in Mali and in the Senegal

Thanks to  satellite time series analyses , the scientists were able to determine that the vegetation density in the West African Sahel increased from 1982 to 2010. This development is especially pronounced in the Senegal and in western Mali. Here there are clear regional differences with respect to plants that have multiplied over time: Not only does one observe the wild growth of trees, bushes and grass, but also foremost the expansion of crops and plants due to farm and forest management measures. In total one notices that in the West African countries, with the exception of Gambia and the Ivory Coast, the forest levels have decreased markedly even though the vegetation density has increased as a whole.

Bayreuth_2The field research work by Martin Brandt (left) concentrated on two regions: the Senegal and Mali: The region surrounding the city of Bandiagara in southern Mali has seen a complete transformation of its vegetation over the last 50 years: Many tree and bush types that were still common in the 1960s have disappeared. Periods of drought did not alone damage the plants through a lack of water, but also it was because income from agriculture fell due to poor harvests, and so the people tried to compensate by felling trees and selling lumber. However in the meantime, a vegetation-rich landscape has since appeared – and not only because the precipitation amounts have been increasing for two decades and extended periods of droughts have failed to occur. “A targeted reforestation and planting of trees on agricultural land have changed the landscape considerably,” reports Brandt, and adds: “Without a sound botanical and ecological knowledge by the local population, this development would not have been possible.”

The transformation to an agricultural landscape  was also found by the scientists from Bayreuth at another region – one located in the Senegal, north of the city of Linguère. This region is mainly settled by nomads belonging to the Fulbe ethnic group who practice intensive pasture farming. In order to feed their livestock with leaves during dry periods, they cut or fell trees and bushes during dry periods. Nevertheless, state-sponsored reforestation and protective measures have led to a considerable increase in vegetation over the last two decades and it has become more adaptable to climate fluctuations. Today three especially robust tree types make up more than 90% of the vegetation found in the region surrounding Linguère. “Alone in the immediate proximity of the city there is a fenced-in area of at least 5000 hectares on which a special species of acacia has been placed,” says Martin Brandt. However he also points to the unmistakable damage in some places arising from the overuse of the tree stock. This completely bare ground is very difficult to regenerate – an example of how intervention into the vegetation by man can be destructive when it is not approached with ecological farsightedness.

Intervention by man stimulates a differentiated agricultural landscape – Plea for research without blanket buzzwords

The newly published scientific results refute the claims that the West Sahel is being hit by a growing desert that is a consequence of a global climate change. However they also refute the suggestion that the “greening of the desert” will take off by itself due to the increasing annual precipitation. The moderate trend reversal after a severe period of drought indeed does entail an increase in vegetation density. But it neither means a return to the conditions that existed before these extreme climatic events, nor does it automatically mean a widespread growth of green vegetation. Moreover, anthropogenic factors – and conversely their absence – have had a decisive impact on landscape and vegetation. Targeted farm and forest management measures that are oriented on scientific knowledge can significantly foster a differentiated man-made landscape.

Martin Brandt, who will soon receive his doctorate at the University of Bayreuth, also here sees reason for hope: “Should the climate prognoses of the UN IPCC do come true, the living conditions in some arid and semi-arid regions of West Africa – foremost in the region of the Sahel zone – will worsen. Appropriate concepts for land and forest management and for environmental protection however do offer the opportunity to adapt quickly enough to such climate developments in order to soften the impacts on poeple..”

International support project

The research works in Mali and in the Senegal were part of an international project: “Climate Change, Environmental Changes and Migration: Social-Ecological Conditions of Population Movements with the Example of the Sahel Countries Mali and Senegal (micle)”. The “micle” research project was funded from 2010 to April 2014 by the German Ministry for Education and Science (BMBF) and coordinated by the Institute for Social Ecological Research (ISOE) in Frankfurt. The Geographical Institute of the University of Bayreuth – together with the Institute for Geography and Regional Research of the University of Vienna – was involved as an associated partner. The leadership of the sub-project “Physical Geographical Perspectives ” was done by Prof. Dr. Cyrus Samimi, who today leads the research group for climatology at the University of Bayreuth. Prof. Dr. Martin Doevenspeck , Professor Regional-Related Conflict Research at the University of Bayreuth, was responsible for the sub-project “Social-Geographic Perspectives”.


Martin Brandt, Aleixandre Verger, Abdoul Aziz Diouf, Frederic Baret and Cyrus Samimi, Local Vegetation Trends in the Sahel of Mali and Senegal Using Long Time Series FAPAR Satellite Products and Field Measurement (1982–2010), in: Remote Sensing 2014, 6, pp. 2408-2434 DOI:10.3390/rs6032408.

Photos: University of Bayreuth


Modeling Soil and Woody Vegetation in the Senegalese Sahel in the Context of Environmental Change

We have another interesting article about environmental change in the Sahel online. For this article, we used a Random Forest classifier and remote sensing products, to model a soil map in the Senegalese Sahel. If people are interested, I can put the R code online.

Brandt, M.; Grau, T.; Mbow, C.; Samimi, C. Modeling Soil and Woody Vegetation in the Senegalese Sahel in the Context of Environmental Change. Land 20143, 770-792.

it’s open access:

Abstract: Climatic stress and anthropogenic disturbances have caused significant environmental changes in the Sahel. In this context, the importance of soil is often underrepresented. Thus, we analyze and discuss the interdependency of soil and vegetation by classifying soil types and its woody cover for a region in the Senegalese Ferlo. Clustering of 28 soil parameters led to four soil types which correspond with local Wolof denotations: Dek, Bowel, Dior and Bardial. The soil types were confirmed by a Non-metric Multidimensional-Scaling (NMDS) ordination and extrapolated via a Random Forest classifier using six significant variables derived from Landsat imagery and a digital elevation model (out-of-bag error rate: 7.3%). In addition, canopy cover was modeled using Landsat and a Reduced-Major-Axis (RMA) regression (R2 = 0.81). A woody vegetation survey showed that every soil type has its own species composition. However, 29% of Bowel regions are deforested (i.e., degraded) and interviews revealed extensive environmental changes and a strong decline and local extinction of woody species. The differences between the soil types are significant, showing that vegetation changes (i.e., degradation and greening), resilience to climatic stress and human activities largely depend on soil properties. We highlight that spatial heterogeneity is an important aspect when dealing with environmental changes in the Sahel, and local knowledge can be well used to classify spatial units by means of public Earth observation data.


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:

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.