Environmental change in the Sahel, Research results

Tree cover promoted in semi-arid Sahelian farms – new publication in Nature Geoscience

More people equal more trees in semi-arid West Africa – Our new study published in Nature Geoscience questions ‘received wisdom’ as concerns the relationship between human agency and woody vegetation of West Africa. We demonstrate that in low-rainfall areas woody cover is denser in cultivated areas than in savannas, and close to settlements rather that further away, thus rejecting simplistic Malthusian ideas of a negative relationship between population density and woody cover.

These findings challenge the traditional view on agricultural expansion in semi-arid lands and this has implications for the understanding of effects of agricultural expansion on ecosystem services, including carbon sequestration. Also, these findings throw light upon the process of land degradation/desertification which contradicts commonly believed narratives on human expansion in drylands causing fuel-wood crisis, deforestation, soil depletion, erosion and desertification.

 The study is part of an unprecedented NASA project (lead by CJ Tucker), which aims at applying commercial DigitalGlobe satellite imagery with a spatial resolution of 50 cm to map the size of each individual tree and shrub in African dryland ecosystems. Our team had access to thousands of these images, and this study is the first allowing a wall-to-wall map of woody cover based on individual trees.

In contrast to traditional case studies prone to sampling errors and bias by the prevailing societal discourse, the woody cover map includes 40,000 villages, passing a technical tipping point in dryland environmental research.

Article in Nature Geoscience:

Brandt, M., Rasmussen, K., Hiernaux, P., Herrmann, S., Tucker, C.J., Tong, X., Tian, F., Mertz, O., Kergoat, L., Mbow, C., David, J.L., Melocik, K.A., Dendoncker, M., Vincke, C., Fensholt, R., 2018. Reduction of tree cover in West African woodlands and promotion in semi-arid farmlands. Nature Geoscience 1. https://doi.org/10.1038/s41561-018-0092-x

Further read in Nature Geoscience News (summary by Niall Hanan):

https://www.nature.com/articles/s41561-018-0112-x

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Environmental change in the Sahel, Research results

New publications!

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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.

Environmental change in the Sahel, Research results

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.

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Environmental change in the Sahel, Research results

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.

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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.

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cseb

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.

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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.

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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

Environmental change in the Sahel, Research results

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):

1

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.

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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.

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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
Environmental change in the Sahel

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.

Environmental change in the Sahel, Research results

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.