Desertification in the Sahel: Towards better accounting for ecosystem dynamics in the interpretation of remote sensing images

To date, the interpretation of remote sensing images has not revealed wide-spread degradation of the vegetation in the Sahel. However, the interpretation of spectral information depends on a range of assumptions regarding the dynamics of the Sahelian vegetation as a function of rainfall variability and human management. Recent papers have presented diverging views on the vegetation dynamics of the Sahel and how these can be analysed with remote sensing images. We present a further analysis of the vegetation dynamics of semi-arid rangelands, in particular the Sahel, and the subsequent implications for the interpretation of remote sensing images. Specifically, the ecological processes driving the response of the Sahelian vegetation to rainfall variation are re-examined, and a regression analysis of NPP versus rainfall data is carried out. It is shown that the relation between the interannual variation in NPP and rainfall in the Sahel is non-linear and that this relation differs between sites with different average annual rainfall. It has been common practise in remote sensing studies for the Sahel to aggregate data from various Sahelian sites in order to obtain an average relation between rainfall, NPP and Rain Use Efficiency, and to assume these relations to be linear. This paper shows that this approach may lead to a bias in the interpretation of remote sensing images and that further work is required to clarify if wide-spread ecosystem degradation has occurred in the Sahel.

[1]  Jennifer Small,et al.  Can human-induced land degradation be distinguished from the effects of rainfall variability? A case study in South Africa , 2007 .

[2]  S. Sullivan,et al.  On non‐equilibrium in arid and semi‐arid grazing systems , 2002 .

[3]  P. Hiernaux,et al.  Modelling tillering of annual grasses as a function of plant density: Application to Sahelian rangelands productivity and dynamics , 1994 .

[4]  L. Hein,et al.  Desertification in the Sahel: a reinterpretation , 2006 .

[5]  Osvaldo E. Sala,et al.  Long-term dynamics of water and carbon in semi-arid ecosystems: a gradient analysis in the Patagonian steppe , 2000, Plant Ecology.

[6]  Le Houérou,et al.  Rain use efficiency: a unifying concept in arid-land ecology , 1984 .

[7]  H. L. Houérou,et al.  Relationship between the variability of primary production and the variability of annual precipitation in world arid lands , 1988 .

[8]  Lars Hein,et al.  The impacts of grazing and rainfall variability on the dynamics of a Sahelian rangeland , 2006 .

[9]  S. Zaman.  Effects of rainfall and grazing on vegetation yield and cover of two arid rangelands in Kuwait , 1997, Environmental Conservation.

[10]  E. Christie.  Ecosystem Processes in Semiarid Grasslands. I Primary Production and Water Use of two Communities Possessing Different Photosynthetic Pathways , 1978 .

[11]  P. Hiernaux,et al.  Sahelian rangeland response to changes in rainfall over two decades in the Gourma region, Mali , 2009 .

[12]  Frédéric Baup,et al.  Trends in productivity of crops, fallow and rangelands in Southwest Niger: Impact of land use, management and variable rainfall , 2009 .

[13]  C. Tucker,et al.  Expansion and Contraction of the Sahara Desert from 1980 to 1990 , 1991, Science.

[14]  L. Milich,et al.  GAC NDVI interannual coefficient of variation (CoV) images: Ground truth sampling of the Sahel along north-south transects , 2000 .

[15]  L. M. Haines,et al.  Influence of precipitation and species composition on phytomass of a semi‐arid African grassland , 2001 .

[16]  Stephen D. Prince,et al.  Evidence from rain‐use efficiencies does not indicate extensive Sahelian desertification , 1998 .

[17]  Mike Hulme,et al.  Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.1083 COMMENT THE RECENT SAHEL DROUGHT IS REAL , 2004 .

[18]  D. Pollard,et al.  Decadal variability of rainfall in the Sahel: results from the coupled GENESIS-IBIS atmosphere-biosphere model , 2004 .

[19]  Sharon E. Nicholson,et al.  On the question of the ''recovery'' of the rains in the West African Sahel , 2005 .

[20]  Benjamin W. Heumann,et al.  AVHRR Derived Phenological Change in the Sahel and Soudan, Africa, 1982 - 2005 , 2007 .

[21]  Osvaldo E. Sala,et al.  CONTROLS OF GRASS AND SHRUB ABOVEGROUND PRODUCTION IN THE PATAGONIAN STEPPE , 2000 .

[22]  J F Reynolds,et al.  Biological Feedbacks in Global Desertification , 1990, Science.

[23]  O. Sala,et al.  Grassland Precipitation-Use Efficiency Varies Across a Resource Gradient , 1999, Ecosystems.

[24]  A. Diouf,et al.  Monitoring land-cover changes in semi-arid regions: remote sensing data and field observations in the Ferlo, Senegal , 2001 .

[25]  C. Tucker,et al.  Desertification in the Sahel: a reinterpretation of a reinterpretation , 2007 .

[26]  H Breman,et al.  Rangeland Productivity and Exploitation in the Sahel , 1983, Science.

[27]  J. Paruelo,et al.  Interactions of Water and Nitrogen on Primary Productivity Across Spatial and Temporal Scales in Grassland and Shrubland Ecosystems , 2019, Dryland Ecohydrology.

[28]  C. Tucker,et al.  Analysis of Sahelian vegetation dynamics using NOAA-AVHRR NDVI data from 1981–2003 , 2005 .

[29]  P. Rosnay,et al.  Multi-scale soil moisture measurements at the Gourma meso-scale site in Mali , 2009 .

[30]  Henrik von Wehrden,et al.  Long‐term degradation of Sahelian rangeland detected by 27 years of field study in Senegal , 2010 .

[31]  P. Hiernaux,et al.  Suivi du développement végétal au cours de l'été 1984 dans le Sahel Malien , 1986 .

[32]  E. Eyal,et al.  Plant and sheep production on semiarid annual grassland in Israel. , 1974 .

[33]  D. Milchunas,et al.  Production and rain use efficiency in short‐grass steppe: grazing history, defoliation and water resource , 1995 .

[34]  J. Ardö,et al.  A recent greening of the Sahel—trends, patterns and potential causes , 2005 .

[35]  S. Vetter.  Rangelands at equilibrium and non-equilibrium: recent developments in the debate , 2005 .

[36]  Lionel Jarlan,et al.  Testing a sahelian grassland functioning model against herbage mass measurements , 2006 .

[37]  C. Tucker,et al.  Desertification, Drought, and Surface Vegetation: An Example from the West African Sahel , 1998 .

[38]  Andrew W. Illius,et al.  ON THE RELEVANCE OF NONEQUILIBRIUM CONCEPTS TO ARID AND SEMIARID GRAZING SYSTEMS , 1999 .

[39]  H. Breman,et al.  Manual sur les pâturages des pays sahéliens. , 1991 .

[40]  Abdou Ali,et al.  Recent trends in the Central and Western Sahel rainfall regime (1990–2007) , 2009 .

[41]  K. Findell,et al.  Simulation of Sahel drought in the 20th and 21st centuries. , 2005, Proceedings of the National Academy of Sciences of the United States of America.

[42]  C. Nunes,et al.  Land-Use and Land-Cover Change (LUCC): Implementation Strategy , 1999 .

[43]  Lars Hein,et al.  The local impacts of climate change in the Ferlo, Western Sahel , 2009 .