, Resilience and Re-greening in the African Sahel – A matter of the observation period?

Hannelore Kusserow

5 Department of Earth Sciences, Institute of Geographical Sciences, Remote Sensing and Geoinformatics, Freie Universitaet Berlin

Correspondence to: H. Kusserow ([email protected])

Abstract. Since the turn of the millennium various scientific publications have been discussing a re-greening of the Sahel after the 1980’s drought mainly based on coarse-resolution satellite data. However, own field studies 10 suggest that the situation is far more complex and both paradigms, the “Encroaching Sahara” and the “Re- greening Sahel”, need to be questioned. The article discusses the concepts of desertification, resilience and re-greening by addressing four main aspects: (i) the relevance of edaphic factors for a vegetation re-greening, (ii) importance of the selected observation period in the debate of Sahel greening or browning, and (iii) modifications in vegetation pattern as 15 possible indicators for ecosystem changes (shift from originally diffuse to contracted vegetation patterns). The data referred to in this article cover a time period of more than 150 years and include the author’s own research results from the early 1980s until today. A special emphasis, apart from field work data and remote sensing data, is laid on the historical documents. The key findings summarised at the end show the following: i) vegetation recovery predominantly depends on 20 soil types; ii) when discussing Sahel greening vs. Sahel browning, the majority of research articles only focuses on post-drought conditions. Taking pre-drought conditions (before 1980s) into account, however, is essential to fully understand the situation. Then botanical investigations and remote sensing based time series clearly show a substantial decline in woody species diversity and cover density compared to pre-drought conditions; iii) self- organised patchiness of vegetation is considered to be an important indicator for ecosystem changes. 25

Keywords Desertification, Re-greening, Resilience, Self-organised patchiness, Ecosystem engineering.

30 1 Introduction

The long-lasting and sometimes heated scientific debates on Sahel greening vs. Sahel browning inspired the author to critically analyse the existing research results from the viewpoint of over 30 years of own research activities in the African Sahel. The article discusses four key indicators relevant for the greening-browning 35 discussion: (i) the relevance of edaphic factors for a vegetation re-greening, (ii) plant species changes as a function of the selected observation period, (iii) remote sensing based vegetation changes as a function of the selected observation period, and (iv) modifications in vegetation pattern as possible indicators for ecosystem changes (shift from originally diffuse to contracted vegetation patterns). The discussion is based on own research results and contributions that are put into context of the scholarly debate of the issue. Thus, each section starts 40 with a description of own contributions followed by an analysis of results of other authors. The article is structured as follows: Besides an introduction into the motives of the author and the overall structure of the article, the current Sect. 1 provides a brief overview of the African Sahel and research methods developed and applied by the author. Sect. 2 provides a general overview of the history of the desertification debate and concepts of desertification, 45 resilience and re-greening. Sect. 3 discusses the relevance of edaphic factors for a vegetation re-greening with examples from Mali, Burkina Faso and Niger. Sect. 4 deals with plant species changes with examples from Mali and Dafur. Sect. 5 analyses the concept of a “re-greening Sahel” based on NOAA-AVHRR, GIMMS3g, MODIS, SPOT VGT studies with examples from Mali. Sect. 6 discusses changes in vegetation pattern and self-organise