Landscape changes of desertification in arid areas: the case of south-west Algeria.

This study aims to monitor the arid Algerian High Plateaus, a key region for pastoral activities which has suffered harsh and widespread degradation from the eighties. This area is not sufficiently known by the international scientific community. For this purpose, we considered phytoecological inventories and thematic maps that have been carried out during 30 years. Available data for the study are vegetation maps derived from aerial photographs (1975-1978) and from satellite imagery (2006). The parameters considered include vegetation, flora, and soil surface properties. The study area is part of the ROSELT/OSS (ROSELT: Réseau d'Observatoires de Surveillance Ecologique à Long Terme (Long Term Ecological Monitoring Observatories Network); OSS: Observatory of the Sahara and the Sahel) network observatory (OSS 2008). To assess land degradation, we used landscape ecology parameters. These include the number and surface area of vegetation units, synthesized by the large patch index and the Shannon landscape diversity index. All parameters reflect an increase in landscape heterogeneity. The largest decline is observed for Stipa tenacissima vegetation units constituting 2/3 of the landscape in 1978 and occupied just 1/10 in 2006. Vegetation units linked to degradation, such those dominated by Salsola vermiculata, inexistent in 1978, now dominate the steppe. Another result of the ongoing landscape degradation on the plateaus between 1975 and 2006 is the decrease of vegetation cover. In 1978, 1/3 of rangelands only had low vegetation covers, inferior to 15%. Presently 9/10 present the same class cover. This can be explained by severe spells of drought combined by an exponential rise of livestock during the last 30 years. This has in turn greatly undermined the fodder potential of the steppe. Results suggest that the "greening-up" described by several authors in the Sahel over the last 40 years is not observed in the Algerian, nor in the North African steppes. On the contrary, the desertification is still ongoing and the threshold of irreversibility seems to be imminent.