The Maghreb (North Africa) rangelands evolution over forty years: re-greening or degradation

Abstract

The Sahel region is subject of several controversies over the current trends of its ecosystems. Some consider that despite the rain recovery, this region remains in a regressive dynamics, while others consider that they are in re-greening, according to the studies using remote sensing. However, in North Africa, and more specifically in the Maghreb countries, despite numerous studies, this controversy is rarely discussed. The question asked here is whether this region which include most of the North African arid steppes and the largest desert of the world (the Sahara), is in regressive or progressive dynamics . In 1975, three Maghreb countries, Algeria, Tunisia, and Morocco, have launched national programs for monitoring and assessing ecosystems dynamics through the establishement of several observatories becoming fully operational in 1978. Afterwards, they had joined the international ROSELT network (Long Term Ecological Monitoring Observatories Network) implemented and coordinated by the OSS (Observatory of the Sahara and the Sahel) created in 1992. It relies on national studies, at regional or local level (observation stations) and standardize their monitoring methodologies to obtain comparable data. The results showed that this area has undergone profound changes of both physical and biological components. The parameters considered include vegetation, and soil surface properties. In particular, the results show that land degradation, especially from 1980 onwards, is the main feature of this evolution due to the combination of an outsized livestock and the occurrence of repeated and exceptional drought. The former vegetation communities dominated by Stipa tenacissima, Lygeum spartum and Artemisia herba alba have been mostly disappeared. New species, like Atractyis serratuloides, Salsola vermiculata, Hammada scoparia, and Noaea mucronata, which were rare in 1978, became dominant, in relation to degradation. The largest decline was observed for Stipa tenacissima vegetation units constituting 33% of the landscape in 1978 and occupying just 2% in 2011. The total vegetation cover has also significantly declined from more 34% in 1978 to less than 20 % in 2011. They also been subject to significant biodiversity loss and reduced richness. The number of species inventoried were 234, 134 and only 91 respectively in 1978 , 2005 and 2011. Since 2007, precipitation has been considerably improved but this was not accompanied with significant increase of vegetation cover and biodiversity even if the sandy fractions become less represented in the soil surface. Remote sensing, however, showed that the trend, as in the Sahel, is moving towards either towards a regreening or a stationary state. There is therefore an apparent contradiction between the “remote sensing” and the field data. It is considered that the soil moisture, the soil surface and the roughness, have probably much more influence on the spectral response than chlorophyll in vegetation, causing confusion in the data interpretation. This emphasizes the limits of the use of NDVI in arid zones, especially in North Africa.