Landscape Changes of Desertification in Arid Areas: the Case of South-West Algeria
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Environ Monit Assess DOI 10.1007/s10661-010-1744-5 Landscape changes of desertification in arid areas: the case of south-west Algeria Aziz Hirche · Mostefa Salamani · Abdelkader Abdellaoui · Salima Benhouhou · Jaime Martínez Valderrama Received: 4 February 2010 / Accepted: 4 October 2010 © Springer Science+Business Media B.V. 2010 Abstract This study aims to monitor the arid matic maps that have been carried out during Algerian High Plateaus, a key region for pas- 30 years. Available data for the study are veg- toral activities which has suffered harsh and etation maps derived from aerial photographs widespread degradation from the eighties. This (1975–1978) and from satellite imagery (2006). area is not sufficiently known by the interna- The parameters considered include vegetation, tional scientific community. For this purpose, we flora, and soil surface properties. The study area considered phytoecological inventories and the- is part of the ROSELT/OSS (ROSELT: Réseau d‘Observatoires de Surveillance Ecologique à Long Terme (Long Term Ecological Monitor- A. Hirche (B) · M. Salamani ing Observatories Network); OSS: Observatory Laboratory of Vegetal Ecology, Faculty of Biological of the Sahara and the Sahel) network obser- Sciences, University of Sciences and Technology vatory (OSS 2008). To assess land degradation, Houari Boumediene, BP 32 El Alia, we used landscape ecology parameters. These in- Bab Ezzouar, 1611, Algiers, Algeria e-mail: [email protected] clude the number and surface area of vegeta- tion units, synthesized by the large patch index A. Hirche · M. Salamani and the Shannon landscape diversity index. All Scientific and Technical Research Center on Arid parameters reflect an increase in landscape het- Regions, Front de l’oued, BP n◦1682 R. P, 07000, Biskra, Algeria erogeneity. The largest decline is observed for Stipa tenacissima vegetation units constituting 2/3 A. Abdellaoui of the landscape in 1978 and occupied just 1/10 UFR Human Sciences Laboratory, Geography in 2006. Vegetation units linked to degradation, Department, University of Paris XII, 61 Avenue du Général de Gaulle, 94010, Créteil, France such those dominated by Salsola vermiculata,in- existent in 1978, now dominate the steppe. An- S. Benhouhou other result of the ongoing landscape degrada- Ecole Nationale Supérieure Agronomique, tion on the plateaus between 1975 and 2006 is Hacène Badi-El Harrach, 16200, Algiers, Algeria the decrease of vegetation cover. In 1978, 1/3 of J. M. Valderrama rangelands only had low vegetation covers, in- Arid Zone Experimental Station (Estación ferior to 15%. Presently 9/10 present the same Experimental de Zonas Aridas)/CSIC (Consejo class cover. This can be explained by severe spells Superior de Investigaciones Científicas), Carretera de Sacramento s/n La Cañada de San Urbano, of drought combined by an exponential rise of 04120, Almeria, Spain livestock during the last 30 years. This has in Environ Monit Assess turn greatly undermined the fodder potential of geographic information systems (GIS) and remote the steppe. Results suggest that the “greening-up” sensing tools. The interest of the approach is to described by several authors in the Sahel over the consider potential landscape heterogeneity as the last 40 years is not observed in the Algerian, nor result of land degradation. in the North African steppes. On the contrary, the In 1975, the Centre de Recherches sur les desertification is still ongoing and the threshold of Ressources Biologiques et Terrestres (CRBT) irreversibility seems to be imminent. carried out a major vegetation monitoring pro- gram in the south Oran steppe producing thematic Keywords Change detection · Heterogeneity · maps of the entire area. Also, permanent observa- Landscape ecology · Desertification · Remote tion sites have been set up to collect more accurate sensing data. The latter have been gathered for 30 years of observations. Results from similar studies will provide a gen- Introduction eral overview for the North African countries and it would be possible to carry out rigorous Land degradation, long an important environmen- comparisons with landscape changes within the tal issue in arid and semi-arid lands, is now acute Sahelian region. Researchers of both the Sahelian in Algeria’s high plateaus. The Algerian steppes and North African regions collaborate presently are the most widespread rangeland of the North in the same major research project, “DeSurvey”, African countries. They occupy a pivotal posi- which constitutes an ideal framework to exchange tion between the hilly and humid north, called scientific data and experiences. the Tell, composing 5% of Algeria’s surface, and the south formed by the Sahara covering 86% of the nation’s territory. The Sahara is the largest Material and methods desert of the planet (Tucker et al. 1991). Never- theless, relatively few scientific works had been The study area edited in English up to now (Benhouhou et al. 2003). Early studies based on remote sensing data The study area occupies two thirds of Algeria’s and techniques undertaken with the objective of south-western High Plateaus. It is administered by monitoring land conditions found no evidence the wilaya of Nâama (Fig. 1). The average altitude for extensive degradation in the Sahelian region is around 1,000 m and it is primarily occupied (Helldén 1984, 1991;Princeetal.1998; Tucker and by Quaternary polygenic glacis. It includes saline Nicholson 1999; Rasmussen et al. 2001). On the depressions such as Chott Chergui. The area is contrary, recent studies of the Sahelian vegetation surrounded by chain of mountains such as the based on remote sensing data even emphasize Antar djebel which culminates at 1,700 m. “the greening-up” of the Sahel (Anyamba and Tucker 2005; Helldén 2008; Helldén and Tottrup Climate 2008). At the North African area, the results are globally in contradiction with these results. The Climatic data was gathered from the National aim of this study is to contribute some elemental Meteorology Office and cover a long period from reflections to this issue by evaluating qualitatively 1907 to 2004 (ONM 2008). The climate is char- and quantitatively the phenomenon of land evo- acterized by low annual rainfall with averages lution in a representative area in the south Oran ranging from 263 mm in Mecheria (the center steppe (about 10 million hectares). The study area of the study area) to 287.5 mm in El Bayadh covers approximately 400,000 ha and the time (a mountainous area surrounding the north-east). horizon considered is 30 years. Data analyses are The distribution of rainfall is very irregular. based on indices borrowed from landscape ecol- The coefficient of variability reached 43.2% in ogy: the large patch index (LPI), the Shannon Mecheria and 31.3% in El Bayadh. Winters are landscape diversity index (SHDI), and the use of cold and harsh while summers are hot and dry. Environ Monit Assess Fig. 1 General locating of survey zone To illustrate the climatic context, the evolution the 1983–1985 period after the first drought period of rainfall since the beginning of the century is that began in 1968. graphed (Fig. 2). Since the end of the 1970s, the mobile aver- The rainfall evolution shows an alternation of ages are almost always below the average rain- dry and humid periods with no clear trends during fall. Temperatures show average minimum val- approximately the first half of the century. At the ues for the coldest month (January) of 1.5◦C end of the second half, dry periods become no- and −1.8◦C for Mecheria and El Bayadh, re- tably more frequent and intense, especially during spectively, and average maximum values of the the 1981–1988 years (Hirche et al. 2007) consid- hottest month (July) of 35.1◦C and 33.5◦C. For all ered to be the driest period of the last century. It stations, the dry period (Bagnouls and Gaussen is remarkable to note that this drought period is 1953) extends over 6 months and the bioclimate partially the same as for the Sahel which has been belongs to the medium-arid class with cold winters affected by a notable large-scale drought during variant. Fig. 2 Evolution of 700 MECHERIA BAYADH annual rainfall since the MEDIAN MECHERIA MEDIAN BAYADH beginning of the century MOBILE AVERAGE 7 Per 600 MOBILE AVERAGE 7 Per BAYADH MECHERIA 500 400 300 Rainfall (mm) 200 100 0 1 9 3 1 9 3 7 1 9 07 15 95 03 9 9 927 935 943 947 95 955 95 96 967 97 975 97 98 98 99 9 99 0 1 1911 1 1919 1923 1 1931 1 1939 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 Environ Monit Assess Sand storms are rare in El Bayadh by the CRBT (1978), Djebaili (1978), Aidoud- (3.1 days year−1) but are more frequent in Lounis (1997), and Kadi Hanifi-Achour (2003)de- Mecheria (7.8 days year−1). Dust storms are much scribe the vegetation dynamics at work in steppic more frequent with an increase in the number of regions. A synthesis based upon these different dust days since 1990 (Nouaceur 2001). Between studies is given in the following flowchart (Fig. 3). 1990 and 1997, an average of 30 days per year was It appears that the Algerian steppe was ini- recorded in El Bayadh reaching 119 days per year tially colonized by forests. Their gradual dete- between 2001 and 2004. The same was observed in rioration led to the colonization of xerophytic Mecheria, where 58 days were recorded between vegetation consisting of a mixture of herbaceous 1990 and 1997 and 136 days between 2001 and and perennial plants. The most emblematic is the 2004. This increase seems to be important, even herbaceous Stipa tenacissima, which colonized the if the period considered (2001–2004) is too short majority of flat zones, corresponding to the ge- for statistical support.