Flora and vegetation of the Ougarta Mountains, a still little-known biogeographical crossroads
Read here the second article of Sandscript 32nd issue
** French version **
The umbrella thorn (Acacia tortilis var. raddiana) represents the most widespread tree element in the Saharo- Arabian region. It combines with desert grass (Panicum turgidum) to form the famous Acacia-Panicum desert savanna. A real cornerstone of Saharan wadis, it extends from the Atlantic coasts to the eastern shores of the Red Sea. Located in the north-western part of the Algerian Sahara, the Ougarta Mountains represent a remarkable transition area in which floristic elements of diverse biogeographical origin overlap. The savanna that develops in these mountains reflects the complex interplay between geomorphology, lithology and microclimate. The combination of these biogeographical and ecological characteristics is expressed by the presence of a flora that remains little known yet is rich and diversified.
In the collective imagination, the Sahara represents an immense expanse of dunes. However, these sand seas – known as ergs – only make up 20 to 25% of the total surface area of the Sahara. Above all, the landscape is dominated by vast stony and rocky plateaus (regs and hamadas), such as the famous Tanezrouft to the west of the Hoggar Mountains (Algeria). The richest habitats, the mountains or djebels, and their associated river systems, the wadis, cover a much more limited area. The best-known mountain massifs are Hoggar and Tassili, while the Ougarta Mountains are much less well known.
Geographical, geological and climate context
The Ougarta Mountains are located 610 km as the crow flies south of the Mediterranean coast and 1,235 km east of the Atlantic coast and peak at 850 m (Joly et al., 1991). They cover a surface area of 6,000 km2 and are represented by a horizontal rocky structure from the Cretaceous period, with outcrops corresponding to Cambrian and Silurian rocks (Aït-Ouali & Nedjari, 2006). These mountains are under the influence of a Sahara-type desert climate characterised by irregular rains that very rarely exceed 30 mm per year. The maximum temperatures are reached in July, regularly bordering on 45°C, and the lowest temperatures vary around 5°C during the months of December and January (Dubief 1959).
The Acacia-Panicum desert savanna in the Ougarta Mountains
The djebel-wadi complex represents the habitat most conducive to the establishment of plant cover in which the plant communities are closely linked to their geomorphological habitat. The vegetation develops by means of species’ strategies to adapt to water stress through physiological, anatomical and ecological mechanisms. It is common practice to distinguish two major plant categories in desert areas: permanent and temporary vegetation. Permanent vegetation is represented by woody perennials (trees, shrubs and low bushes). With the rains, temporary vegetation appears, represented primarily by annual plants known locally as “acheb”. They develop surprisingly quickly (1 to 4 months) and constitute key resources for fodder. Accordingly, and unlike regs and hamadas, where the plant cover is very sparse with an extremely poor suite of species, the wadis – and to a lesser extent the djebels – undeniably represent the richest habitats in terms of number of species and rate of vegetation ground cover. Moreover, the plant communities on the wadi beds are the only environments, apart from palm groves, where the tree element can grow.
In the Ougarta Mountains, the wadis with a gravelly loam soil are sufficiently deep to allow a variant of the Acacia savanna to become established. This is distinguished by the Acacia tortilis var. raddiana, Panicum turgidum and Foleyola billotii community (Quézel 1965, Benghanem et al., 2016). On the rather steeply inclined, fissured rocky slopes of these mountains, a particular community dominated by endemic shrubs is observed: Ouarouari (Withania adpressa) and Rass El-Khadem (Ceratolimon feei) (Guinet, 1958; Benhouhou et al., 2003). This is sparse vegetation in which the tree element is absent.
Biogeographical and heritage context
From a biogeographical perspective, the flora of the Ougarta Mountains is dominated by a Saharo-Sindian pool of species. The map of the Sahara’s phytogeographical subdivisions shows the position of these mountains in the north-western Sahara and illustrates the spread of Mediterranean elements from the north and tropical elements from the south (Médail and Quézel, 2018). A real biogeographical crossroads, these mountains represent the southern limit of the distribution area for Mediterranean taxa such as Saharan eryngo (Eryngium ilicifolium) or Saharan broom (Calobota saharae), and the northern limit for tropical taxa, such as Amatellel (Cocculus pendulus) or Ttarah (Gymnosporia senegalensis).
The floristic richness of the Ougarta Mountains amounts to 152 taxa recorded during different explorations (Guinet, 1958, Benghanem, 2020). This figure equates to an areal richness of 25 x10-3, i.e. ten times more than for Hoggar! Corresponding to 15% of this flora, the degree of endemism is remarkable there. This high level is due to the Ougarta Mountains’ geological originality, and the topographical barriers formed by the Atlas Mountain ranges to the north and west and the Great Western Erg, a vast sand sea stretching almost 80,000 km2, to the east. This endemism is also linked to a climatic isolation indicated by the isohyet 20 mm to the south of these mountains.
Among the endemic species inventoried, nine taxa are strictly located in the Ougarta Mountains and found in the southern parts of the Moroccan Atlas Mountain ranges. They are primarily herbaceous perennials (Carthamus duvauxii, Deverra battandieri, Perralderia coronopifolia subsp. coronopifolia, Salvia pseudo jaminiana), shrubs (Ceratolimon feei, Foleyola billotii, Rhanterium adpressum), an annual (Pseudorlaya biseriata) and the only bulbous plant in the northwestern Sahara (Battandiera amoena).
During future explorations, the proximity of the Ougarta Mountains to the southern foothills of the Atlas Mountain range could reveal the presence of an even greater number of endemic taxa. Among the most remarkable, mention can be made of several shrubs such as Trabut’s bindweed (Convolvulus trabutianus), Afarfar (Crotalaria vialattei) or Afzaz (Warionia saharae). These explorations could potentially make it possible to find new taxa for science, as was recently the case with the discovery of a new chenopodium species in Hoggar (Chenopodium hoggarense) (Chatelain et al., 2022).
Abdelkader Nabil Benghanem PhD, Lecturer and Researcher – ÉCOLE NATIONALE SUPÉRIEURE AGRONOMIQUE – Algiers
Salima Benhouhou PhD, Lecturer and Researcher – ÉCOLE NATIONALE SUPÉRIEURE AGRONOMIQUE – Algiers
