Biodiversity and biogeography of Moroccan argan tree communities.

摘要

The argan tree (Argania spinosa), a member of the tropical family Sapotaceae, is an endemic of southwestern Morocco appreciated for its edible, high nutritional oil, extracted from the kernels of the drupe-like fruit. The argan tree may be shrubby or reach up to 10 m, occasionally 20 m, with a main trunk which may be a fusion of several interlaced stems. The rough bark is grooved longitudinally and transversely. The branches are spiny. The leaves, oblong-spatulate to oblanceolate, are alternate, clustered and simple. They are bright green below, dark green above. In the axils of the spines and leaves are small clusters of sessile, greenish flowers about 5 mm wide, each flower subtended by two bracts; the corolla is bell-shaped and deeply five-lobed. The olive-size fruits (circle or ovoid to conical or spindle-shaped) turn bright yellow when ripe. Argan forests cover approximately 800 000 hectares of calcareous arid or semiarid land bounded by mountains (Atlas and Anti-Atlas mountains), the Atlantic Ocean and the Sahara. The argan tree is widely distributed from Safi (North) to the Draa River (South) and isolated populations extend as far as Tindouf, well inside the Western Sahara. The altitudinal range of the argan tree extends from sea level up to 1300-1500 m. The climate of the Argan tree zone benefits from more temperate oceanic influences with annual precipitations between 150-400 mm and frequent fog throughout the year. Argan trees are subjected to drought from April to October. The diurnal stomatal conductance of leaves remains notable during the dry season. However, the maximum conductance decreases as the soil dries out. Measurements of transpiration performed at the beginning of, and during, the dry season gave maximum values of 200 mmol m-2 s-1 and 120 mmol m-2 s-1, respectively. Water use by the argan tree is not as low as one would expect. The values of the water potential and transpiration showed that water stocks exist between the soil and leaves, preventing any excessive fall of the water potential during day-time. The plant biodiversity of the argan zone reflect the biogeographic heterogeneity where Mediterranean, tropical, succulent and endemics taxa coexist. The costal and sub-coastal area of South Western Morocco is one of the most remarkable regions of North Africa as a region of great biodiversity. However, this area must not be included in the Macaronesian sub-region; it belongs to the Mediterranean sub-region. The recording of biocenoses with argan trees has been the subject of numerous publications over the past years. Such data is often vital for a rational conservation of this vegetation. The biodiversity of South Western Morocco is presently under pressure from man and his cattle. The original argan forest covered approximately 1 400 000 hectares, but human activities has reduced this area by half, with argan trees often scattered as small clumps or single trees. Innovative solutions have to be found to resolve this dilemma between conservation and production..