Arabian Wadi Vegetation

Wadis are ephemeral watercourses carrying water only for short periods and subject to sporadic flooding. Their vegetation varies from place to place and often includes plants of differing water requirements living side by side. In addition the vegetation also tends to vary with the mechanical constitution of the substratum, which changes gradually from the heads to the mouths of wadis. Nevertheless, Retama raetum and various Tamarix species particularly T. aphylla and T. nilotica are often characteristic of this habitat. Other local dominants may include Atriplex halimus, Atriplex leucoclada, Ephedra alalta, Salsola tetrandra, Thymelaea hirsuta, Zilla spinosa and Zygophyllum coccineum. Among endemic or near endemic species associated with this habitat are Artemisia judaica (Asteraceae), Kickxia floribunda and Verbascum fruticulosum (Scrophulariaceae) and Polygonum palaestinum (Polygonaceae).

South Arabian Wadi Riparian Forest

Wadi ecosystems are based on periodic runoff following heavy rains, and although there are no permanent rivers in this BioProvince, the larger wadis such as Wadi Rhima, Wadi Siham, Wadi Surdud, Wadi Mawr and Wadi Zabid, which originate in the mountains of southwestern Arabia, have permanent water along parts of their lengths. These often deeply incised river valleys provide habitat for some of the best riparian forests in Arabia. The characteristic trees include Breonadia salicina, Celtis toka, Combretum molle, Cordia africana, Mimusops laurifolia, Phyllogeiton discolor, Tamarindus indica, Terminalia brownii, Trichilia emetica, Ziziphus mucronata, Z. spina-christi and various species of Ficus. Among the lower growing trees and shrubs are Maesa lanceolata, Myrica salicifolia, Nuxia congesta and Phoenix reclinata. Wadis are also characterized by the genus Tamarix, and in fact, the Arabian Peninsula is the centre of diversity for this genus with many species showing circum-Arabian distributions. Typical species include Tamarix aucheriana, T. mascatensis and the endemic or near endemic T. arabica (Tamaricaceae). They have an interesting adaption to desert conditions. In addition to salt, the salt glands of these plants also excrete an alkaline solution that can absorb moisture hygroscopically. The few endemic or near endemic taxa found in wadis include Acacia johnwoodii (Fabaceae), the fan palm Livistonia carinensis (Arecaceae) and Tarenna graveolens subsp. arabica (Rubiaceae).

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