1.3.12 Desert vegetation
Shrubs and trees of this category are confined to wadis (Fig. 1.3.104). However, on the broad boundary between the desert and the steppes zone, [Anabasis articulata] and [Zygophyllum dumosum], the typical desert semi-shrubs, grow in a diffused pattern. Chalk and marl outcrops are populated with the halophyte communities of [Suaeda asphaltica] (Fig. 1.3.105), [Salsola tetrandra], and [Haloxylon negevensis]. The nearly monospecific communities of semi-shrub halophytes are accompanied by varied assemblages of herbaceous plants, which grow on the leached soil only in rainy years. It is worth dealing with this limited vegetation according to the sequence of sections dominated by specific growth forms (Fig. 1.3.106): e.g., the highest parts of the wadis receive small amounts of water, which enable the growth of annuals only in rainy years (Fig. 1.3.107). The common species here are: [Aaronsohnia factorovskyi], [Anastatica hierochuntica] (Fig. 1.3.108), and [Trigonella stellata].
A lower part of the wadi gets larger quantities of water and supports small, short-lived semi-shrubs such as [Pulicaria incisa], [Helianthemum kahiricum], [Helianthemum lippii], and [Diplotaxis harra], which may function locally as annuals. Further down, there are larger, longer-lived semi-shrubs such as [Anabasis articulata], [Zygophyllum dumosum] and [Gymnocarpos decander]. A section dominated by shrubs, such as [Lycium shawii], [Atriplex halimus], [Retama raetam] (which may reach a height of 3 m.), and [Lycium shawii] prevail further down the wadi system. In its very lowest section, trees – mostly [Acacia] or [Tamarix] species – can be found.
The nature of plant communities and the sequence in which they occur along the wadis are closely aligned to rock and soil types, which greatly influence the moisture, salinity, and nutrient regime in the wadi systems (Lipkin, 1971). Different plant communities occur in wadis on alluvium composed of flint pebbles, flint rocks, limestone outcrops, magmatic rocks, or marl. An example of the diversity of the plant communities in such an area near Hazeva, in the Arava Valley (Fig. 1.3.106), is discussed by Rudich & Danin (1978). Using aerial photographs, the patterns of wadi branching and the colors of the areas among the wadis are the main features used to divide heterogeneous areas into homogeneous units (Danin 1983).
The most common dominant shrub in the large desert area south of Ma’an as far as the Saudi Arabian border is [Anabasis articulata]; it is accompanied by the tree [Acacia pachyceras] at the fifth-order section of the wadi system. Large areas of similar gravel plains in central Sinai support vegetation similar to that south of Ma’an. However, the companion tree often seen here is [Acacia raddiana].