Biq'at 'Uvda and its surrounding hills
‘Uvda Valley and the Shayarot Ridge east of it are a unique landscape with vegetation found nowhere else in the Negev. This is an area of extreme desert conditions where one would expect to find only the poor vegetation that is restricted to wadis. However, there are large areas of wide wadis, covered with dense vegetation, while in some areas the vegetation appears to be scattered. For many years carpets of lush annual vegetation have developed in Nahal Hayon, in the northern part of the valley; green or richly-colored, in a manner unknown anywhere else in the Middle East. This unique situation is a result of the specific structure and geological-geomorphological history of the area. The vegetation of ‘Uvda Valley and the large wadis crossing the Shayarot Ridge are influenced primarily by the old drainage system of these hills. Thus the wide valleys of Nahal Yitro, Nahal Kasuy and Nahal Ayah are influenced by their large width and their minimal slope over a long distance. Sand and loess, transported through the ‘Uvda Valley by wind from the southern Negev and from Sinai, were deposited in the various wadis at different depths and intensity (Fig. 6.1.1).
The specific combination of these environmental conditions may be considered endemic to the area of the study and not found elsewhere. Superimposed on these specific conditions were 15 consecutively very dry years, which killed many of the species that are dependant upon a local supply of water. In this extreme desert there is a high level of randomality in the water supply and many rain events are very local. The rainy season in the extreme desert is not fixed. In a study of the distribution of Tamarix aphylla in the Middle East we concluded that in the extreme deserts of Israel, Sinai, and Jordan, storms causing local floods can be expected during every month of the year (Danin & Orshan 1999: p. 7-11).
Biq’at ‘Uvda is a rift valley (graben) constrained by the Milhan fault on its eastern side, delimiting the valley with the Shayarot Ridge. ‘Uvda fault is delimited in the west by the ‘Uvda cliffs. This valley functions as a relatively low drainage basement with water arriving from different directions. In its southern part there is an area of ca. 16 km2, where several wadis split into small secondary wadis, sometimes wider than one km (Figs. 6.1.2, 6.1.3). Nahal Uvda passes through the center of the Uvda valley, and in its northern part joins Nahal Hiyyon on its way to the Dead Sea. Several wide wadis (nahal-nehalim in Hebrew) drain into Nahal Uvda: N. Shaharut, N. Isaron, N. Tse’ela, N. Reuel, N. Yitro, N. Kasuy, and N. Ayah (Figs 6.1.4, 6.1.5). These wadis are the result of a long period of erosion in a stable limestone terrain. North-facing slopes are sites of constant sand sedimentation (Fig. 6.1.6). The hills function as wind-breaks and the air-borne grains are sedimented in the protected slopes. Local erosion by water brings varying quantities of sands to the wide wadis (Fig. 6.1.7). The special combination of wide wadis filled with loessial-sandy soil constitutes a unique ecosystem in this part of the extreme desert.
Earlier studies showed that most soil types support vegetation outside of the wadis (=diffused vegetation) in the climatic range where mean annual rainfall is above 70 mm (Danin, 1977, 1983). Below this threshold vegetation remains in the diffused pattern on sandy soils and on outcrops of hard rocks with few fissures. Runoff water from the hard rock into the few fissures enables the growth of shrubs on the rocks in a diffused pattern. An association of Zygophyllum dumosum and Diplotaxis acris develops outside of wadis in the most extreme areas of the southern Negev such as near Shaharut, Har Hizkiyahou, and the limestone hills above En Netafim. This association was recorded in March 1994 and published officially (according to the rules of the International Association for Phytosociology in Europe) as part of our book on the desert vegetation of Israel (Danin & Orshan, 1999: p.184-189).
The average cover for shrubby vegetation was 3% and of herbaceous plants 4%. The companions found in more than 60% of the records of the two leading species in the book were: Helianthemum kahiricum, Reaumuria hirtella, Pteranthus dichotomus, and Stipa capensis. When I visited the area near Shaharut in March 2011 there were no annual plants, as there was insufficient rain in 2011. Most shrubs were dead following 15 years of drought.
One of the impressive phenomena of my visit in 1994 was the development of diffused shrubby vegetation on soft rock with no outcrops of hard rocks. Soft calcareous rock in the extreme desert is a saline habitat that does not allow the development of shrubs in most of its area. The diffused vegetation was found on sand covering these soft rocks (Figs. 6.1.8-6.1.11). Water penetrates into this substratum more efficiently than into lithosol that has a fine-grained layer at the soil surface. Following the rain, therefore, salts are leached from the soil, and water accumulates in the soft rock. The dominant semi-shrubs in these associations were typical halophytes (salt-resistant plants) such as Traganum nudatum and Salsola tetrandra.
During my visit in 2011, I found no living specimens in a few stands of this kind of vegetation, only dead skeletons. The skeletons of these shrubs have typically intricate and twisted branches (Fig. 6.1.12). The dead trunks become blackish as a result of fungal activity. I saw a similar sight in Gebel Igma in Sinai in a belt where Salsola tetrandra was growing on the slopes in good years. At the lower part of the S. tetrandra belt there were only dead S. tetrandra shrubs; visiting a higher elevation, the number of living S. tetrandra shrubs increased. At the top of Gebel Igma were skeletons of dead Artemisia sieberi. Near each dead plant of the two altitudinal belts was a specimen of Atriplex leucoclada. I therefore gave the latter the “identity card” of a colonizing species. I then understood the origin of the carpets of A. leaucoclada at the roadside between Mizpe Ramon and Sede Boqer in the years immediately after the construction of the new road. I was unable to make out which was the pioneer or colonizer able to establish itself so soon after the climatic catastrophe of the previous two decades when so many shrubs were killed.