Dust Trapping in the Judean Desert and in Extreme-Desert Springs
Poa eigii and its companions in the Judean Desert
Dark shrubs of Suaeda asphaltica cover hills in the Judean Desert near the sea-level sign on the Jerusalem – Jericho road (Figs. 7.1.22-7.1.24). In a normal winter, the area among the shrubs becomes green a few days after the first effective rainfall. Looking closely at the green carpet one may recognize short narrow leaves resembling those of a well nurtured English lawn. The plants forming this lawn in the desert and in England belong to the genus Poa. They are perennial, and evidently Poa eigii (Fig. 7.1.24) in the Judean desert is more resistant to drought. Air-borne dust becomes deposited among the green or dry leaves of the Poa which grow above the dust layer (Fig. 7.1.25). Mosses, liverworts, and lichens grow among the Poa tufts. These three kinds of organisms sustain during drought dormancy and resume activity a few minutes after efficient wetting. In a study we conducted on the chalk hills in that area, we found fine-grained soil without stones at a depth of 50-60 cm (Fig. 7.1.26, right). As in the study of soil trapped in mosses above the basalt, no chalk fragments were found in the soil of the northern slope in the Judean Desert. When studying the soil of the Zygophyllum dumosum plant community on the south-facing slope, we found soil rich in chalk stones and poor in fine-grained particles (Fig. 7.1.26, left). Hence, we may conclude that herbaceous vegetation that traps dust grows above the area with more than 100 mm mean annual rainfall.
The results of this study support the results of the moss-soil study in Makhtesh Ramon. The biological crust, developing on desert soils under a mean annual rainfall of 70 to 300 mm, plays an important role in trapping air-borne particles over large continuous areas. In Israel, this is the main area with loessial soils whose main component is air-borne particles. There are places where the loess soil is more than 5 m deep. When we pass to the extreme desert areas the role of plants in trapping dust becomes restricted to special sites and habitats.
Trapping dust in springs of extreme desert area
In 2001, I was invited by Prof. Thomas Levy to visit his field school in archaeology near Wadi Feinan and the village Greig’gra in SW Jordan. Prof. Levy and his students from the University of California at San Diego studied a small hill with remnants of dwellings. 5,000 years ago the people in that place were digging copper ores. When searching for the water sources of these people we used plant indicators for water. I was happy to discover a hidden spring and an ancient water channel dug in the hard rock near the spring. We extended the observation area towards the Arava Valley and I was surprised to find spring plants such as Desmostachya bipinnata (Figs. 7.1.27, 7.1.28), Imperata cylindrica, and Saccharum spontaneum growing on straight crests 2-5 m in both width and height, along dozens to hundreds of meters (Figs. 7.1.29-7.1.33). The soil among the plants was moist and the assumption was that we are dealing here with seepage lines where the water today does not reach the surface but serves to support the spring plants. These plants are perennials with tough leaves. They are protected from drought by growing above seepages. They reduce wind velocity in their immediate vicinity and cause sedimentation of air-borne particles within the plant tuft and its wind shadow. The trapped soil is rich in silt and clay and poor in stones when compared to the local soil of the area. The sharp differences between the soils of the two habitats here constitute excellent conditions for proving the role of desert spring-plants in trapping fine-grained soil. The spring soil differs considerably from the gravel-plain soil.
The night after this discovery, I dreamed of an interdisciplinary study in which archaeologists would dig in the spring-soil crest and find remnants of human activity from the beginning of the formation of these crests. On the morning of my departure to the lecture hall in Jerusalem, I asked Prof. Levy to start the study I had dreamed about. His agreement did not stop the evacuation of the field school after one or two days, when all the residents were taken to the hospital in Elat for treatment against an intestinal problem that affected all of them. For nine years I tried to reach “my crests” again but failed.