Rakhama ridge: Loessial Serozem Terrace

פורסם: January 16th, 2011 | עודכן: 14/01/15

Rakhama Ridge is an anticline stretching from northeast to southwest (Fig. 2.3.1). The rock layers incline gently north-westward and their inclination angle towards the south-east is much steeper (Fig. 2.3.2). As a result many rock types are exposed in the ridge flanks facing south-east (Fig. 2.3.3). At the beginning of our investigations, in preparation for mapping the vegetation of the Negev (in the laboratory for Geobotany, the Negev Institute For Arid Zone Research 1962-1967), we learnt that the rock type should be taken into consideration when we wish to understand vegetation distribution in the desert. The high edaphic diversity of the eastern flanks of the Rakhama Ridge is reflected in a high diversity of plant communities.

Rakhama Ridge is located in the transition zone between the relatively moist part of the Negev Highlands dominated by [Artemisia sieberi] (mean annual rainfall P = 100-250 mm) and the drier part (P

Fig. 2.3.1: A view of Rakhama Ridge from north-east showing the steeply inclined rock layers. The flint layer is the darkest and the flint outcrops look triangular, where the layer has been cut by descending dry water courses (wadis).

Fig. 2.3.2: The flint of Mishash Formation is rather weathering-resistant whereas the soft chalk of Menukha Formation weathers easily causing a clear “step” on the ridge flanks. The bedded limestone of Netser Formation is seen above the flint and chalk layers.

A special feature typifies the flint layer (No. 2 in Fig. 2.3.3). During the processes of biogenic weathering, a thin layer of manganese and iron oxides is deposited on the rock surface. This dark color brings about an increase in the absorption of solar heat radiation, which creates “islands” of warm rocks in the cold Negev Highlands.
The discussion of the landscape units and their vegetation is numbered according to the units in Fig. 2.3.3.

Fig. 2.3.3: A schematic cross section in the inclined layers of Rakhama Ridge.

3.1. Loessial Serozem Terrace of Nahal Revivim

The loess layers were deposited in the valleys among the hills by wadis which functioned in the past. The parent material is air-borne dust that was formed by weathering of rocks in the Middle Eastern deserts and was deposited all over the area. The inter-bedding of pebbles and the loess indicates transportation of the material by flood water. The loess undergoes soil-forming processes typical for the local climate. The soil type formed under the local conditions is known as “Loessial Serozem” and is characterized by the presence of layers of concretions of calcite (Bca), gypsum (Bcs) and sodium chloride (Bs). These layers are formed as a result of long leaching by rain water and their depth varies from place to place. The rain water contains 8 ppm (parts per million) salts from a Mediterranean Sea origin. The presence of a salty layer influences the situation where most shrubs seen in this area are of [Haloxylon scoparium] (Fig. 2.3.4) The latter is a halophyte, i.e. resistant to soil salinity. A microbiotic crust of mainly filamentous cyanobacteria covers the entire area. In sites where there is no trampling or other type of disturbance, a system of fissured polygons may be seen (Fig. 2.3.5).

Fig. 2.3.4: A river terrace of Nahal Revivim where Haloxylon scoparium dominates on Loessial Serozem.

Fig. 2.3.5: Microbiotic crust where filamentous cyanobacteria grow in the upper soil layer, which breaks into permanent polygons, as long as the site is not trampled.

The soil among the shrubs is leached in rainy years and two types of annuals patches may be found. In sites where soil is leached, [Erucaria microcarpa] is dominant and is accompanied by many herbaceous plants.
An interesting geophyte is [Leontice leontopetalum] (Fig. 2.3.6), which used to be regarded as a weed, growing exclusively in wheat or barley fields. The loessial area here and many other areas like it, has not been cultivated during the last tens or even hundreds of years. If the [“Leontice leontopetalum” L. leontopetalum] had not adapted, enabling it to live in the Loessial Serozem it would have disappeared from this habitat long ago. According to the studies of J. Galil (in his book Kinetics of Geophytes – in Hebrew), the germinating seedling leads to a penetration of its renewal bud, situated on the corm (Fig. 2.3.6, left side) to a considerable depth. At this depth the corm is protected from drought but also from the plow. In this way the plant became one of the weeds typical of cultivated fields and is not influenced by agro-technical human activity (Fig. 2.3.7). Surprisingly, one of the plants that prosper in rainy years is the geophyte [Ixiolirion tataricum] (Fig. 2.3.8).

Fig. 2.3.6: Leontice leontopetalum – a typical plant of pristine loess in the Negev Highlands. The close-up (left) shows its special corm seen through a soil profile.

Fig. 2.3.7: A ploughed field of loessial soil planted with wheat. L. leontopetalum continues to grow there.

Shrubs of [Haloxylon scoparium] function as an efficient local barrier to wind and lead to sedimentation of air-borne dust below the shrubs. There are places where the activity of animals hiding below the shrubs leads to improvement of the nutrient condition of the soil. In rainy years a circle of ruderal plants, such as [Hordeum glaucum] and [Malva parviflora], develop under such shrubs. The annual herbaceous vegetation has a much darker green color in these places. In rainy years, patches of annual halophytes develop in sites where the calcite layer or the saline soil layer is close to the surface. [Gymnarrhena micrantha] (Fig. 2.3.8), is a halophyte and the structure of the patch dominated by it resembles the entire area where the halophyte shrubs of [“Haloxylon scoparium” H. scoparium] prevail.

The number of individuals of other species is low and they are often halophytic annuals such as: [Spergularia diandra], [Spergula fallax], [Nasturtiopsis coronopifolia], [Pteranthus dichotomus], [Aizoon hispanicum], and [Mesembryanthemum nodiflorum]. In the valley of Nahal Revivim, at the foot of Rakhama Ridge, there are carpets of [Nasturtiopsis coronopifolia] (Fig. 2.3.10) which become prominent in rainy years. Shrubs of [Anabasis syriaca], which are also salt resistant, recycle salts by absorbing them from deep in the soil and depositing them on the soil surface together with the dry stems. When the litter disintegrates, the soil surface becomes highly saline. Thus, empty circles are formed in the [“Nasturtiopsis coronopifolia” N. coronopifolia] carpets (Fig. 2.3.10), with [“Anabasis syriaca” A. syriaca] in their center. In dry years most of the area appears without companion plants. In the autumn months the blooming of the [“Haloxylon scoparium” H. scoparium] and [Anabasis syriaca] shrubs starts. Their flowers are not colorful and are recognized only by their groups of five stamens per flower (Fig. 2.3.11, right side).

During the months after pollination, wings start to develop from the back of the five tepals in [“Haloxylon scoparium” H. scoparium] (Fig. 2.3.11, left side) and three in the [Anabasis]. The wings color changes from yellowish to purple and selected [Haloxylon] shrubs may function as “pot flowers.” Flowering and fruiting may take place both in dry and in rainy years. For [“Haloxylon scoparium” H. scoparium] and [“Anabasis syriaca” A. syriaca] there are no competitors for the soil water, as both of them use saline water not used by other plants. The quantity of [“Anabasis syriaca” A. syriaca] shrubs near Yerokham is small and increases with the increase in altitude. Near Mizpe Ramon the Loessial Serozem is dominated by the latter semi shrub. A few small wadis have developed on the loess terrace and they are recognizable from afar by tall shrubs of [Thymelaea hirsuta]. The latter grow also in sites where ancient farmers made local dams in order to accumulate run-off water.

When observing the large river-terrace from the top of the flint layer crest, the effect of human activity in the past becomes prominent. The water of the wadi, passing between the flint and chalk layers (No. 2 & 3 in Fig. 2.3.3) and Netser Formation (No. 4) was diverted by the ancient farmers. A channel of over 2 km brought the water to three terraces that are clearly visible due to varying density of [“Haloxylon scoparium” H. scoparium] shrubs. Near the transition point of the channel above Mishash formation (No. 2), the ancient farmers who cultivated the land built their houses; the latter were discovered by archaeologists long ago. Near the mountain foot the [Haloxylon] shrubs are accompanied by [Zygophyllum dumosum] shrubs. There are many flint stones in the soil in these sites. The herbaceous companions differ from those that prevailed in the Loessial Serozem plain.

Fig. 2.3.8: Ixiolirion tataricum becomes prominent in moist years. It may also become a weed in ploughed loess fields.

Fig. 2.3.9: Gymnarrhena micrantha becomes dominant in saline loessial soil. Very few other species accompany the dominant plant here.

Fig. 2.3.10: There are saline patches where Anabasis syriaca replaces Haloxylon scoparium. During relatively rainy years the soil is leached and the annual halophyte Nasturtiopsis coronopifolia creates yellow carpets. A. syriaca recycles salts and makes the soil around it barren.

Fig. 2.3.11: Flowers and fruits: 1. Anabasis syriaca flowering, 2. Haloxylon scoparium bearing fruits; the wings developing after fertilization.