Artemisia Monosperma - continued; Retama raetam
4. [Artemisia monosperma] and its activity – continued
[Cyperus macrorrhizus] which often accompanies A. monosperma belongs to a group of typical desert sand plants which have roots with “rhizosheaths” (Figs. 4.1.44, 4.1.45). Dense root-hairs cover the roots and remain connected to the mother root even when it matures. These root-hairs produce mucilage which attaches them to sand grains (Figs. 4.1.45, 4.1.46). Similar roots, studied in Western USA deserts, displayed a water holding capacity 3-4 times higher than that of ordinary sand, in the nano-habitat formed in the rhizosheath. In this area there is high activity of nitrogen fixing bacteria. Nitrogen fixation ameliorates the nutrition regime in the newly arriving sand that is poor in resources. Air-borne dust deriving from deserts is also trapped, and as the years pass it makes the sandy soil more fertile.
5. [Retama raetam] and its activity
[Retama raetam] shrubs may be found in places where the vegetation dominated by A. monosperma is aging. The penetration of Retama into the system is an increase in the life-form diversity and is accompanied by a substantial increase in species diversity. Retama shrubs bring about considerable changes in the soil texture and composition and consequently in the vegetated landscape. Belonging to the Papilionaceae (pea family), Retama has “nitrogen nodules” on its roots. The bacterial activity in these nodules causes an increase in the nitrogen-containing compounds in the plant’s body. The main assimilation activity of Retama plants takes place in the thin green stems after their minute leaves fall (Fig. 4.1.47). Thin dry branches accumulate below the Retama shrubs (Figs. 4.1.48, 4.1.49). Water and nutrient regimes in these sites are enhanced and mosses develop on the piles of rotting stems (Fig. 4.1.50/left).
Mosses improve dust fixation by growing above the dust particles trapped among their stems. Sand color at a depth of half a meter is light, like that of the mobile sand. The sand among the shrubs is lighter than the sand below the Retama, but darker than the mobile sand. The shade below the Retama shrubs functions as a special habitat and has an important role during the succession. It is discussed further below, under the Retama raetam -Helianthemum stipulatum association. In the landscape where Retama raetam is present, the two dominants are [Artemisia monosperma] and/or [Helianthemum stipulatum] (Figs. 4.1.51). Areas covered by A. monosperma seem to be younger than those of H. stipulatum.
4.6. Retama raetam – [Helianthemum stipulatum] associations
In this group of associations the semi-shrubs and shrubs are accompanied by annuals that are confined to infertile soils in the first stage. These companions are [Senecio joppensis] (Fig. 4.1.52), [Plantago sarcophylla] (Fig. 4.1.53), [Maresia pulchella] (Fig. 4.1.54) and [Rumex pictus] (Fig. 4.1.55). The sand under the Retama shrubs is darker than the sand of previous stages and the sand among the shrubs. In a more advanced stage the dominant shrubs are accompanied by species adapted to stable sand: [Trifolium palaestinum], [Aegilops sharonensis] (Fig. 4.1.56), and [Bromus rigidus].
Being a tall shrub Retama is in a favored position for birds’ visits. While standing on these shrubs and watching their environment, the birds excrete seeds of plants they consumed in the maquis. Vines that reach the stabilizing and changing dune in this manner are: [Ephedra aphylla], [Ephedra foeminea], [Asparagus horridus], [Asparagus aphyllus], [Rubia tenuifolia] and [Smilax aspera]. Important maquis shrubs transferred in this way are: [Rhamnus lycioides], [Rhamnus alaternus] and [Pistacia lentiscus]. The increasing density of the vegetation makes dust trapping more efficient. Consequently, well-organized shading improves humus formation under the shrubby plants. The circles of mosses below the Retama shrubs support specific flora which are not found in other sandy microhabitats. This niche includes: [Umbilicus intermedius] (Fig. 4.1.57), Sedum species, [Cyclamen persicum] and [Arenaria leptoclados] – all of which also grow in rocky habitats. The quantity of water in the rock crevices is limited and the plants are indicative of the unique water regime of the Retama shade.
Species composition of plants growing in this microhabitat changes from shrub to shrub; they function as islands of special conditions isolated by the large areas of less fertile sands surrounding them. If there are researchers of plant populations amongst our readers, who wish to take on a detailed study of “island vegetation” I suggest the following challenge: consider the Helianthemum – Retama association as an ocean and the Retama-with-mosses niches as isolated islands. There are Retama shrubs with [Chrysanthemum viscosum] coloring them with their yellow-orange flowers. In Rishon LeTsiyon sands ([localvegd2 Fig. 4.1.19/5]) there are Retama shrubs supporting [Stellaria pallida], [Geranium molle], [Sonchus oleraceus] and [Solanum nigrum]. All the latter species are confined to secondary habitats (resulting from human activity) in the Mediterranean area. The mosses-in-the-Retama-shade may be considered as their primary habitat.