Plant Adaptation in the Extreme Desert in Israel

By Dr.  Ori Fragman-Sapir

Head Scientist, the Jerusalem Botanical Gardens

Translation by Julie Baretz

More than half of Israel ’s area has a desert climate. Many of the plants that developed in this region exhibit special adaptations; this article will focus on the most important ones.

The desert in Israel can be divided into two main climatic units: (1) The semi-desert, which receives 70-300 mm average annual rainfall. It is characterized by vegetation scattered over the entire area (slopes and riverbeds). The semi-desert of Israel includes the higher section of the Judaean Desert , all of the northern Negev Desert and the Central Negev Highlands . The landscapes of scattered vegetation are reminiscent of the cold deserts of Asia , from Syria , Iraq and Iran to the foothills of the Himalayas in western China . (2) The extreme desert, which receives less than 70 mm average annual rainfall. This area is characterized by riverbed vegetation only, and in extreme cases by sterile areas with no vegetation at all. Israel ’s extreme desert is located along the southern end of the Dead Sea , along the Arava Valley , in the southern Negev and the Eilat Mountains .

It is difficult to generalize regarding plant adaptation to the extreme desert; different plants make different adjustments.

The important limiting factor in the desert is the quantity of water available to the plant. In order to survive in the extreme desert, a plant must, on one hand, utilize the little rainwater available to it, and on the other hand, survive the very prolonged dry periods. The lack of rainwater is not the only problem; the rainfall does not arrive regularly. If the average annual rainfall in a particular place in the southern Negev is 40 mm, this does not mean that 40 mm can be expected each year. The common model is random, so that often there is no rainfall for a number of years, or one year it rains very little and the next year or two it rains more than the average. Hence, plants and animals must survive in an uncertain environment.

An additional problematic factor in the extreme desert is the salinity level – the scant rainfall is absorbed by the upper soil layer and evaporates very rapidly, so that more and more minerals build up after each rainfall and the soil grows saltier.

One of the more common adaptations that characterize most desert plants is low plant surface area, with mainly small leaves or no leaves at all. Limited surface area results in less water loss for the plant. Retama raetam is the plant best know for this mechanism. Retamais a desert shrub with small leaves, blooming in early winter, but the leaves fall off very rapidly and the plant remains leafless most of the year. Photosynthesis is carried out by its green stems. In addition to a lack of leaves, Retama’s stomata are sunken into canals so that water is saved from evaporation. The phenomena of lack of leaves and sunken stomata is called spartioism. Other spartioid plants in Israel include Polygala negevensis (a rare plant of the Eilat Mountains and the southern Negev ), species of Ephedra and others. Other plants with especially small leaves are Gypsophila arabica,found in all Israeli deserts , Kickxia spartioides , which grows along the Arava Valley .

Retama raetam Polygala negevensis

Kickxia spartioides Gypsophila arabica

Another important adaptation is being succulent. The most famous succulents in the world are, of course, cactuses but these are American plants and not wild Israeli plants (with the exception of Opuntia, naturalyzing after planted for its edible fruit and as a natural fence). Some of the Israeli succulents are: Caralluma sinaica , a plant with fat finger-like stems that grows along the Dead Sea and in the southern Negev ; species of Suaeda; species of Zygophyllum; and species of Anabasis. When people are asked what the advantage of succulents is in the desert, most answer water storage for the dry seasons. This answer is only partially correct. Besides the lack of water, there is an additional problem in the desert of high soil salinity. Storage of rainwater in the plant’s body allows the salts to be dissolved. This process is important because almost all life processes (respiration, photosynthesis and enzymatic processes) cannot take place in conditions of high salinity. As mentioned, succulent leaves form a system in which the plant stores water that enables it to survive the dry seasons as well.

Caralluma europaea Anabasis atriculata

Zypophyllum dumosum Suaeda fruticosa

Other plants have developed different adaptations for conserving fluids. Hair growth is such an adaptation. The layer of hairs cools the plant, or at least prevents its temperature from rising. It also provides volume that insulates the plant body from its surroundings, so the plant is less influenced by the outside temperature. Usually the hairs of desert plants are a light-colored, whitish-gray. They brighten the plant. Light colors reflect most of the sun’s radiation so the plant is heated less and loses less water. Examples of hair-covered plants in our extreme desert are Salvia lanigera; Astragalus sparsus (a rare plant that creeps over the ground on rocky slopes of the south eastern Negev and the Eilat Mountains ); andLotus lanuginosus, a plant that grows in extreme desert river beds (this Lotus is unique for its beautiful reddish flowers).

Species of Anabasis and Zygophyllumhave an additional adaptation: their green organs are divided into units. In times of drought a certain number of the units dry up and fall off so that the remainder of the plant is protected and does not wilt completely. In Ababasis , the stem is divided into joints, a phenomenon that gives the plant its Hebrew name. In Zygophyllumthe leaf is divided into a petiole on which sit two leaflets, each of these three units can separate and fall off.

In addition to the morphological-physiological adaptations, an additional important adaptation of extreme desert plants is full or partial dormancy during the dry seasons. Annual plants are the big “specialists” in this case – they germinate with the rain’s arrival, bloom and the fruits ripen rapidly and remain as dry and dormant seeds during the dry period, which may last a year or a few years. This phenomenon is notable along the Dead Sea shore; usually this region is dry in winter and spring also. Every few years an above-average quantity of rain falls and millions of seeds that were “waiting” for the rain germinate, creating beautiful carpets of flowers.