The Wadi Araba Acacia
The acacia tree - with its chestnut-colored twisted trunk and glossy dark green leaves - is a native of the Wadi Araba in Palestine. We take its low-lying branches for granted, yet this once prevalent tree is in danger of becoming extinct.
The two species of acacia, Acacia raddiana and Acacia tortilis, growing wild in the Araba, are the major tree types in that region. Jujubes and Doum palms can be found scattered among them, but they are mere statistical curiosities when compared to the vast number of acacias. The acacia also constitutes a major part of the Araba ecosystem because it is a major food source for a wide variety of animal species. Even at the end of a hot and arduous Araba summer, the acacia remains green; each tree is a tiny oasis capable of nourishing ibex, deer, hyraxes, and even domesticated animals such as camels and goats. Numerous insects live among its leaves, and birds, some feeding on the insects, build their nests in its boughs. Many acacias are also hosts to a parasitic plant known as the strap flower, the preferred dietary staple of several animal species.
Clearly, when one acacia dies, an entire ecological system dies with it.
It was first noticed that acacias were dying in large numbers in the early 1980s, in the area between Wadi Shlomo and the Notza Spur in the southern Araba. In Wadi Roded, 20% of the acacias had died. In the area of Ein Evrona, south of Beer Ora - with its ancient water systems known as fugaras - the fatality rate was 30%. In Wadi Ketura, the figure had reached 70%. It was particularly noticeable that in areas where the trees were dying, no young trees were developing. It was noted that such a large percentage of acacia fatalities could not possibly be a naturally occurring phenomenon. In order to ensure the species' continued existence, It was decided to look into the causes of this high mortality rate.
The death of the trees was gradual. First, some of the branches would die. At the end of this stage, new growth could be observed in the upper or lower branches, with very few flowers or fruit. New branches would appear low on the trunk of the tree, but their leaves would turn yellow and then fall off, after which the ends of the branches would dry up. The dead branches would immediately be attacked by wood beetles which live on dry wood.
There is no way to verify the age of acacias, as they don't sprout the yearly rings by which age is measured in other species. When aerial photos were examined taken in 1945 however, It matched up many of the trees presently growing in the area with trees in the photographs.therefore it was assumed that some of the trees currently found there are adults, and may be hundreds of years old.
The Botanist Ruchama Berliner first noticed that acacias were dying in the mid-1970s. She theorized that the cause of the fatalities was the drop in the ground water level. Upon examination, it was found that there is no connection between the two phenomena. The scientists suggested a connection between the rising salinity of the soil and the tree deaths, but this theory also lacks solid grounding.
The sites of dead acacias in the southern Araba valley were mapped out.No correlation between the dying trees and their location were found. In some places, an adult tree was thriving 5 meters away from a dead tree. Living trees, planted in various kinds of soil, stood among those that were dying, and no other plant species growing in the vicinity of the dying acacias exhibited unusual fatality rates. The parasitic strap flower could not be blamed for the phenomenon - many of the dead trees had not hosted this parasitic species. Additionally, neither diseases nor pests were found to be the cause of the acacia's accelerated fatality rate.
A correlation was eventually established between the altitude of the area and the number of dead acacias: in Wadi Shlomo, when moving higher above sea level, more acacia fatalities appeared. This fact suggested a connection between the water flow and the death of the acacias.
The scientists turned to the characteristics of acacia root systems, a subject lacking much scientific study. The common belief has been that acacia roots run deep, some twenty meters or more below ground. This information was based on random observations made by the two Professors Michael Zohari and Heinrich Mendelssohn, and simply passed from generation to generation without ever having been verified.
In order to research the matter more fully, the scientists collected additional data from trees that had been torn from the earth during floods and also had nineteen acacias uprooted with a tractor. The tractor dug a ditch on one side of each tree, into which the tree was then carefully pulled for examination of its root system, a method which prevented any damage.
The scientists came to a clear conclusion; acacia roots reach a depth of only 1.5 meters. The roots face the direction of the water flow from the riverbed. They spread out over a broad area, often wider than the expanse of the top boughs of the tree, sometimes even as much as double the treetop area. It was concluded that the Araba acacias are dependent upon surface runoff, a hypothesis proven by examining a tree's growth rate when measured against the amount of yearly rainfall.
Surface runoff is a critical factor affecting the distribution of acacia trees. Acacias thrive in the Araba's riverbeds, and don't need a great deal of rainfall to survive. The amount of rainfall need only be sufficient for young seedlings to take root. On the other hand, flooding will uproot the tallest of the seedlings. But the moment that the water supply is compromised, the acacias cannot rally and survive, and the water supply is compromised whenever the riverbed is diverted or blocked.
What causes riverbed diversion or blockage? In a growing country like Palestine, it's large scale development, such as the construction of the new Voice of America radio transmitter or the dams near Hatzeva, that tolls the death knell for tens of thousands of ancient acacias. A conservative estimate of the number of trees affected is 20.000, and some estimates are closer to 80.000. The regulations controlling the transmitter construction determined that if the project was abandoned, the affected landscape would be rehabilitated. So far, 300.000 US Dollars has been allocated for this purpose, a sum insufficient for the desired goal.
The Araba acacias, as products of an arid climate, are very sensitive to any change in the flow of water in their riverbed environment. Tracks left by even one ATV can cause the death of a century-old acacia. Depending on the area in which the tree is situated, the dying process can continue for many years. A tree that does not get enough water produces increasingly fewer leaves, flowers and fruit. The animal life that depends upon the tree is then reduced accordingly, eventually resulting in the deaths of large numbers of animals.
Any road-building that involves detouring the riverbed, however slightly, must be carefully engineered. Additional water-conducting channels beneath roads must be planned to avoid riverbed blockage. In the absence of these considerations, the entire landscape and ecology of the Araba could change, spelling disaster for the entire region.

Biological Background:Twisted acacia, Acacia raddiana,is the most common of Palestine's acacias, as well as the principal representative of the Sudanese Vegetation Enclave in Palestine. The tree attains a height of 4 to 7 meters, and has a flat canopy with bare younger branches. Each leaf splits into six pairs on either side of its stem, each leaf then splitting again into five to twelve additional pairs of leaves. Thorns emerge at the joints where the leaves divide. The twisted acacia has yellow flowers and a long narrow seed pod which curls around once or twice. This species begins to bloom in May and finishes flowering in December, its fruit ripening between September and July. It grows in most parts of the Naqab, Araba, along the shores of the Dead Sea, and in the southern Sinai.
Umbrella acacia, Acacia tortilis, is a short acacia tree, with several trunks branching off close to the ground. The canopy of the tree is flat and the leaves are small, growing paired on either side of each stem. Each leaf divides into four to eight leaves, each of them in turn producing about ten more pairs of leaves. The younger branches are fuzzy, giving the treetop a whitish tinge. Its yellow flowers start blooming in April and last until August. Fruit appears in May, remaining on the tree through September. The umbrella acacia usually grows in large- and medium-sized river valleys, and sometimes on wide stone terraces that provide access to rainwater flowing into the stream. The northernmost trees of this species in Palestine (and in the world) grow in the region of the Dead Sea.