In order to research the matter more fully, I 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 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. I concluded that the Arava 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.
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. In a growing country like Israel, it s usually 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.