More on Pistacia atlantica in the Negev Highlands
Seedlings and their habitats
I do not know the reasons why people who have not seen young trees of P. atlantica, say that this tree is not germinating in the Negev at all. This opinion is strange to me because I have seen hundreds of P. atlantica seedlings in the Negev Highlands and shown them to many people. During the times I counted "annual rings" (with which I'll deal later) I found a seedling 3 years old and others 50 years of age and more. Obviously, taking a wider view of the age situation, a tree of 100 years old may be regarded as a seedling near trees 800 or 1000 years old. The habitat where many P. atlantica plants establish themselves is soil pockets and crevices of smooth-faced rock outcrops (Figs. 7.4.1 and 7.4.2). The small seedling of Fig. 7.4.1 is resting in the shade of a Chiliadenus iphionoides. I named the plant community supporting P. atlantica seedlings after these two species. It develops at altitudes of 700 to 1000 m above sea level in the Negev.
The ability to sustain tree seedlings in the desert derives from the fact that a rain shower of 1 mm may cause run-offs, water that may fill up soil pockets or soil at the foot of rock outcrops. The Pistacia seeds are eaten by birds that transfer the non-digested seeds and excrete them in their living environment. Here is where a seed may germinate and send roots to soil pockets carrying sufficient water for its establishment. There are not so many appropriate soil pockets in the Negev Highlands and this is the reason for the rare occurrence of such seedlings. The number of individuals from the same age group in this species is smaller than in plants with fewer limitations of germination and establishment. An additional habitat where many seedlings were found is water courses at the Negev Highlands where many trees occur (Fig. 456).
In a detailed study I carried out with Dr. Yoav Avni of the research institute at the "Ramon Science Center" and the Geological Institute, we found young plants of P. atlantica in watercourses of all the "orders" – from 1 through 5. Seeds germinate and seedlings establish themselves in watercourses where there are short events of temporary high water availability from time to time. There is a biological advantage for trees that develop deep roots and survive many years after the rain that brought about their germination. The Pistacia may survive for as long as several hundred years. During such a period erosion caused in the alluvium may lead to changes in the local topography and trees may be seen high up on old terraces of such wadis.
Habitats of adult trees
Trees that germinated and became established in smooth-faced rock outcrops, and have roots that penetrate to moist layers, develop into trees such as those in Figs. 7.4.4 and 7.4.5. A large tree in fall is seen on the rocky slope to the right of the shrubs growing in the wadi (Fig. 7.4.6). A closer view of this tree displays its ability to break the supporting rocks when growing up and expanding. The shrubs in the wadi are seven individuals of Amygdalus ramonensis, an endemic tree of the Negev Highlands. It is rare, but may be found growing in rocks and low-order water courses (Figs. 7.4.6 and 7.4.8), and in larger wadis (Fig. 7.4.9). The location of the Pistacia tree closest to the local water-divide of the mountains depends on the nature of rock at the local catchment area.
An area that contributes water efficiently is expected to be smaller than an area of fissured limestone. While counting 1,400 trees using aerial photographs, 7 trees that have rocky catchment areas, were found to have an average area of 1,500 m2. Nine other trees, growing at the foot of slopes with fissured rocks, had an average catchment area 250,000 m2 (Fig. 7.4.10). A few Pistacia trees die when their time comes (Fig. 7.4.11). In a few of them beetle caterpillars were found in the trunk, and these attack the trees as part of their life cycle. Attempts to show that human activity caused the death of these trees did not convince me. I see the population of P. atlantica in the Negev Highlands as a natural population with germination of new plants, growth of adult trees (Figs. 7.4.12-7.4.14), and death of old trees or others that are affected by various insects.