Ecological Approach for Afforestation
in Arid Regions of the
Northern Negev Desert, Israel
Moshe Shachak, and
The average annual rainfall gradient in the Negev desert is from 50 to 350 millimeters. The 200-millimeter isohyet is accepted as the transition zone between the northern semiarid and the southern arid deserts of Israel (Bruins 1990) (Figure 11.1). The landscape mosaic of the northern part is composed of human settlements, agricultural fields and orchards, human-planted for- ests, and open shrublands, which are used as rangeland. In the south the landscape is composed mainly of grazed shrublands. The natural shrubland in the Negev is a mosaic of two types of patches (Noy-Meir 1985; Pickett and White 1985; Shachak, Sachs, and Moshe 1998). One patch type is character- ized by a matrix of biological soil crust and the other type is a small patch of dwarf shrubs.
The properties of the biological soil crusts are (1) a microphytic commu- nity consisting of cyanobacteria, bacteria, algae, mosses, and lichens cover- ing the surface (Eldridge and Green 1994; Johansen 1993; Shachak, Sachs, and Moshe 1998; West 1990; Zaady, Groffman, and Shachak 1996b), about 10 to 15 millimeters thick (Zaady, Gutterman, and Boeken 1997); (2) a tightly structured surface (Fletcher and Martin 1948), primarily due to the binding of soil particles by polysaccharides excreted by cyanobacteria and soil algae (Bertocchi, Navarini, and Cesaro 1990; Schulten 1985); (3) a surface highly resistant to soil erosion due to the mucilagenous sheath on the surface that lightly glues together the soil particles (Baily, Mazurak, and Rosowski 1973; Metting 1981; Metting and Rayburn 1983), and thus plays an essential role in