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A model forest system under study at the Blaustein Institutes for Desert Research (BIDR) is the Yatir Forest, in the southern part of the Hebron mountains, which was planted by the Keren Kayemeth LeIsrael - Jewish National Fundbeginning in 1964. It now covers an area of 3000 hectares (ha) (30,000 dunams) and consists primarily of pine trees (Pinus halepensis) at a density of about 400 trees/ha. Because the forest lies in the transition zone between the Negev and Judean deserts and the more humid Mediterranean climate, it provides a unique site for probing forest functions under conditions of relative stress, as well as the effects of stress on water balance.
In arid regions, in which there is little plant cover or trees, most raindrops fall directly onto the soil, break down the aggregates and create a surface crust that reduces water permeability. Although some rainwater still penetrates into the soil, most of it will run off in a flow that erodes soil and washes away the earth. Planting a forest radically changes this picture. Rainfall, for example, is first trapped by thecanopy of leaves and does not directly hit the ground. This prevents crust formation and decreases runoff. Evaporation off the land surface decreases, but water take up by water-thirsty trees increases.
Researchers ask: What is the fraction of precipitation that is trapped by the tree canopy, evaporates and never reaches the ground? How much rain is lost to runoff and does not penetrate the tree roots? Do the trees and other forest plants remove water from shallow water tables?
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