Complex Geo-Ecological Responses to Quaternary Climatic Changes in the Northern Negev Desert

Drylands are regarded as highly sensitive to climatic changes. A positive relationship between rainfall and environmental factors (water availability, productivity, species diversity, etc.) is often assumed for areas with an average annual rainfall of 100-300 mm. This approach disregards the fact that a climatic change in arid and semi-arid areas is not limited to climatic factors. This change is often accompanied by a parallel change of surface properties, such as deposition of loess during a wet climatic phase and of sand during a dry phase. The alteration of surface properties may have opposite effects on the water regime and ecosystem characteristics. Data on rainfall, runoff, soil moisture regime, soil properties and vegetation cover were collected at several research sites in the Negev desert along a rainfall gradient of 86-160 mm average annual rainfall. Data obtained revealed complex responses to climatic changes. Loess deposition resulted in an increase in salt input (by dust and rainfall) and a limited leaching depth related to the high water absorption capacity of the fine grained material. The combined processes led to soil salinization and desertification. An opposite trend occurred during the following period. The negative effect of rainfall decrease was counteracted by sand deposition that allowed deep water penetration and water preservation. However, when sandy areas are considered; rainfall increase had contrasting effects on the water regime and vegetation cover. The findings are attributed to the decisive role played by biological topsoil crusts in sandy arid areas. Under wet conditions a thick biological crust develops.This crust absorbs all rainwater at most rainstorms; limiting thus depth of water penetration. Under dry conditions, a very thin crust allows deeper water penetration. This process is further enhanced by a high frequency of runoff generation over the thin topsoil crust.