Geomorphic changes leading to natural desertification versus anthropogenic land conservation in an arid environment, the Negev Highlands, Israel

摘要

The Negev Highlands in southern Israel are currently under an erosive regime causing degradation of soil and vegetation; a process which has often been attributed to land mismanagement and overgrazing caused by the local Bedouin population. To estimate the anthropogenic role in the erosional processes in the Negev Highlands, two similar drainage basins were selected and studied, one undisturbed with almost no human impact and the other with intensive human modification including the establishment of Roman to Early Islamic agriculture. Field observations and luminescence dating indicate that during the Late Pleistocene glacial period (OIS 4 and 3) deposition of fluvio-loess sediments, with minor erosion cycles, occurred in the Negev Highlands. Severe erosion started during OIS 2 and continued into the Holocene. As the climate shifted during the termination of the Pleistocene to the present interglacial phase, higher rain intensity generated the incision of gullies and channels into the fine-grained alluvial sediments of the previous phase, causing extended soil erosion and reducing the natural biomass and the agricultural potential. Establishment of runoff-harvesting farms in the 3rd century interrupted the Holocene natural erosion and gully incision, and led to the redeposition of up to 3.5 m of fine alluvial loess sediments originating from Late Pleistocene loess sections. This accumulation is not related to any late-Holocene pluvial climatic phase and is solely the result of farming. We conclude that since the end of the Pleistocene a dynamic change in the soil/rock ratio related to the long-term process of adjustment of the geomorphologic system to the Holocene climate has been taking place within the drainage basins in the Negev Highlands. The fluvio-loess sediments deposited in the region during OIS 4 and 3 have been eroding since the latest Pleistocene throughout the Holocene. This process causes degradation of the biomass and agricultural potential and leads onto natural desertification of the region. The historical intervention by establishment of runoff-harvesting agriculture, which as a by-product resulted in the accumulation of redeposited loess sediments, counteracted the natural trend of soil erosion. This was in fact a land-conservation act, applied by the ancient farmers in the semi-arid regions of the Middle East deserts. This activity and its geomorphic consequences are in contrast to the well-documented land degradation trend generated by recent anthropogenic impact on marginal lands elsewhere. In any case, the human impact, either contributing to land degradation or to soil conservation, is super-imposed on the natural long-term trend leading toward desertification.