Effects of gully formation and headcut retreat on primary production in an arid rangeland: Natural desertification in action

Geomorphic processes may have significant effects on vegetation productivity. This study reports on the effects of gully formation on rangeland productivity in the central arid Negev desert. The study region is hilly, with wide valleys transverse the landscape. The research was conducted at two sites: Boqer and Zipporim wadis, in which gullies are developing in valley floors. At both sites, we measured the amount of herbaceous biomass and woody vegetative cover and the number of woody species. Data were collected in the upper valley (upstream of the gully head) and in the lower valley (downstream from the gully head). In the upper valley, we collected data on the loess plain. In the lower valley, we collected data from three geomorphic units: the loess plain, the gully banks and the main channel. In the lower valley, we collected data in three segments along its length axis, each comprised of 100 m. Naturally occurring gully formation was found to be inducing degradation of the arid ecosystem. As a consequence of this process, primary production of vegetation on the valley floor decreased. We also found that there has been no rehabilitation of the vegetation in the lower valley since the initiation of the gully.