The response of Acacia tree populations on small alluvial fans to changes in the hydrological regime: Southern Negev Desert, Israel

Two species of Acacia—A. tortilis and A. raddiana—inhabit channel beds of ephemeral streams in the hyperarid region of Eilat, Southern Negev, Israel. A survey was conducted on two adjacent small alluvial fans in this region in order to investigate the effect of an abrupt change in the hydrological regime on patterns of mortality and regeneration of Acacia tree populations. Changes in the Acacia tree population of the Nahal Yael alluvial fan, which was dammed at its apex 19 years ago, were compared with those in the adjacent Nahal Naomi alluvial fan which was left in its natural, unaltered state. The results show a 2.3 times higher mortality and a 4.1 times lower regeneration in the tree population of the Nahal Yael alluvial fan compared to that of Nahal Naomi. Patterns of mortality and regeneration of the Acacia tree population in the surveyed area are primarily influenced by surface flows, in contrast to groundwater, as generally assumed.