Anthropogenic causes of high mortality and low recruitment in three Acacia tree taxa in the Negev desert, Israel

Abstract

Large-scale mortality of native Acacia trees has been reported in the Negev desert, Israel. These trees may act as keystone species in this ecosystem due to their nitrogen-fixing abilities. Total mortality was variable and ranged from 0–61%. Contrary to widely-held opinion, aquifer depletion for agriculture and other human uses was not found to be a likely cause of this mortality. We found that the major probable cause of mortality was the absence of culverts under roads cross-cutting the ephemeral river beds where the Acacias are mostly found. Culverts allow water to pass from the upper to lower parts of the river during winter floods. In river beds without culverts, an average of 25% total mortality of trees was recorded downstream from the roads as compared with 12% upstream from the roads. We found that an average of five species of perennial plants disappeared downstream from the roads. An additional cause for concern is the rarity of recruitment of Acacia trees. This may be due to the absence of large mammalian herbivores (both wild and domestic) which eat seed pods and enhance the germination capacity of seeds through scarification of the hard seed coat during digestion.