Recruitment and decay rate of Acacia seedlings in the hyper-arid Arava Valley, Israel

• A pioneer study of acacia recruitment and mortality rates in hyper-arid regions
• The study showed an extremely high mortality of young seedlings in dry riverbeds.
• Mathematical analysis included data fit to exponential and Gaussian functions.
• The main factor in seedling mortality is water shortage during the dry season.
• Another mortality factor is soil erosion and deposition on the valley floor.

Acacia trees, including Acacia pachyceras, Acacia raddiana, and Acacia tortilis, constitute some of the main keystone species throughout the hyper-arid Arava Valley of Israel. Several studies performed over the last several decades have revealed drastic changes in the acacia populations, with high mortality rates and low recruitment rates. The objective of this study was to examine the patterns of survivability – through measuring the decay rate – of acacia seedlings during the first year after germination. Following the 2012–2013 rainy season, we measured – over one entire year – the survivability of acacia seedlings in 12 ephemeral rivers (wadis). Data analysis revealed that the main impediment to the recruitment and survival of acacia seedlings is their desiccation, resulting in their mortality. This limiting factor was predominant despite the above-average and well-distributed precipitation during the year of the study. Another, secondary impediment is imposed by erosional and depositional processes under heavy flash floods, resulting in either the uprooting of the seedlings or their burial under deposited soil and fine pebble sediments. Therefore, the novelty of this study stems from the identification, quantification, and modeling of two different mechanisms that determine the decay of acacia seedlings: one with a constant mortality rate that is caused by drying, and the second with a mortality rate that grows with time, which is caused by fluvial processes. The mortality due to drying revealed high fitting to an exponential decay, while the mortality due to fluvial processes closely fits a Gaussian decay function.