Recruitment and decay rate of Acacia seedlings in the hyper-arid Arava Valley, Israel I. Stavi a, ⁎, R. Shem-Tov a , Y. Shlomi a , G. Bel b , H. Yizhaq a,b a Dead Sea & Arava Science Center, Ketura 88840, Israel b Department of Solar Energy and Environmental Physics, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, Israel abstract article info Article history: Received 3 July 2014 Received in revised form 5 March 2015 Accepted 8 March 2015 Available online xxxx Keywords: Concentrated flow Drylands Geomorphic processes Fractional exponential decay Precipitation Rain storms 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 mea- sured – 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 iden- tification, quantification, and modeling of two different mechanisms that determine the decay of acacia seed- lings: 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. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The Acacia genus comprises some of the keystone tree species in the hyper-arid Arava Valley of Israel, as well as in other Middle Eastern and North African drylands. During the last several decades, the acacia pop- ulations in southern Israel, including Acacia tortilis, Acacia raddiana, and Acacia pachyceras, have experienced dramatic phenological changes, particularly displaying high rates of mortality. The earliest documenta- tion of this trend was released in the mid-1990s, with emphasis on this problem occurring throughout the Arava region (Ashkenazi, 1995). In addition to the high mortality rates, low rates of recruitment have also been highlighted. Previous studies investigated the causes for these low recruitment rates. For example, Ward and Rohner (1997) noted the decrease in the populations of large mammalian herbivores that consume the pods and enhance the germination capacity of seeds through scarification of their hard coats during digestion. Other studies proposed that infestations of insects, such as the bruchid beetle, increase the vulnerability of seeds to predation, resulting in reduced rates of ger- mination (e.g., Or and Ward, 2003). These studies are in accord with a re- cent research project in several wadis throughout the Arava Valley that revealed little occurrence of young acacias, suggesting a low rate of re- cruitment (Stavi et al., 2014). Drastic demographic changes in the acacia populations have also been observed in other Mediterranean drylands, such as in Egypt (Andersen and Krzywinski, 2007) and Tunisia (Noumi and Chaieb, 2012). Yet, despite the extensive research on trends in the acacia popu- lations, very little is known about the mechanisms that affect the surviv- ability of acacia seedlings after germination. Therefore, this study is of wide interest and relevance to the understanding of ecosystem dynam- ics in African and the Middle Eastern drylands where the Acacia genus has been prevalent. Moreover, the importance of acacias in drylands also stems from their positive impact on the physical quality of the soil underneath their canopy, improving the soil–water status and supporting the establishment of understory vegetation (De Boever et al., in press). We believe that the results of this study are not limited to the region in which it took place but that they also shed light on the general mechanisms of the germination and survivability of tree seed- lings in dryland riverbeds. The 2012–2013 rainy season was relatively wet throughout the Arava Valley, reaching a cumulative precipitation rate that was comparatively larger that the long-term inter-annual means (Central Arava R&D website) and that yielded high germination rates of acacia seedlings. The objective of this study was, therefore, to explore the survivability of Catena 131 (2015) 14–21 ⁎ Corresponding author. E-mail addresses: istavi@adssc.org, istavi@yahoo.com (I. Stavi). http://dx.doi.org/10.1016/j.catena.2015.03.004 0341-8162/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena