Effects of ‘red unit’ deposit on Acacia trees in the hyper-arid southern Israel

• Coarse-textured and stony alluvium stratum limit water availability for vegetation.
• Fine-textured red paleosols occur in some patches in the hyper-arid southern Israel.
• These red deposits increase the soil's available water capacity in the dry riverbeds.
• This alleviates vegetation's water stress, improving tree vitality and survivability.
• This provides the Acacias with a particular advantage in prolonged droughts.

During the last few decades, populations of the Acacia genus across the hyper-arid Arava Valley and southern Negev have faced considerable demographic changes, with high mortality rates as a predominant trend. We suggest that, in addition to the decreasing precipitation rates and the resultant decrease in flood events during this period, these changes are not homogenous across the region but are related to the type of wadis' (ephemeral streams') riverbed. The largest part of the wadis across the region is covered by stony alluvium stratum. At the same time, a small share of the area and some of the wadis are covered by a hard (cement-like) paleo layer of fine-grained reddish sediments (‘red unit’ deposit, also named the Zehiha Formation). Of the most important observable differences is the high rock fragment (particles > 2000 μm) content in the alluvium as opposed to the practically no rock fragment content in the red unit. The Acacia trees and wadi riverbeds (alluvium stratum/red unit deposit) were studied in six wadis during the summer of 2015. Of these wadis, three contain an alluvium stratum and other three, a red unit deposit. The study results revealed very high variability in terms of overall (both alive and dead) tree density in the wadis. Yet, the mean overall tree density was (though not significantly, P = 0.5463) 42% greater in the red unit deposit than that in the alluvium stratum. At the same time, mean percentage of alive trees was significantly (P = 0.0437) and 9% greater in the red unit deposit than that in the alluvium stratum. Generally, characteristics of the red unit deposit indicated much better soil quality than those of the alluvium stratum. These included the texture (clayey in the red unit vs. sandy in the alluvium), hygroscopic moisture content (fivefold greater in the red unit), soil organic carbon concentration (69% greater in the red unit), and calcium carbonate content (24% smaller in the red unit). Above all, means of both water field capacity and permanent wilting point were approximately threefold greater in the red unit deposit than that in the alluvium stratum, resulting in the mean available water capacity to be threefold greater in the red unit than that in the alluvium. Strongly positive and significant (P < 0.0001) correlation was found between the percentage of alive trees and available water capacity (r = 0.91), but not between overall tree density and this variable. It is concluded that as long as precipitation regime and flood frequency are normal (similar to the long-term average) in these hyper-arid drylands, the Acacia similarly grow and establish under both the red unit deposit and alluvium stratum. However, once long-term droughts or climatic change occur, with the resultant decrease in precipitation rates and flood frequency, the red unit deposit alleviates water stress, considerably increasing Acacia vitality and survivability.