Mass mortality of shrubs, especially the Noaea mucronata species, has been observed in the semi‐arid Negev of Israel since the early twenty‐first century. This has followed a long‐term drought episode, and suggests a hysteresis‐like effect. However, recent studies have revealed that the mortality has been varied across the region. Therefore, we assessed the depth and stoniness of the soil profile, in homogeneous and heterogeneous hillslopes. Then, we studied the volumetric moisture content during two consecutive growing seasons, in the topsoil of shrubby patches and of inter‐shrub spaces in these hillslopes. The study shows that geodiversity – characterized by shallow soil, a high content of stones in the soil, and a high cover of rock fragment on the surface – reduced shrub mortality. This was attributed to the soil moisture content, which was considerably greater in the heterogeneous hillslopes, than that in the homogeneous hillslopes. It is proposed that the shallow soil halted the growth of herbaceous vegetation in the inter‐shrub spaces of the heterogeneous hillslopes. Therefore, under rainstorms, this hillslope configuration results in considerable generation of overland water flow in the inter‐shrub spaces. The water accumulates in the shrubby patches, allowing them to thrive, even during long‐term dry episodes. In hillslopes with a deep soil layer, no stoniness in the soil profile, and no cover of rock fragments, the herbaceous vegetation is well developed, covering a considerable share of the inter‐shrub spaces. This negates runoff formation and source–sink relations, limiting water availability for the shrubs, and resulting in their mass mortality. Despite no direct pastoral value for livestock, the shrubs play an important role in overall ecosystem functioning. This is due to their capacity to transect hydraulic connectivity, and negate ecosystem collapse. We propose a conceptual model for demonstrating the role played by geodiversity in alleviating drought stress in drylands. Copyright © 2018 John Wiley & Sons, Ltd.

Deserts are the most frequent locations of terrestrial crude oil contaminations. Nevertheless, the long-term effects of petroleum hydrocarbons on desert ecosystems are still unknown, which makes risk assessment and decision making concerning remediation difficult. This study examined the long-term effects of petroleum hydrocarbons on perennial desert vegetation. The study site was a hyper-arid area in the south of Israel, which was contaminated by a crude oil spill in 1975. The contaminated area was compared to uncontaminated reference areas. The composition of perennial plants 40 yr after the oil spill was not significantly affected by the contamination. However, the size distribution of the two most dominant shrub species, Baker and (Moq.) Iljin., and the only tree species, Savi and (Forssk.) Hayne, were different from the reference. These differences can be explained by decreased recruitment. The estimated recruitment of in the last 40 yr post oil spill was 74% less than recruitment in the reference area. Low recruitment of may in the future lead to the loss of tree cover, which would change the entire ecosystem, as are keystone species on which a number of microorganisms, plants, and animals rely. Remediation of oil spills and preventative measures are recommended.

Geodiversity refers to the variety of geological and physical elements as well as to geomorphological processes of the earth surface. Heterogeneity of the physical environment has an impact on plant diversity. In recent years, the relations between geodiversity and biodiversity has gained attention in conservation biology, especially in the context of climate change. In this study, we assessed the spatial and temporal change in plant’s community structure in a semi-arid region, Sayeret Shaked Long Term Ecosystem Research (LTER) station, Israel. Vegetation surveys were conducted on different hillslopes, either with or without rock covers in order to study the spatial trends of hillslope geodiversity. The surveys were conducted for two consecutive years (2016 and 2017), of which the second year was drier and hotter and therefore permitted to investigate the temporal change of plant’s community structure. The results of the spatial trends show that (1) geodiversity increases vegetation biodiversity and promotes perennial plants and those of the temporal change show that (2) the positive effect of geodiversity on plants’ community structure and species richness is greater in the drier year than that in a wetter year. The main insight is that in these drylands, hillslopes with higher geodiversity appear to buffer the effect of drier years, and supported a more diverse plant community than lower geodiversity hillslopes.

Aims In plant eco-physiology, less negative (enriched) carbon 13 ( 13C) in the leaves indicates conditions of reducing leaf gas exchange through stomata, e.g. under drought. In addition, 13C is expected to be less negative in non-photosynthetic tissues as compared with leaves. However, these relationships in δ 13C from leaves (photosynthetic organs) to branches, stems and roots (non- photosynthetic organs) are rarely tested across multiple closely related tree species, multiple compartments, or in trees growing under extreme heat and drought. Methods We measured leaf-to-root 13C in three closely related desert acacia species (Acacia tortilis, A. raddiana, A. pachyceras). We measured δ 13C in leaf tissues from mature trees in Southern Israel. In parallel, a 7-year irrigation experiment with 0.5, 1.0, or 4.0 L plant -1 day -1 was conducted in an experimental orchard. At the end of the experiment, growth parameters and δ 13C were measured in leaves, branches, stems, and roots. Important findings The δ 13C in leaf tissues sampled from mature trees was ca. -27 ‰, far more depleted than expected from a desert tree growing in one of the Earth’s driest and hottest environments. Across acacia species and compartments, δ 13C was not enriched at all irrigation levels (-28‰ to ca. -27‰), confirming our measurements in the mature trees. Among compartments, leaf δ 13C was unexpectedly similar to branch and root δ 13C, and surprisingly, even less negative than stem δ 13C. The highly depleted leaf δ 13C suggests that these trees have high stomatal gas exchange, despite growing in extremely dry habitats. The lack of δ 13C enrichment in non-photosynthetic tissues might be related to the seasonal coupling of growth of leaves and heterotrophic tissues.