The study was conducted between February 2016 and May 2017 in a fenced area of the Sayeret Shaked Park (~20 ha). The park is located in the semi-arid north-western Negev of Israel (31°17′N, 34°37′E; 200 m.a.s.l.), where annual average rainfall is ~165 and standard deviation of 58 mm/year . The site’s soil is aeolian loess, with sandy loamy to loamy sand texture . The park is covered with dwarf shrubs (0.1-0.6 m tall) such as Atractylis serratuloides Cass., Thymelaea hirsuta (L.) Endl., and the dominant N. mucronata shrubs. The site has a large number of annuals, geophyte (e.g., Asphodelus ramosus L.) and hemicryptophyte species, as well as a rich community of biological crusts [34,36–39].
Three low-geodiversity hillslopes, with a thick (> 1 m) and non-stony soil layer (homogeneous: HM; Fig. 1 A), and three high-geodiversity hillslopes, with a thin (~ 0.1 m) stony soil layer (heterogeneous: HT; Fig. 1 B) were selected for the study. Distance between two adjacent hillslopes was at least 100 m. On each of these hillslopes, a 400 m2 (20 x 20 m) plot was established for data collection. To study the effect of geodiversity on plant community, we assessed the plant diversity at a once-a-month frequency over the growing season (November through June) of two sequential years (2016–2017). In each of these years, the last cycle of data collection took place when the annual plants were dried out, and at the point where the N. mucronata‘s ‘winter leaves’  have disappeared.
Figure 1 A view of a homogeneous (a) and a heterogeneous (b) hillslopes. In the homogeneous hillslopes, the white spots are piles of snails in the vicinity of dead shrubs. The piles indicate the location of dead shrubs. At the same time, most of the shrubs in the heterogeneous hillslope are alive, and survived the prolonged drought of 2008-2009.
The N. mucronata was the only perennial plant present in both HT and HM hillslope, making it the best-fit model species to assess the year-round differences in plant viability. In each plot (n=6) we studied three individual shrubs, to a total of nine individuals per hillslopes type. In order to monitor the shrub’s morphological changes during the year, we measured the maximum length of green branches. Also, we measured the plant's size by measuring the maximum length from a green part to another on a north-south and east-west axes, as well as the shrub height. We multiplied the three axes to calculate the maximum plant size. In addition to these nine N. mucronata plants, we sampled leaves (winter leaves only) from other nine randomly selected N. mucronata plants to estimate their physiological condition, through measuring the relative water content (RWC), membrane stability (EC), carbon-nitrogen ratio (C:N), and chlorophyll content.
To measure plant diversity, we identified all the plants to the species level along 3-m transects (one transect per plot), counted the number of individuals per species, and recorded the total cover per species. The monitoring of transects was conducted at a once-a-month frequency throughout the growing season (Feb–June 2016 and Feb–May 2017: a total of 11 sampling cycles). Due to overlapping plants, vegetation’s total cover could exceed 100%. We classified all plant species into three life forms: annual, perennial, and hemicryptophyte. The use of any plants in this study was in accord with national guidelines. The formal identification of the plant material was performed by Prof. Rachmilevitch. We did not use voucher specimen.
In order to determine the plant diversity in the different hillslope, we calculate species richness (n) as the number of species present at the site, and species abundance as the total number of plants present at the site (N). In order to determine the diversity in the communities, we calculate the Shannon Diversity Index and Shannon Evenness Index and the Simpson Index, [41,42]
Relative water content (RWC)
We added 3–5 gr of young leaves of each individual to a 50ml vial with a wet tissue to maintain humidity. The leaves were weighted for fresh weight using Sartorius AG Göttingen CP225D, Germany. The samples were submerged in de-ionized water for 24hr and then weighted for turgor weight. Additionally, the samples were dried at 65ºC for 24hr in the oven for dry weight.
Few leaves from each shrub were collected for total organic carbon (Corg) and total nitrogen content (Ntot) analysis, dried at 65°C for 12h and manually ground by mortar and pistil. Of these samples, 20 mg were put in a C-N analyzer (CHNS elemental analyzer, Thermo Scientific, USA).
Membrane stability index (MSI)
From each plant, 20–30 leaves were collected and placed in 50 ml vials filled with 20 ml of double distilled water (DDW). The electrolyte’s electric conductivity (EC) was measured with a probe (EUTECH Instruments, CON 510, Singapore), as initial leakage (Ci). The samples were then placed on a shaker for 12 h at 200 rpm and the EC was measured again as Cr. The samples were then autoclaved to blast cell membrane, and the maximum conductivity (Cm) was measured. Membrane stability index (MSI) was calculated according to Equation 1:
10 leaves of N. mucronata were collected and add into a 2 ml Eppendorf with 1ml of Dimethyl sulfoxide (DMSO), covered with aluminum foils. In the lab, all vials were kept in a 65 º C incubator for 72 h and then centrifuged for 14000 rpm at 20oC for 10 min. 200 ml of the supernatant was added to a 96 wells plate and the absorption was read with Epoch™ spectrophotometer (BioTek Instruments, Inc., Vermont, USA). Chlorophyll a (Chl a), contents was calculated using Equation 2 :