Plant diversity, seasonal dynamics, and vegetation-soil relationship of Rawdhat Khuraym, Saudi Arabia: a biodiversity hotspot region within a hyper-arid region

The Rawdah ecosystem in arid regions is characterized by its unique vegetation and biodiversity that support wildlife, buffer climate, and provide ecosystem services that benefit humans and nature. Understanding the floristic composition of the Rawdah ecosystem and its ecological dynamics is crucial for informing sustainable management. Rawdhat Khuraym, a vegetation hotspot zone in the Riyadh region of Saudi Arabia, has recently become part of the Imam Abdulaziz bin Mohammed Royal Reserve. For 23 years, the vegetation composition of this cultural and ecological important rawdhat has not been studied; hence, to inform suitable conservation and effective restoration strategies this study aims to (1) update the floristic composition of Rawdhat Khuraym in the dry and wet seasons, (2) determine the dominant plant communities within distinct parts of Rawdhat Khuraym, (3) determine the soil variables controlling the various plant communities, (4) assess the extent to which the reserve’s protection since 2020 has impacted plant communities, and (5) explore how these trajectories may alter in response to future climatic changes. Three distinct zones were identified in Rawdhat Kkuraym, and based on a random stratified sampling design, a total of 21 plots were selected. Surveys were conducted in the dry and wet season. In each plot, three replicate quadrat surveys were conducted, and soil samples were collected. For historical vegetation cover, satellite-based NDVI and climatological variables were analysed. The present study revealed a rich plant diversity in Rawdhat Khuraym, in particular, the dense growing trees and shrubs that support the plant communities in this ecosystem during the dry season, such as Ziziphus nummularia, Capparis decidua, Vachellia. gerrardi, Capparis spinosa. During the wet season, grasses like Cynodon dactylon and herbs like Malva parviflora and Calendula arvensis flourished after rains and became dominant. The floristic analysis showed 89 plant species (57.3% annuals and 42.4% perennials), belonging to 30 families, where Asteraceae, Poaceae, Fabaceae, and Brassicaceae were prominent (50.6%). Six life forms were recorded, with therophyte as dominant (49.4%). The Saharo-Arabian element was the most represented (55.1%), followed by the Irano-Turanian element (31.5%). In the wet winter-spring season, the northern part of Rawdhat Khuraym had three main plant communities (two dominated by C. dactylon and one by Z. nummularia). The central part also had three communities, two dominated by M. parviflora and one dominated by C. arvensis. The southern part showed four plant communities (Zilla spinosa, V. gerrardi, Rhazya stricta, and C. dactylon community). As can be expected, in the dry summer-fall season, the number of plant communities within each part became more limited; the northern part attained two communities (Z. nummularia and C. spinosa-C. dactylon), the central part showed one community (C. spinosa-C. dactylon), while the southern part had three communities (Z. spinosa, V. gerrardi, and R. stricta). Soil variables controlling the plant community differed in the three distinct parts. in the northern part communities showed a positive correlation with HCO₃, Mg, Ca, salinity, SO₄, clay, Na, NO₃, K, porosity, field capacity, and NH₄. In the central part, communities showed a positive correlation with silt, CaCO₃, and pH. In the southern part, communities revealed a positive correlation with sand and the bulk density of the soil. The comparative analysis between the previous studies and the present study, 5 years after it became a protected area, revealed a recovery of perennial species, and most of the identified species are drought-resistant and show moderate salinity resistance. The NDVI analysis showed that the northern part had stable vegetation cover (average 63%) from 1986 to 2025, while central and southern parts attained lower levels of 38 and 40%, respectively. The fluctuation of NDVI can be attributed to water availability, as it showed a positive correlation with the soil wetness index, relative humidity, and precipitation. To predict future trajectories of plant communities, it is recommended to perform a detailed modelling study of the vegetation dynamics in correlation to the environmental and regional predicted future climatic conditions to be able to account for future changes in the vegetation composition of the Rawdhat Khuraym in restoration programs.