Seasonal Patterns and Environmental Drivers of Plant Communities in the King Abdulaziz Royal Reserve, Saudi Arabia

The King Abdulaziz Royal Reserve (KARR), one of Saudi Arabia’s eight royal reserves, displays strong seasonal vegetation turnover typical of arid-zone ecosystems, yet its environmental drivers remain largely unquantified. This study aimed to establish a seasonally resolved, statistically validated baseline of plant community composition in KARR and to quantify the soil and topographic variables associated with this compositional variation. To this end, we surveyed a defined section of KARR across all four seasons of 2023 (763 plots: 189 winter, 213 spring, 188 summer, 173 autumn) using a systematic 10 km sampling grid. A total of 289 vascular plant species from 47 families were recorded, and species richness, Shannon and Simpson diversity, and functional dispersion were quantified per community; the flora was dominated by therophytes (48.4%) and chamaephytes (21.5%) and by Saharo-Arabian phytogeographic affinities. For each season, Ward’s hierarchical clustering (k = 4) identified four plant communities, validated by PERMANOVA (p = 0.001 in every season) and distinct characteristic species. Soils across the study area were predominantly sandy loam, with texture, pH, electrical conductivity, organic matter, and macronutrients quantified per community. Canonical Correspondence Analysis showed that elevation and soil properties (clay, sand, potassium) significantly explained community composition each season (p = 0.001), though explained variance was modest (4–8%); variance partitioning showed a substantial spatial fraction exceeding the pure environmental fraction in every season. Functional dispersion was consistently lowest in one community each season; characteristic-species composition resolved this into two recurring functional types—a woody, disturbance-associated assemblage (indicated by Rhazya stricta) in winter and summer, and a psammophytic, dune-associated assemblage (indicated by Artemisia monosperma and Moltkiopsis ciliata) in spring and autumn—both reflected in Rao’s quadratic entropy (a closely related metric, r = 0.95). These results provide a seasonally resolved, statistically validated baseline of plant community composition and its environmental correlates for KARR, supporting future vegetation monitoring and habitat-based management, and contribute a data point from an under-studied hyper-arid region to the global understanding of dryland vegetation–environment relationships and grazing-driven degradation.