Phytogeographic Transition Areas Hide Floristic Diversity in Hyper-Arid Environments (Sharaan Natural Reserve, NW-Saudi Arabia)

Phytogeographic transition zones are often considered areas rich in biodiversity due to their ecological and landscape diversity. This study constitutes the first floristic investigation of the Sharaan Natural Reserve (SNR) located in the northwest of the KSA, which is supposed to be a phytogeographic transition zone due to its location between the central zone of the country dominated by Saharo-Arabic flora and the northern zone characterized by a Mediterranean and Irano-Turanian floristic affinities. The floristic studies carried out in the vegetation season from 2022 to 2024 led to the identification of 166 plant taxa belonging to 38 botanical families. The flora of the SNR is dominated by Therophytes, while the Saharo-Arabian element dominates the chorology of the taxa. The comparative phytogeographic analysis of SNR flora compared to the available literature on published floristic inventories in different phytogeographic regions of the country has confirmed the floristic affinities of the flora of the reserve, especially with the northern and Nefud regions. Among the identified plants, 13 taxa were not mentioned in the 44 consulted references and seem more or less exclusive to the reserve/phytogeographic region. Also, 12 taxa are considered of Least Concern according to the IUCN red list while the presence of Calligonum comosum L’Hér. allows the classification of its natural habitat (dunes) as a Critical Habitat according to the PS6 criteria. This study reports that SNR constitutes a phytogeographic and floristic transitional area by the presence of a significant number of Irano-Turanian (43) and Mediterranean (21) taxa mixed with the dominant Saharo-Arabic species. The SNR region contains a rich and diverse flora due to the biogeographical situation as a transitional zone and the protection due to the closure. This conservation should be strengthened and extended to other neighboring areas with the same biological wealth.