Abstract
Mountains are biodiversity hotspots, and due to their unique microclimatic circumstances, they host a vast range of endemic species. There are two main hypotheses, the Rapoport and the mid-domain effect hypotheses, which explain how elevation and species richness are linked. The current study was conducted in the Al-Hada escarpment, which is a unique area in the world. It is located on the border of Eurasia and Africa, where there are a lot of plant species from both places. The study aimed to detect the effect of elevation on the floristic composition of the study area. The obtained results showed that the Al-Hada escarpment flora consists of 297 species belonging to 194 genera and fifty-seven distinct families. Only two families, Poaceae and Asteracae, had 22% of the whole recorded species. The obtained results showed that with increasing elevation, the numbers of species, genera, and families increased, in accordance with the Rapoport hypothesis. The numbers of families increased by 62% from the lowest elevation to the highest one, while the numbers of both species and genera increased by more than two-folds. Therophytes exhibited the maximum number, which was 44%, and Chamaephytes came in second with 25%. Phanerophytes and hemicryptophytes made up 13% and 11% of all life forms, respectively, while geophytes made up just 7%. Monoregional elements represented 33.2% of the total recorded species, where Saharo-Sindian species had the most monoregional species, accounting for around 13% of the total species. At the highest elevation, succulents accounted for 7.6% of the research area, whereas N-fixing plants accounted for 6.2%. At the highest elevation, they had the lowest value, and at the middle elevation, they had the highest value. With the rise in elevation, the neotropical, endemic, and Mediterranean elements rose. Succulents and N-fixing species did not show a clear relationship with the elevation but exhibited the lowest value at the highest elevation, and vice versa. Therophytes and geophytes increased while the number of hemicryptophytes decreased with elevation. Surprisingly, phanerophytes did not show any relationship with elevation, while, with the rise in elevation, the pan-tropical, endemic, and Mediterranean elements rose. Neotropical and Saharo-Sindian elements decreased with the rise in elevation. Considering our results, we can conclude that the relationship of taxa diversity with the different altitude of the arid subtropical regions' mountains, whose elevation does not exceed 2000 m (without human disturbance), confirms the Rapoport hypothesis.