Flora

The flora of Saudi Arabia reflects the geographical position of the Arabian Peninsula between Africa, Asia and Europe. Consequently, the flora has many elements of two of the eight global terrestrial realms; namely the Palaearctic (Europe and Asia) and the Afro-tropical (Africa south of the Sahara) as well as a smaller complement of elements from the Indo-Malayan terrestrial realm. It is thus an area of ecological and academic significance.

At the broadest scale, Saudi Arabia divides naturally into seven terrestrial physiographic regions:

1- Tihamah
2- Western Highlands
3- Arabian Hinterland
4- The Cuesta Region (Sedimentary Najd)
5- Aeolian Sands
6- As-Summan and Widyan Plateaus
7- Arabian Gulf Coastal Region
Saudi Arabia is generally an arid country with a few exceptional sub-humid regions on the south-western escarpments and is divided into three chorological units: the Saharo-Sindian, Somali-Masur, and Afro-Montane.

Approximately 24 different ecosystems comprise the functional elements of the major floristic units. Most of these ecosystems cover large areas and are widespread in the Kingdom. Ecosystems range from those of the saline areas along the Gulf Coast and in the inland drainage basins through the systems of the dune seas of the Nafud and Rub al Khali to the complex and species-rich woodlands of the western highlands. Most of the ecosystems are comparatively simple in structure and species diversity, reflecting the aridity and high summer temperatures experienced by most of the physiographic regions.

The vegetation of most ecosystems of the Saharo-Sindian region is generally sparse and about 60% of the vegetation, mainly in the low lying areas, is represented by annuals. Population density of these annual species varies from year to year, depending on the amount of rainfall and the amount of seed remaining from previous years. The western region is rich in vegetation when compared to the central and eastern region. The north-western mountains are rugged and floristically poorer than the south-western mountains, with affinities to the Mediterranean and North African floristic regions. The south-western region is the richest in terms of species diversity, with elements of all chorological units being found. The African element is especially noticeable, adding many species. The south-western area also contains the highest concentration of endemics, despite the fact that these high altitude areas are heavily populated with human settlements dating to ancient times.

The flora of Saudi Arabia is moderately well known at the taxonomic level and species richness of the 15 Protected Areas administered by the National Commission for Wildlife Conservation and Development, as well as many of the areas under the administration of the Ministry of Agriculture is well documented. Hence the relationships of the flora to surrounding areas, as well as the numbers of endemic taxa are well established. The 2,250 species of flowering plants in Saudi Arabia belong to 132 families and 837 genera. About 105 species inhabit sand dunes, 90 are halophytes, 75 are trees and 12 are aquatic plants. No families or genera of flowering plants are endemic, but there are some 246 species that are considered regionally endemic. The influence of the floras of neighboring countries, particularly Yemen and Oman, is high on the flora of Saudi Arabia.

About 450 species (18%) of flowering plants have direct benefit to man and 45 species (1.8%) are poisonous. Some 334 species (13.4%) are used in folk medicine or are known to have medicinal value. Thirty-eight species are important palatable fodder plants, 6 are important as fuel-wood, 25 species are human food plants and 47 species are used as ornamentals or for other purposes.
Related to the modest numbers of species and levels of endemism, most families of flowering plants have only a very small proportion of their worldwide total numbers of species found in Saudi Arabia. Exceptions are two small families, the Ceratophyllaceae and Barbeyaceae, in which all known species occur in the Kingdom.

In contrast with the flowering plants, gymnosperms, pteridophytes, bryophytes, and algae are not as well known. However, as the bibliography and annexes show, some ground-breaking work has been done on some groups.

Although the species richness is quite well known, other measures of plant diversity have not been widely determined for the Kingdom. Consequently, the Red List status of plant species is very poorly known at this stage. For example, the 2000 IUCN Red List of Threatened Species did not include a single record for Saudi Arabia. Even with increased the emphasis that is being placed on improving the state of knowledge of biodiversity in the Kingdom, the 2003 Red List only contains the five species listed in the table below. Non- Red List literature sources however list 14 flowering plants as endangered, 11 vulnerable, 23 critically endangered, 1 rare and 14 extinct. Unfortunately, since these figures are often based on single collection reports and do not include any quantitative assessments of population sizes and total distribution, it is difficult to determine their significance and therefore to assess status adequately. However, it has been estimated that about 20% of the flora, including the rare and endemic species, are present in small populations in their respective niches. Given the extent of over utilization of rangelands in the Kingdom and the rapid growth of human population in the most botanically diverse regions, it is most likely that the real numbers of threatened plant species is considerably higher than the currently available figures suggest.