The Hail (Hayil) region of Saudi Arabia, located between 250 35` and 290 00` N longitudes and 390 01` and 440 45` E latitudes, is an exposed complex of Precambrian igneous and metamorphic rocks. It is also a part of the vast phanerozoic formations that overlap the northern and eastern edge of the Arabian Shield. The Hail region covers an area of 118,322 sq. km (= 445,684 sq. miles). It is named after the large Wadi Hail (formerly called Al-Odair valley) in the Shammar Mountain region and is bordered in the north by Al-Jauf and Northern Frontier Emirates, to the west by Tabuk and Medina Al-Munawarra regions, to the south by Qassim Province and to the east by Central and Eastern Provinces. The study area is characterized by several landscape units, such as isolated mountains, peneplains, escarpments, wadis and sand-seas. The Hail valley extends to the north-east through a narrower corridor linking the capital city with the Shamrah Mountain, whereas the Great Nafud desert with rolling sand dunes forms northern part of the Hail region.
The Shammar Mountains form a major feature of Hail region and consist of two great mountain ranges namely Aja, made up of granite and Salma composed of basalt. The entire region is about 1000 m high and the height of the mountains varies from 100-600 m. Jabal Aja which is situated between Hail town and the An Nafud desert is the largest mountain in the study area. The principal part of Hail is An-Nafud desert, which covers about 64,000 sq. km and is composed of reddish sand and lies at an elevation of 900 m from sea level. The sands are stained red because of the iron oxide coating on the sand grains. The desert is believed to be the product of Quaternary episodes of increased Aeolian activity and is composed of 60-120 m high barchanoid and transverse dunes. Major parts of the desert are formed of sand dunes. However, interdunal depressions can also be observed with relatively thin sand cover. An-Nafud, the second largest desert in the Arabian Peninsula, is connected to the largest Arabian Desert known as Rub’a al Khali (the Empty Quarter) by Ad-Dahna belt which is separating the central Najd from Eastern Arabia.
Hail Region was reported to be a relatively excellent area for pasturage. Over the years, due to intense grazing and unsustainable use of the region's resources, the area has changed into a less productive one with a few shrubs and trees (mainly Acacias) in wadis and foothills. However, the region's mountains housed a number of endemic and rare species, some of which are not present in any other part of Saudi Arabia.
The weather system in the Hail Region is generally arid to extra arid. It is influenced by two main pressures, namely Siberian high in winter and tropical low in summer months. It is also influenced by Saudi Arabia’s unique topography; mountains in the west bordering the Red Sea and desert land in the interior (Najd) eastwards. The sun-rays, as in other parts of Saudi Arabia, are intense and seldom diffused by clouds. Summer temperatures typically rise as high as 500 C in the day time with diurnal variation of about 250 C. Winter temperatures hover around freezing at night especially at higher altitudes although the ground occasionally freezes and daytime temperatures nearly always reach 250 C in the sun. The wind in the Hail emirate comes from the north or northwest and is a great evaporative force hence causing immense physical damage. At certain times of the year, especially during spring, the wind builds up 4-5 days severe dust storms known as ‘Shamals’ in which air is full of grit (sand + silt) to a height of hundreds of meters. The rainfall is erratic and there is no regular pattern. The main source of precipitation in the region is from the winter cyclones originating from the Mediterranean Sea and eastern Atlantic Ocean. Although most of the precipitation of this region is from winter storms, summer storms from the south western region also have a very minor share in some years. Snow fall and hail storms are also reported during the winter months.
It is evident from the studies that the entire sparse vegetation in the mountains are influenced by the floras of Mediterranean countries, whereas the vegetation in the An Nafud sand seas and in the open plains and wadis are related to the floras of Saharo Arabian and Irano-Turranean phytogeographical regions. Though the vegetation of various landscape units are clearly distinguished in most places, the borders between units in certain areas (such as in the north-east and south-east of the study area) are difficult to demarcate because of overlapping. Rhus tripartita is one of the major components of the vegetation of the mountains followed by Ficus palmata. Other important perennials include Cymbapogon commutatus, Periploca aphylla, Gomphocarpus sinaicus, Pergularia tomentosa, Blepharis ciliaris and Trichodesma spp.. The east facing mountainous sides are somewhat steep and generally dovoid of any appreciable number of species. Nevertheless, species such as Rhus tripartita, Aerva javanica, Cenchrus ciliaris, Ephedra foliata and Astragalus spinosa forms into small communities in shades and crevices. The shaded areas of the mountains on the north western parts, on the other hand, have a richer floristic composition. Although the vegetation is spread over the shaded areas, majority of plants can be seen along the ravines and ditches where sand and silt accumulate during rainy season. Representative plants in these areas include: Noaea mucronata, Heliathemum kahiricum, Gymnocarpos decandrum, Ballota undulata, Silene schweinfurthia Paracaryum rugulosum, etc.
At lower altitudes the terrain is composed of large boulders and gravels. Vegetation in this part is dominated by Ochradenus baccatus, Zoegea purpurea, Aerva javanica, Pennisetum setaceum, Astragalus spinosus, Teucrium polium, Lycium shawii, Fagonia indica, etc., whereas in the foothills Haloxylon salicornicum is dominant among the shrublet category, which is associated with Zilla spinosa, Rhanterium epapposum, Astragalus sieberi, etc.
The wadi system in the Hail Region, in general, is part of the Wadi Rimah drainage. Acacia gerrardii is the major component dominating in these wadis, along with occasional shrubs such as Lycium shawii, Ochradenus baccatus, Rhazya stricta, Convolvulus oxyphyllus, Zilla spinosa, Senna italica, Pulicaria undulata, Rhanterium epappsoum, etc. Some of the previous studies categorized the vegetation of An-Nafud desert into four groups. They are annual vegetation cover, Haloxylon persicum-Artemisia monosperma-Stipagrostis drarii community, Calligonum comosum-Artemisia monosperma-Scrophularia hypercifolia community and ecotone communities. The ecotone community is further divided into Hamada-Calligonum-Piturathos triradiatus-Scrophularia community and Ranterium-Calligonum-Pituranthos-Scrophularia community. These communities have remarkable representation in sand dunes such as lower dune and upper dune or middle dune areas. However, Calligonum-Artemisia-Scrophularia community is the most widely spread plant groups in the entire Nafud area.
A floristic analysis shows that majority of plants in the study area are annuals while the minority group is in the tree category which represents only 1%. Members of Composite dominate the flora (Asteraceae-63 species) followed by Gramineae (Poaceae –33 species), Cruciferae (Brassicaceae – 27 species) and Leguminosae (Fabaceae - 20 species). 24 families are monotypic. The largest genus in the flora is Plantago (Plantaginaceae) with 10 species, followed by Astragalus (Leguminosae) with 9 species