Saudi Journal of Biological Sciences 15 (1) 159-176 June, 2008 ISSN 1319-562 X The Official Journal of theSaudi Biological Society htt:www.saudibiosoc.com Phytogeography of Saudi Arabia Abdulatif H. Al-Nafie Department of Geography, Faculty of Social Sciences, Imam Muhammad Ibn Saud Islamic University, P.O. Box 5760, Riyadh, 11432, Saudi Arabia, E-Mail:
[email protected] This paper presents the phytogeography of Saudi Arabia and examines its phytogeographic location within the complex plant geographical regions of the Middle East. It reviews and discuses works have that been published so far by plant geographers and biogeographers on the phytogeographical regions of Southwest Asia and Northeast Africa, and determines the location of Saudi Arabia within it. The delimitation of the frontiers between plant-regions in Saudi Arabia, especially the south western part, which has always created some difficulties for biogeographers as well as phytogeographers who have studied the region. Key words: Phytogeography, Saharo-Arabian, Sudano-Zambezian, Nubo-Sindian Afromontane, Saudi Arabia. Abstract Saudi Arabia appears to have been overlooked and neglected by many biogeographers and plant geographers, though it is a very significant region for both. The importance of the phytogeography of Saudi Arabia is the result of its location at the meeting point of two continents and two or three major plant geographic regions. Its land plays a great role as an avenue by which plant species could penetrate and are exchanged between these continents and regions. However, unfortunately, most of Saudi Arabia’s lands have been regarded by many scientists, as well as non-professionals, as a ‘lifeless’ environment. The main purpose of this paper is to examine the phytogeographic location of Saudi Arabia within the complex plant geographical regions of the Middle East. It should be indicated that it is premature to conduct quantitative study, since the checklists of the flora of countries of the Arabian Peninsula are far from complete, and the distribution as well as range of many plant species are uncertain in many neighbouring regions. The delineation of phytogeographic regions of Saudi Arabia in this paper will follow the traditional phytogeographical hierarchy and will be based mainly on environmental characteristics as well as taxa that dominate various areas of the country. Environmental Setting Saudi Arabia extends over an area of 2,026,213 sq. km, Saudi Journal of Biological Sciences Vol. 15, No (1) June, 2008 Introduction or about two-thirds of the Arabian Peninsula. This huge of landmass makes Saudi Arabia the tenth largest country in the world, covering about 1.47% of the Earth’s land surface. It is about 5% of Asia, and nearly half the size of Europe. Saudi Arabia extends over approximately 160 degrees of latitude, from 160 22’ 46” at the borders with Yemen in the south; to 320 14’ at the Jordanian border in the north, and between 340 29’ 30” E and 550 40` E. Longitude. Geologically Saudi Arabia is divided into two geological structural provinces: 1- The Arabian Shield 2- The Arabian Shelf The Arabian Shield is an ancient land mass consisting of igneous and metamorphic rocks of the Precambrian age. It covers the central and western parts of Saudi Arabia, along the Red Sea. It covers roughly about 750,000 sq. km, or roughly 34% of the total land area of Saudi Arabia. The surface of the shield was later covered in some parts by Lower Palaeozoic basal sands. Basaltic or volcanic rocks, resulting from volcanic activities and floods of basic lava since the Mid-Tertiary, are also found in the form of lava fields (Harrat) spread over its western parts. Since the Palaeozoic era, the shield has been relatively stable and only the surface sediments have been affected by erosion forces. At the beginning of the Tertiary period, the Arabian Shield was separated from its extension, the African Shield by the formation of the Red Sea. Generally, the shield 159 have a variety of complex shapes. The Great Nafud is the second largest body of sand. 1995). 15. As a result of its location. 1966. These wadis are not continuous. This coastal strip is an irregular land surface covered with marshes. Only Nafud Al Urayq and a few smaller sand dune fields are scattered over the Arabian Shield. salt flats and narrow sandy plains (Al-Nafie. is at Jabal Saudah near Abha in the south-western part of the country. These plateaus are transected by large wadis and their tributaries. Despite its reputation as a sandy desert. Saudi Arabia comprises several distinct physiographical regions. extends 250 km northeast of Ar Riyadh. Eastwards of the mountains is the central plateau region including the Pediplain Najd. north east.015 metres. (Powers et al. 1995).Abdulatif H. star and sand mountains. 2. Al-Nafie. km or about 24% of the total area of Saudi Arabia. 3.The Great Nafud 3. The hard rock plains of As Summan Plateau lie between the Ad Dahna sand dunes and the Arabian Gulf coastal region. such as Wadi As Sirhan. On the other hand. 2008 160 . Al-Nafie. km. including transverse. Its highest peak. These mountains are characterised by steep slopes towards the Red Sea and very gentle slopes towards the interior of the country.9% of the interior part of Saudi Arabia. km. Its average elevation is 840 metres above sea level and 240 metres above the nearby plains. Wadi Ranyah. 1995).Madinah Al-Munawarah. constituting one of its most prominent physiographic features. The mountainous south-western part of the country enjoys a Saudi Journal of Biological Sciences Vol. The Arabian Shelf extends to the east of the Arabian Shield. According to Meigs’s classification of arid and semi-arid homoclimates. which together form a high land terrain that composed of local mesas. These sedimentary sequences were deposited in a shallow sea (the Tethys) over the crystalline basement that forms the eastern and northern flanks of the shield.200 kilometres from south east of the Great Nafud to the northern areas of Ar Rub Al Khali and covers 35466 sq. covering about 65504 sq. except for Asir Province. The Al Aramah Escarpment. Chapman. is made up of a nearly parallel sequence of several prominent crescent-shaped northsouth escarpments. After passing the sand dunes. 2004). These west-facing escarpments extend along the eastern margins of the Crystalline Shield between Ad Dahna sand dunes in the east and the central plateau region in the west. Saudi Arabia belongs to the zone of deserts that extend from the Atlantic Ocean in North Africa to the Takala-Makan and Gobi deserts in China and Mongolia. Ar Rub Al Khali extends over 486245 sq. and they are at times covered and buried by the central sand dunes.. eastward to the plains of Najd. 1978). longitudinal. Sand accumulations represent the most recent and distinct geomorphological feature of Saudi Arabia. and east. under a thin sedimentary core in the eastern part of the Arabian Peninsula. buttes and lava fields. that dominates the topography of central Saudi Arabia. Most of the country is characterised by hot and almost rainless summers. The presence of these large wadis reflects the wetter climate that existed in the region in the past (Chapman. These sand dunes and others that intersperse the interior part of the Saudi Arabia. Hisma Plateau. with a maximum elevation of about 540 metres above sea level and 120 metres above the nearby plains (Chapman. The cuesta region. is classified as an “arid province” within Thornthwaite’s global climatic classification. 1978. Wadi Ar Rumah. responding to the general slope of the land. The Tuwayq Escarpment (or as it is locally called. These wadis flow from the higher mountains in the west. winter is mainly cold with limited scattered atmospheric precipitation. These sand dunes are: 1. and as “dry climates” in Koppen’s classification. Some of these magnificent red coloured sand dunes might reach heights of 300 metres above the general land surface.130 metres north west of Al.Ad Dahna 4. extends for about 1200 km. Wadi Ar Rumah emerges as Wadi Al Batin. Eastward from the narrow coastal plains extend along the Red Sea. Al-Nafie. 1978. most of Saudi Arabia except Asir Province and Ar Rub’ Al Khali is within an “arid climate. Al-Nafie slopes very gently toward the north.A group of sand dune bodies that extend mainly along the western side of Tuwayq Escarpment. and HijazAsir Plateau. Wadi Bishah.” (Schyfsma. For example. some of these wadis emerge under new names. Sand seas extend over about 36. The overall climate of the country. It is made of a sequence of continental and shallowwater marine sedimentary rocks that range in age from Cambrian to Pliocene. averaging 2. which extend approximately 1. Wadi Tathlith and Wadi Ad Dawasir. No (1) June. Ar Rub Al Khali and the Great Nafud are connected by the Ad Dahna sand belts. Tuwayq Mountain).Ar Rub Al Khali (The Empty Quarter) including Nafud Al Jafurah. 1978. which is mainly of hard marine limestone capped with upper Jurassic limestone. They cover about 37% of the entire land surface of the country. which is capped with upper Cretaceous limestone. the western mountain range rises steeply from the sea level. ecologists and biogeographers in order to stress their particular approach to plant distribution on a global scale (Wickens. as Good (1974) indicated: “is that branch of botany that deals Table 1. Group Pteridophyta Spermatophyta Total Families 14 135 149 % 9. This is a common feature of desert flora. and Family Representation One of the main characteristics of the vegetation cover of Saudi Arabia is its low floristic diversity. 2004).6 (840/2172). Commelinaceae. or 24.6 100 Genera 18 822 840 % 2. extinct and species that have not been identified yet (Collenette. 1586 species belong to 23 families or 15. The number of plant species that recorded in the country is 2172 species. Araceae. Number of endemic plant species in the Kingdom is 107 species or 4. Number of families. These plant species represent 73% of the total species in the Kingdom. The number of species might be increased to 2250 by adding subspecies.9% of the total species reported. These species belong to 840 genera and 149 families (Al-Nafie.4 90. 15.6% of the total number of families are represented by single genus per family.7 100 (Al-Nafie. Central.2% of the total number of families. 2004). Plant families account only for 29% of the families in the world. phytogeography or plant geography. It is an indication that only a few of the large number of species that belong to these old plant families have adapted and survived in this harsh environment Other species that could not survive have become extinct.9 100 Species 27 2145 2172 % 1.Phytogeography of Saudi Arabia pleasant summer temperature. Endemic taxa in the interior part of the country are not common. Al Hijaz and Madian mountains as well as their surroundings in western Saudi Arabia which have 88 endemic species or 82. The number of species per genus in Saudi Arabia averages about 2. genera. 2004) Saudi Journal of Biological Sciences Vol. with more rain and cold dry winters.4% of the total families. The low number of plant species in these areas is due mainly to harsh environmental conditions.3 98. approximately 74% of the total plant species of Saudi Arabia. Celastraceae.4% of the total plant species in the Kingdom. genera. many of which are in the wetter areas of its south-western part. An outstanding feature of floristic composition of the flora of Saudi Arabia is that a few families are of importance floristically. 1999). 36 families. eastern. 2008 161 . No (1) June.2% of the total endemic species in the Kingdom (Al-Nafie. are represented by one species per family. and northern parts of Saudi Arabia have only 26% of the total number of plant species in the Kingdom. The number of genera appears to be very high compared to the number of species. No endemic families or genera are recorded and only 19 species can be regarded as endemic such as Carnulaca arabica and Calligonum crinitum which are found in Ar Rub Al Khal. Gramineae (262 species). is found in the mountainous western area due mainly to a greater rainfall. Most of endemic species are found in As Sarawat. Genus. 1977a). 46 families or 30% of the families in the country such as Aloaceae. and Burseraceae are found only in As Sarawat mountains (AL-Nafie. which is also known as plant geography. and species are very low compared to Saudi Arabia’s vast land area. for example. and species differs in its geographic distribution from one part of the Kingdom to another according to natural and human factors. the result of the harsh environmental conditions that prevail in the Saharo-Arabian region which covers vast area of the country. Aristolochiaceae and Barbeyaceae). has been the subject of a number of definitions and different interpretations by different phytogeographers. which is probably. as well as the presence of vast areas of sand dunes. To botanists. 68 families or 45. Most of Saudi Arabia’s land area does not seem to be an important centre of diversity. Among 149 families. Floristic Composition of the Flora of Saudi Arabia Species. and Papilionaceae (165 species) are represented by 660 species or 30. The greatest plant diversity. 2004). most plant species of Saudi Arabia belong to a limited number of plant families. Numbers of families. As in most tropical and subtropical deserts. Phytogeographical Background Phytogeography.1 97. Number of families. Compositae (233 species). (Examples: Neuradaceae. Genera and Species forming flora of Saudi Arabia. Some regions can be regarded as a recipients (e.. 15. ecology and biogeography are inevitable. 5-Regions might differ from one another in respect of being recipients or donors. is concerned with the presence or absence as shown by distribution records rather than with the association between species or their relative abundance (Wickens. Chorology. explaining their relationships with the environment. White (1983) proposed the replacement of the traditional phytogeographical hierarchy (Floral Empires. while ecology [in the Anglo-American sense] is concerned with the segregation of taxa into communities within the common pool [phytochoria] due to environmental factors”. Some parts of the study have been spatial-synoptic (so overlapping with phytogeography and zoogeography). in the Continental European school of phytosociology. Other botanists and plant geographers. etc. on a purely floristic classification. Sectors. stratification and size of species present which are the reflection of the ecological conditions such as climate and soil that are present in the region.g. These natural chorological units or phytochoria were delineated by some authors. In his study about Africa phytogeography. and examining the interactions between their constituent species. Ecology. 1992). Irano-Turanian) that can be called donors. Al-Nafie with the spatial relationships of plants both in the present and the past.Abdulatif H. etc. Its aim is to record and then if possible to explain. However. which investigate vegetation at lower level and in more detail at individual plant and plant species levels.g. 6-Horizontal-vegetation units endemic to each region also play a greater role in drawing boundaries between neighbouring regions. Early plant geographers and phytogeographers in the last century and the first half of this century delimited the globe into natural chorological units or phytochoria according to different bases. genera and species that are endemic to a particular region. which might be also confused with phytogeography. 1994). 4-The floral history and past geological events which might affect the floral composition in the region and make it distinct from others can also be considered one of its main characteristics and markers. et al. In addition. No (1) June. although the community approach. presence and absence of certain families. reliable boundary lines must be based initially on the climatic zones than the taxa dominating these zones. 2008 162 . and with their arrangement in a hierarchical classification according to the degree of floristic similarity between them (Clayton and Cope. hence overlapping with ecology”( Goudie. Zohary (1973) has also discussed the diagnostic markers that delineate and characterise plantgeographical regions. It should be indicated that. 2-Each delineated region should have floristic stock where the proportion of endemics to the total number of species is high. Districts. life-form. is also concerned with describing and mapping plant communities. the term biogeography “has meant the study of the biosphere and of human effects on plants and animals. Leonard (1988-1989) and White and Leonard (1991) sorted out the main phytochoria covering south west Asia and part of Africa. White (1983) subdivided the continent into regional centres of endemism separated by regional transition zones and by regional mosaics. continued to dominate the philosophy of vegetation description and analysis. while other parts have been orientated towards processes such as Quaternary ecology and human impact.) by a new system which attempts to establish phytochoria based chiefly on the richness of their endemic flora at the species level. Saharo-Arabian) since they have collected their species from other regions (e. Wickens (1977b) defined the terms phytogeography or biogeography and ecology are as follows: “Phytogeography [Biogeography] is primarily concerned with the changes in floristics of the common pool resulting from historical factors such as climatic shift. According to him: 1-Each region should have a large number of endemics. 3-Phytogeographical regions must also be speciation areas and centres of certain groups of taxa (centre of diversity).. 1980). the distribution of plants over the world’s surface” To geographers. Following White’s system. Domains. A regional centre of endemism should have a total of more than 1000 endemic species. it must have more than 50 percent of its species confined to it. Plant communities differ markedly from one region to another although they might display Saudi Journal of Biological Sciences Vol. where the classification is based on external morphology. Provinces. genetic shift. 1977a). has become widely used and accepted in plant ecology since 1975 (Kent and Coker. namely in accordance with the distribution. Regions. which put more emphasis largely at the community level. It is clear that some overlapping between the three allied disciplines such as phytogeography. such as Schouw (1823). continental drift. this school has been less concerned with the processes at work than with the precise composition of communities. applied the physiognomic or structural approach. Zohary (1973) indicated that in delineating a biogeographical region. the individualistic approach. such as Grisebach (1884) and Schimper (1903). all other desert regions in these classifications are situated between two or more floristic realms or regions. The bioclimatic approach can be used to delimit the world into broad climatic zones such as Arctic. 2-The Middle Saharo-Sindian sub-region. Boreal. Each of the six major floristic realms is divided into several sub-regions.Phytogeography of Saudi Arabia many common species. 1973). while others have classified the northern parts of the Arabian Peninsula as being within the Holarctic realm rather than the Palaeotropic. etc. and White and Leonard (1991) the Arabian Peninsula is comprised mainly of two phytogeographical regions that cover much of the Middle East and north Africa. Areas above 1800 m are called Afromontane archipelago-like regional center of endemism (Leonard. Ahti et al. (1988). besides 163 . The floristic approach. 1983). relies on the distribution pattern of significant groups of families. and Takhtajan (Floristics + endemics) (1986) have suggested six floristic realms in the world. Konig. (1974). of which. which includes the northern part of the Arabian Peninsula. will be used to identify these regions in the forth coming discussion in this paper. 15. 3-The Eastern Saharo-Sindian sub-region (Figure 1). No (1) June. Nubo-Sindian local centre of endemism and the Arabian regional subzone are in Saudi Arabia. (Eig. 1979. create difficulties in the delimitation and the alignment of the frontier of these realms. He extended the northern boundary of Nubo-Sindian local centre of endemism towards the eastern region to cover most of western Najd plateau. Such desert regions. Zohary. the Arabian Peninsula. White (1983). Saudi Arabia falls within the Middle Saharo-Sindian sub-region. other factors such as soil must also be considered. 1977b). 1-The Saharo-Sindian Region. where Saudi Arabia is located. edaphic-topographic or floristic. Phytogeographic Sub-divisions of the Arabian Peninsula For most authorities such as Eig (1931-1933). The Australian. The Saharo-Sindian regional zone is divided into tree subzones. 1988-1989. 1986) or SudanoZambezo-Sindian Region (Konig. falls between the Palaeotropic and Holarctic realms. it should be indicated that the altitudinal zonation complex of each region is peculiar and is another characteristic of the region. due to its being a floristic part of the Saharo-Arabian region. Some phytogeographers have classified it as falling exclusively in the Palaeotropic realm. 1931-1932) or Saharo-Arabian Region (Zohary. as indicated earlier. GruenbergSaudi Journal of Biological Sciences Vol. According to Eig’s classification. As an example. (1973) White. the general approach and terminology of Zohary (1973) for the Saharo-Arabian and Sudanian regions.. This approach is very useful for understanding the past plant distribution from fossil evidence. 2-The Sudanian Region (Gruenberg-Fertig 1954) or Sudano-Zambezian Region (Takhtajan. 7-Although zonation complexes and higher elevation areas might suggest that plant-geographical regions are not uniform areas and plants in these areas might resemble those in adjacent regions. This region is bordered by the Mediterranean and Irano-Turanian regions in the north and the Sudano-Deccanian region in the south. For determining smaller phytogeographical units that can not be determined by climatic data alone. The Capensis (South African) and The Antarctic. The Palaeotropical. genera and species. The effect of both historical and ecological factors should also be considered (Wickens. which shows clear relationships with the Holarctic (Walter. Alfarhan (1999) identified three chorological units in Saudi Arabia based on the richness of endemism. Good (Floristics). It can be noted that with the exception of the Australian and the Central Asian deserts. (1968) suggested that phytogeographical regions can be classified according to one of three approaches: bioclimatic. therefore. 1988). as follows: The Holarctic (Boreal). Leonard (1988-1989) and White and Leonard (1991) designated areas between 1500-1800 m above the sea level in the northern eastern parts of Sudano-Zambezo-Sindian Region in Africa and south-western Arabian Peninsula as a separate region which is named Somalia-Masai regional centre of endemism. 2008 Fertig (1954). The Neotropic. provinces and other smaller units. Walter (Floristics + physiognomy) (1979).Saharo-Arabian 1-1 Region names and boundaries Eig (1931-1933) defined the Saharo-Sindian region as the great desert belt that extends from the Atlantic coast in Africa to the Taher desert in India. 1. Temperate zones. For the purpose of this paper and to avoid the diverse conceptions of different authors which have resulted in different names for the two main regions in Saudi Arabia. 1973) and SaharoSindian-Regional Zone (White. Zohary. Thes are Saharo-Sindian regional zone. which are well known. He divided the Saharo-Sindian region into three sub-divisions: 1-The Western Saharo-Sindian sub-region. The edaphic-topographic approach is dependent on ecological similarities and can be. accordingly. White and Leonard 1991). considered as ecological classification. Somalia-Masai regional centre of endemism and Afromontane archipelago-like regional centre of endemism. (1983). in later investigations. This region gets most of its rainfall during the short winter and spring seasons. with an average of 40 mm to 150 mm in most parts. especially in the higher parts. some of them are sandy loams. 2008 164 . Zohary indicated that what Eig had defined as the East Saharo-Sindian is more Palaeotropic than Holarctic. On the other hand. Phytogeographical regions of the Northern Hemisphere of the old world. and denoted this region as the Saharo-Arabian (Figure 2). mainly through the Mediterranean cyclones that pass through the Middle East during this time. low temperatures in winter. After Guest. The Arabian regional subzone covers the interior parts of Saudi Arabia. 1-2 Environmental Characteristics of the SaharoArabian Region The Saharo-Arabian region is climatically characterised by very high temperatures during the summer and very Fig 2. Leonard (1988-1989) and White and Leonard (1991) reintroduced the idea of the Saharo-Sindian region zone similar to that designated by Eig (Figure 3). parts of lower Palestine. and Nubo-Sindian local centre of endemism stretches in Saudi Arabia over a narrow a strip around the Red Sea and Arabian Gulf coasts. ravines. an Arabian regional subzone and a Nubo-Sindian local centre of endemism. southern and central Jordan and lower Iraq. others are gravely sands. Zohary (1973). 1966. The soils of this region are mainly classified as Torripsamments / Calcatric Arenosols in sandy deserts such as Ar Rub Al-Khali. He excluded the East Saharo-Sindian sub-region from the Saharo-Sindian. They can be classified as Torriorthents / Lithic and Eutric Leptosols in hills and pediments. the East Saharo-Arabian Subregion of Saharo-Arabian Region covers the interior parts of Saudi Arabia. resulting from the very intensive sunshine and very dry air. 1999). the summer season is very dry and long. High and extreme fluctuations in the diurnal and seasonal temperatures are also characteristic of this climate. No (1) June. Soils are dry in most of the year and have very low organic matter and nutrient levels due to the sparse vegetation cover. Evaporation is very high. Al-Nafie Fig 1. Saudi Journal of Biological Sciences Vol. pediments and wadis are also distributed all over the region. 15.Abdulatif H. Phytogeographical regions concerning the Middle East based on Zohary`s (1973) conception. This region extends from the Atlantic Ocean to the Sind desert and is divided into three main chorological parts: a Sahara regional subzone. Precipitation and atmospheric humidity are normally very low. Soil in hills. the Great Nafud as well as Ad Dahna and other small sand dune bodies. and Torrifluvents Calcaric Fluvsols in wadis. According to Zohary (1973). 1966. and should not be included with the Saharo-Arabian region. renamed this broad region and redefined its limits. Sinai. mainly based on Eig`s (1931) and Zohary`s (1950) conceptions. Egypt. After Guest. and low depressions (Ali. The annual rainfall ranges from 0 mm to 100 mm. vegetation cover might then dry up. Mandaville. when moisture is sufficient and temperature is appropriate. They usually bloom and form luxurious growth in rawdhat. of which about 1200 were steppe or desert plants and approximately 310 species are endemic (Takhtajan. and one might travel for some distance to come across with few species. After White and Leonard. these species dry out very quickly. Usually. Annual plants grow following winter and spring precipitation. The Saharo-Arabian region is very poor in species (Takhtajan. where moisture accumulates after sufficient rain. 1-3 Floral Characteristics of the Saharo-Arabian Region The Saharo-Arabian region is characterized by the dominance of shrubs and shrublets (Table 2). General limits of the principal phytochoria established in Africa and Asia based on the areas of 509 species and subdivision of species collected in the deserts and subdeserts of Iran. Ad-Dahna as well as small sand dune bodies scattered in the central part of Saudi Arabia have only 165 species (Al-Nafie. It is also characterised by a very low density of individuals particularly in the sand dunes and surrounding deserts. as well as the lack of moisture and nutrition. At the beginning of summer. sq. Sand dunes that cover vast areas in central and northern Saudi Arabia such as the Great Nafud. Most of the countries which are occupied largely by the Saharo-Arabian region. and the stable sand dunes are generally favourable for perennial species with long root systems which enable them to reach and obtain the required moisture from the water stored in deeper soil layers. As a result. Therophytes (annuals and biennials) not surprisingly account for 50-60% of the total species of the region. which are mainly shrubs and dwarf shrubs is due to the ability of these species to endure high temperature. Species that may behave as annuals. 2004). Fagonia and Farsetia are found. 1984). lower ground areas such as rawdhat. when moisture in the top layer of the soil is almost completely exhausted. This region is very weak in endemic flora at all taxonomic levels. 1969). Eig (1931-1932) estimated the number at 1500 species. 1969). such annuals might not appear. if no more rain falls. No (1) June. 1973). 15. After White and Leonard.6% of the total number of plant species recorded from Saudi Arabia (Al-Nafie. wadis and at the base of stable dunes. wadis. this region is less clearly defined floristically than other areas (Takhtajan. In some years. These annuals are mainly part of the very shallow-rooting herbaceous vegetation. The presence of perennial species. movement and burial by sand. where water usually accumulated. so that the area witnesses a complete or partial absence of vegetation. when there is not enough rain. Ozenda reported 1200 species for the entire Sahara in Africa (Zohary. such as Iraq. only about 20 plant species were recorded.Phytogeography of Saudi Arabia Perennial species that have adapted to this harsh habitat in several ways amount to 40-50% of the total species number. biennials or short-lived perennials. their relatively large size. 2008 165 . Perennial species that have adapted to this harsh Fig 3. it might requires two periods of rain with an interval of some days to allow these annuals to flourish. 1995) which consist about 7. Water conditions in rawdhat. 199. Egypt and the countries in Arabian Peninsula have a low number of species. Vegetation can also be found over stable sand dunes as well as hollows between them and sand sheets that cover hamadas and stony plains. Saudi Journal of Biological Sciences Vol. and their beneficial uses. 1969. wadis and ravines. Plant species in this region show the usual ways of adaptation to aridity and very high temperature in such a harsh environment. depending on the availability of moisture as well as other ecological conditions. Perennial species are the main feature of the Saharo-Arabian vegetation for their presence year round. They complete their life cycle within the cool season. Vegetation cover is mainly concentrated in depressions. 1991. In Ar Rub Al Khali (the Empty Quarter) which covers about 486245 km. No endemic families are recorded and only a few really endemic genera such as Tribulus. constitute only a small number which dose not exceed 3%. Stipagrostis. 15. No (1) June.2% of the total species number. Saharo-Arabian species presented in Saudi Arabia Species Species Aaronsohnia factorovskyi Agathophora alopecuroides Aizoon hispanicum Allium dictyoprasum Allium sinaiticum Althaea ludwigii Anabasis articulata Anabasis setifera Anastatica hierochuntica Anchusa aegyptiaca Anisosciadium isosciadium Anthemis desertii Anticharis glandulosa Anvillea garcinii Arnebia tinctoria Artemisia judaica Artemisia monosperma Asphodelus refractus Asphodelus viscidulus Asteriscus graveolens Astragalus annularis Astragalus bombycinus Astragalus caprinus Astragalus collenettiae Astragalus dactylocarpus Astragalus fruticosus Astragalus intercedens Astragalus kahiricus Astragalus schimperi Astragalus sieberi Astragalus sparsus Atractylis carduus Atractylis mernephtae Bassia arabica Blepharis ciliaris Bromus hausknechtii Calendula tripterocarpa Calotropis procera Carduus getulus Carrichtera annua Carthamus nitidus Centaurea ammocyanus Centaurea scoparia Centaurea sinaica Citrullus colocynthis Colchicum schimperi Convolvulus fatmensis Convolvulus spicatus Conyza stricta Iphiona mucronata Iphiona scabra Isatis lusitanica Juncus punctorius Kickxia acerbiana Lappula sinaica Lasiopogon muscoides Launaea angustifolia Launaea capitata Launaea massauensis Limonium lobatum Linaria haelava Linaria tenuis Lotus lanuginosus Malva aegyptia Maresia pygmaea Matthiola arabica Medicago laciniata Moltkiopsis ciliata Moricandia sinaica Neurada procumbens Nitraria retusa Notoceros bicorne Ochradenus baccatus Ogastemma pusillum Pancratium sickenbergeri Parietaria alsinifolia Paronychia arabica Pennisetum divisum Phagnalon barbeyanum Phoenix dactylifera Picris asplenioides Picris babylonica Picris cyanocarpa Plantago amplexicaulis Plantago ciliata Plantago cylindrica Plantago maris-mortui Polycarpon succulentum Polygala negevensis Pteranthus dichotomous Pulicaria arabica Pulicaria inuloides Pycnocycla saxatilis Reseda arabica Reseda muricata Retama raetam Rhamnus disperma Rhazya stricta Species Cornulaca monacantha Cornulaca spp. depend on the availability of moisture as well as other ecological conditions for their survival. biennials or short-lived perennials. Al-Nafie Table 2. constitute 166 Saudi Journal of Biological Sciences Vol. Crucianella membranacea Cucumis prophetarum Cymbopogon commutatus Cyperus macrorrhizus Deverra tortuosa Deverra triradiata Dichanthium annulatum Dipcadi erythraeum Diplotaxis acris Diplotaxis harra Echinops glaberrimus Echium rauwolfii Ephedra alata Ephedra aphylla Eremobium aegyptiacum Erodium glaucophyllum Erodium neuradifolium Erodium touchyanum Euphorbia grossheimii Euphorbia retusa Fagonia indica Globularia arabica Gomphocarpus sinaicus Gymnarrhena micrantha Gymnocarpos decandrum Haplophyllum tuberculatum Helianthemum kahiricum Helianthemum sanctiantonii Helianthemum sessiliflorum Helianthemum stipulatum Heliotropium arbainense Heliotropium digynum Herniaria hemistemon Hippocrepis constricta Horwoodia dicksoniae Hyoscyamus desertorum Hyoscyamus muticus Ifloga ruepellii Ifloga spicata Species Rumex vesicarius Salsola cyclophylla Salsola incanescans Salsola inermis Salsola jordanicola Salsola schweinfurthii Salsola tetrandra Salvia aegyptiaca Salvia deserti Savignya parviflora Schimpera arabica Sclerocephalus arabicus Scrophularia deserti Scropularia hypericifolia Seidlitzia rosmarinus Senecio flavus Silene arabica Silene hussonii Silene villosa Silene vivianii Spergula fallax Stachys aegyptiaca Stipagrostis ciliata Stipagrostis drarii Stipagrostis raddiana Suaeda aegyptiaca Suaeda vermiculata Tamarix nilotica Teucrium leucocladum Thymus bovei Traganum nudatum Trichodesma africanum Trichodesma boissieri Verbascum decaisneanum Verbascum schimperianum Volutaria crupinioides Volutaria lippii Zilla spinosa Zygophyllum album Zygophyllum coccineum habitat amount to 89. 2008 .Abdulatif H. Species that may behave as annuals. there activities and action such as overgrazing and wood cutting are accountable for some environmental destruction and pressure over this period.Eritreo-Arabian Subregion Province a. 1977). 15. The vegetation cover of the Saharo-Arabian region might have taken its present form since that time. 1986) Konig (1988) named this region as Sudano-ZambezoSindian and divided it into six domains as follows: 1.Sudanian Province 3.Sudanian 3. and neighbouring regions as proposed by Takhtajan. Salt flats and gravel floors between the sand dunes are almost devoid of vegetation. No (1) June. ( Masry.5 M.Zambezian Province 2. Cyperaceae. Sapotaceae. as well as tree forms.8%. 1. Combetaceae.Afro-oriental Domain 5. 10. 1984). 2008 Fig 4. a wide strip along the Arabian Gulf. 1986.Zambezian Domain (Figure 5). B.Socotran Province Subregion 4-Omano-Sindian a.Zambezian Subregion a. including Gramineae. as well as the south-western corner of the Arabian Peninsula. which is believed to be a transitional period climatically between the humid early Tertiary and the arid Late Tertiary and Quaternary environments (Anton. tropical families such as the Meliaceae.Sindian Province (Takhtajan. 6. north of the equatorial rain forest region.Somalo-Ethiopian Saudi Journal of Biological Sciences Vol.Sindian According to Zohary (1973) the Eritreo-Arabian and the Nubo-Sindian provinces of the Sudanian Region are present in Saudi Arabia. The Eritreo-Arabian province extends over Tihama plain on a narrow strip along the 167 . and the Mediterranean and Irano-Turanian regions in the north and north-west. part of tropical Africa. the Oman Gulf and the Arabian Sea. and Caryophylceae. Myrtaceae. Aridity has accelerated during the Holocene. Ceratopteridaceae and Palmae were also present (Mandaville. 1990). but might occur in the adjacent areas (Mandaville 1986). P. Compositae. A large number of the plant families that exist in and dominate some parts of the Saharo-Arabian region today are believed to have existed since the Middle Miocene.Sahelian Province b.Sahelo-Sudanian Subregion a. 2. This region was known as Sudano-Deccanian (Eig. especially in the last 3000 to 2000 years. mainly the Sudanian region in the south. southern Iran and Baluchistan (Figure 2). Annual species.Province of Oman b.South Arabian Province c.Phytogeography of Saudi Arabia only a small percentage.South Arabian Domain Domain 4. 1931-1932) or Sudanian Region (Gruenberg-Fertig 1954) or the Sudano-Zambezian region (Figure 4) and divided into four subregions: and smaller areas as follows: 1. Africa. are almost absent from the inner zones.Y. Human populations inhabited the central and northern parts of Saudi Arabia as early as the early Pleistocene. Many of the Saharo-Arabian elements are derived and developed from the neighbouring regions. Phytogeographical regions concerning the Middle East.The Sudanian Region 2-1 Region names and boundaries The Sudanian region extends to the south of the SaharoArabian region occupies in the northern hemisphere.Sahelian Domain Domain 2. b.South Iranian Province c. In addition. They are developed gradually but discontinuously under the increase of aridity since the Middle Miocene. The Nubo-Sindian province stretches over a narrow a strip on the Red Sea coast north of Makkah and also along the Arabian Gulf coast. spring and summer months. but rainfall is too low to maintain typical savanna or any other tropical arboreal vegetation. No (1) June. Sometimes they are formed of gravel. In the desert plains and lower slopes of the mountains pseudo-savanna comprising species such as Acacia spp. is the dominant vegetation cover. with an average rainfall exceeding 400 mm per year. They are non-saline to strongly saline and have gravely. The Sudanian region is characterised by hundreds of genera. They are mostly loamy and occasionally gravel. South Arabian Province. which are Camborthids/Calcaric Cambisols. Tihama plain has deep loamy soils. Precipitation and atmospheric humidity are normally high. White (1983). Forest and woodland are scattered and still survive in suitable areas all over the region. Africa. or moisture stored in the soil at the end of the rainy period. and neighbouring regions as proposed by Konig. 2008 168 . The Sudanian territories in Saudi Arabia are mainly characterised by tropical vegetation where temperatures are high enough to support it. support Sudanian species. dominantly loamy and show stratification in most of their volume. climatically by a tropical climate with very high temperatures in summer and warm temperatures throughout the rest of the year. and are classified as Torriorthents / Lithic and Eutric Leptosols (Ali. when the rain factor can be compensated by underground moisture. Examples of these areas can be seen along the watercourses of some parts of Saudi Arabia. the soil moisture regime is considered to be “xeric”. In the same investigations mentioned above. 1988. 2-3 Floral Characteristics of the Sudanian Region In contrast to the Saharo-Arabian region. These soils are Torriorthents / Calcaric Fluvisols. 15. 2-2 Environmental Characteristics of the Sudanian Region The Sudanian region is characterised. Rain is evenly distributed through the rainy season and takes place mainly during winter. where summers are mild. Soils in lava fields are shallow to very shallow over bedrock. savanna. and grassland. In high mountains. in the south and south-western of the Arabian Peninsula are Saudi Journal of Biological Sciences Vol. spring and summer months. Nubo-Sindian local center of endemism. Al-Nafie Fig 5. These soils which occur on gentle to steep slopes are actively eroded. as they are devoid of any genetic horizons. In most wadis and their flood plains where alluvial fans are formed. They replace the region by various African phytochoria of which Somalia-Masai regional center of endemism. The deep loamy soils in terraced slopes are Xerorthents / Calcaric Regosols. The Eritreao-Arabian subregion.Abdulatif H. Leonard (1988-1989) and White and Leonard (1991) suggested the abandonment and break-up of the Sudanian region. Areas where rain is scarce. Soils of Sudanian territories differ according to many natural and human factors. High mountains in the area receive rainfall throughout the year but rainfall reaches its peak in the winter. Red Sea coast south of Makkah and below 1800m on As Sarawat Mountains. At higher altitude Juniperus woodlands. sandy to loamy texture. Phytogeographical regions concerning the Middle East. The main vegetation types of this region are open woodland. semi-evergreen bushland and succulent communities are the dominant vegetation cover over the slopes of As Sarawat Mountains in this region. Afromontane archipelago-like regional centre of endemism and Afroalpine archipelago-like region covers parts of western and south-western Saudi Arabia (Figure 7). soils are deep. 1999). as a result of its ambiguity. numerous species and plant communities. Zohary. and Vernonia cinerascen (Table 3 ). and Maytenus forsskaoliana. specially. Telea nobilis. the south-western part of the Arabian Peninsula has a number of Saharo-Arabian species which grow in Saudi Journal of Biological Sciences Vol. Anisotes trisulcus. In addition to plant species that belong to the Sudanian region. In higher areas evergreen trees. Crinum album. Commicarpus helenae. However. In lower areas of this region in Saudi Arabia Acacia spp. the southern part of the Arabian Peninsula can be named South Arabian Domain and considered as a separate domain under the Sudanian region. Cordia sinensis. Riparian forest and woodlands that grow along temporary water courses are bigger in size than those growing in Africa. Dianthus uniflorus. Euphorbia ammak. Leonard and white (1991) ascertained that south and south-western Arabian Peninsula are very poor in tree species which dose not exceed 300 species among those only a few can be considered as regional endemic. 1986. As a result of the presence of number of endemic species in the southern part of the Arabian Peninsula which have strong links to the species of the Sudanian region. 15. Myrsine africana. to which the grasslands are subjected (Wood. The most characteristic plant species of this region in Saudi Arabia are: Anticharis linearis. The most characteristic species of this region are: Abrus bottae. Buddleja polystachya. 1973). under the zone of Juniperus forest. The most important Sudano-Zambezian presented in Saudi Arabia. Euphorbia fruticosa. Melhania ovata. Aloe sabaea. Acacia gerrardi. Carissa edulis. Rosa abyssinica. this area is the richest part of the Arabian Peninsula in terms of endemic species.are the most dominant. Brachiaria deflexa. In As Sarawat mountains the most dominant species are: Juniperus procera. Aloe. Erica arborea. Acacia abyssinica. Aloe vacillans. Acacia origena. The most dominant genera in this region in Saudi Arabia are: Acacia. Tephrosia uniflora. Although Zohary (1973) and Takhtajan (1986) indicated that south and south-western Arabian Peninsula have 225 endemic species and are considered one of the richest areas of the Sudanian (Sudano-Zambezian) territories.Phytogeography of Saudi Arabia the richest and the most complex with its latitudinal zonation and species diversity containing about 225 endemic species (Takhtajan. Cyanotis nyctitropa. Cichorium bottae. and Nuxia congesta. Hypericum revolutum. No (1) June. Euphorbia inarticulate. shrubs and shrublets which grow also in Africa are dominant but with higher density. species Species Abutilon fruticosum Abutilon hirtum Acacia asak Acacia ehrenbergiana Acacia etbaica Acacia gerrardii Acacia hamulosa Acacia laeta Acacia mellifera Acacia oerfota Acacia raddiana Acacia seyal Acacia tortilis Adenia venenata Aizoon canariense Andrachne aspera Balanites aegyptiaca Barleria acanthoides Breonadia salicina Cadaba longifolia Cadaba rotundifolia Capparis decidua Caylusea hexagyna Chloris barbata Chrozophora oblongifolia Cissus quadrangularis Cissus rotundifolia Clematis hirsuta Cleome arabica Cleome droserifolia Cocculus pendulus Combretum molle Cometes abyssinica Commicarpus grandiflorus Commicarpus plumbagineus Commicarpus sinuatus Commiphora gileadensis Commiphora habessinica Commiphora myrrha Cordia africana Cordia sinensis Crotalaria microphylla Dobera glabra Euphorbia cactus Euphorbia granulata Ficus palmata Ficus vasta Species Grewia tembensis Grewia velutina Hibiscus deflersii Hyphaene thebaica Hypoestes forsskalii Indigofera oblongifolia Indigofera spinosa Jatropha pelargonifolia Lotus glinoides Maerua crassifolia Maytenus parviflora Mimusops laurifolia Morettia parviflora Moringa peregrina Pennisetum unisetum Pentatropis nivalis Pergularia tomentosa Plicosepalus acaciae Polycarpaea repens Premna resinosa Rhynchelytrum repens Rhynchosia minima Salvadora persica Sarcostemma viminale Senna alexandrina Senna italica Senna obtusifolia Sida ovata Solanum incanum Solenostemma argel Suaeda fruticosa Suaeda monoica Tamarix aphylla Teclea nobilis Tephrosia nubica Tephrosia pumila Terminalia brownii Tribulus macropterus Tribulus pentandrus Trichilia emetica Trichodesma ehrenbergii Trigonella hamosa Zaleya pentandra Ziziphus mucronata Ziziphus nummularia Ziziphus spina-christi Zygophyllum simplex 169 . Grewia villosa. Grewia tenax. which tolerate grazing. Barleria bispinosa. and Commiphora spp. Caralluma subulata. Commiphora kataf. Hibiscus micranthum. Solanum incanum. and Indigofera. Sida ovata. Crotalaria. Olea europaea. 2008 Table 3. 1997). Silene coniflora. Pimpinella eriocarpa. Poa sinaica. Most of the plant species presented in the south-western highlands of Saudi Arabia today. Scorzonera papposa. Arenaria deflexa. such as: Haplophyllum tuberculatum. Sisymbrium septulatum. Pterocephalus pulverulentus. Linum bienne. Kickxia elatine. It can be stated that these parts of Saudi Arabia is moving closer to the Mediterranean type of vegetation. Juniperus phoenicea and Anagyris foetida. Sinapis arvensis. Hyoscyamus albus. Plant species whose main area of distribution is centered near the Mediterranean Sea are represented in the flora of south-western the Arabian Peninsula and amount approximately 30 species such as the weeds of waste ground which include Echium rauwolfii. It has a large number of plant species which dominate in one (uniregional). The most important Mediterranean woody species found in Al-Hijaz Mountains are: Lonicera etrusca. Roemeria hybrida. Picris longirostris. Diplotaxis erucoides and Sisymbrium irio (Wood. Gagea commutata. Bromus sericeus. two (bioregional) or more (plurirgional) regions. Hedypnois rhagadioloidea. Hypericum sinaicum. Sedum caespitosum. Gypsophila viscosa. Colutea istria. No (1) June. Orobanche aegyptiaca. Valerianella oxyrrhyncha. Ajuga chamaepitys. Paronychia sinaica. Myrtus communis. Bromus danthoniae. Oligomeris linifolia. Avena barbata. Pimpinella cretica. Holosteum glutinosum. Malcolmia chia. Valerianella szovitsiana. Crassula alata. Ferula sinaica. Anthemis rascheyana. Ferula communis. Valerianella sclerocarpa. are derived from the palaeo-African vegetation that occupied the area throughout the Middle-Late Eocene and Oligocene (c. Psylliostachys spicata. Eremopyrum distans. Dianthus strictus. Ochradenus baccatus. Rheum palaestinum. Scorpiurus muricatus. Scorzonera pusilla. Uniregional. Hyoscyamus pusillus. Crucianella aegyptiaca. Bromus fasciculatus. Minuartia meyeri. Eryngium glomeratum. Ambrosia maritima. Bifora testiculata. Crucianella ciliata. Erodium laciniatum. and Althaea ludwigii. Ephedra foeminea. Lavandula dentata. Atractylis cancellata. Cuscuta pedicellata. Echium judaeum. 2008 170 . 50-24 M yrs BP) before the formation of the Great Rift Valley. Al-Nafie the desert areas and Tihama plain. Bromus diandrus. Centaurea eryngioides. Lonicera etrusca. Sonchus Saudi Journal of Biological Sciences Vol. Lycium depressum. Pimpinella peregrina. Atraphaxis spinosa. Stipa hohenackeriana. Rumex pictus. Cymbolaena griffithii. Medicago truncatula. Papaver decaisnei. Noaea mucronata. Anarrhinum forsskalii. Antirrhimim orontium. Stipa parviflora. Emex spinosa. Filago pyramidata. Crepis aspera. Argyrolobium crotalarioides. Ballota undulata. 2003). Ammi majus. Zoegea leptaurea. Malcolmia crenulata. Echium angustifolium. Launaea fragilis. 1997). Minuartia picta. Anthemis haussknechtii. Echinops viscosus. Prosopis farcta. Gypsophila pilosa. Galium canum. Alyssum desertorum. Gagea reticulata. Typha elephantina. Rhazya stricta. which is not separated from the Irano-Turanian region by major climatic or topographic barriers. Salvia spinosa. Lolium persicum. Haloxylon persicum. Valerianella dufresnia. Bellevalia flexuosa. Diplotaxis erucoides. Vulpia persica. Ammi visnaga. biregional. Linum usitatissimum Lolium multiflorum. Artemisia sieberi. 15. Chorispora purpurascens. Lagurus ovatus.Pisum sativum. Uniregional elements Irano-Turanian elements Plants that are dominant and centred on the Caspian Sea region are well represented in the flora of Central and Northern Saudi Arabia or in the Saharo-Arabian region. Scandix stellata. Phlomis brachyodon. Hypericum hircinum. Neotorularia torulosa. Ononis natrix. Ixiolirion tataricum. Alyssum linifolium. Scleranthus orientalis. Alyssum marginatum. Erucaria hispanica. Boissiera squarrosa. Aethionema carneum. Zosima absinthifolia. Linaria albifrons. Anthemis edumea. Juniperus phoenicea. These elements include: Acantholepis orientalis. Lepidium aucheri. Daphne linearifolia. Orobanche pubescens. Capparis spinosa. Pulicaria arabica. Catapodium rigidum. Anthemis hyaline. Leptaleum filifolium. Enarthrocarpus lyratus. Lens culinare. Lotus halophilus. Typical Mediterranean elements found in flora of Saudi Arabia include: Allium pseudocalyptratum. Phagnalon sinaicum. Stipagrostis ciliatea. Pistacia terebinthus. Gypsophila capillaris. Halothamnus lancifolius. Chrysanthemum segetum.Abdulatif H. Convolvulus pilosellifolius. This palaeotropical vegetation cover was the precursor of the Sudanian Region elements dominating the area (Ghazanfar. Clypeola aspera. Ruta chalepensis. Mediterranean elements Plants whose distribution is mainly around the Mediterranean Sea are well represented in the flora of Northern and North Western of Saudi Arabia. Hippocrepis unisiliquosa. Rhamnus lycioides. Astragalus spinosus. Eremopyrum bonaepartis. and Zygophyllum fabago. plurirgional elements In addition to plant species that belong to the SaharoArabian and the Sudanian regions presented in Saudi Arabia. Rhus tripartita. Carthamus persicus. Ridolfia segetum. Tripleurospermum auriculatum. Asclepiadaceae and Euphorbiaceae are well represented (wood. Arnebia decumbens. Stipagrostis obtusa. Dichanthium foveolatum. Asphodelus tenuifolius. Satureja nabateorum. Momordica balsamina. Tropical elements: Plants whose main area of distribution is the tropics are fairly well represented in the flora of south-western Saudi Arabia especially in Tihama plains and escarpments. Ammania auriculata. Eragrostis pilosa. Capparis decidua. Cutandia memphitica. Hibiscus micranthus. Corchorus olitorius. Panicum turgidum. Lepidium sativum. Gnaphalium luteoalbum. Monsonia nivea. Onobrychis ptolemaica. Forsskaolea tenacissima. Cleome amblyocarpa. Chloris pycnothrix. Saharo-Arabian and Sudanian elements The most dominant Saharo-Arabian and Sudanian elements in the flora of Saudi Arabia include: Arnebia hispidissima. Convolvulus arvensis. Anchusa milleri. Plantago ovata. Equisetum ramoisissimum. Pterogaillonia calycoptera. Limosella aquatica. Onopordum ambiguum. Datura stramonium. Canary Islands. Cardaria draba. Stipa capensis. Lycium shawii. Saharo-Arabian and Irano-Turanian: The most dominant Saharo-Arabian and Irano-Turanian elements in the flora of Saudi Arabia include: Achillea fragrantissima. Malcolmia africana. Eragrostis cilianensis. Eruca sativa. Cardamine hirsute. Ipomoea nil. Helianthemum lippii. Halophila ovalis. Adonis dentata. and Thalassodendron ciliatum. and Saharo-Arabian and Irano-Turanian. Aeluropus lagopoides. and Tetrapogon villosus. and Viscum cruciatum. Euphorbia chamaepeplus. Callipeltis cucullaris. Cymodocea rotundata. Coronopus didymus. Filago desertorum. Chenopodium murale. Ipomoea aquatica. and Zoegea purpurea. Anogramma leptophylla. Anagallis arvensis. Cotula anthemoides. Ipomoea pescaprae. Adiantum capillus-veneris. Pulicaria incisa. Lappula spinocarpos. Amaranthus graecizans. Gastrocotyle hispida. Brassica rapa. Ceratophyllum demersum. Leysera leyseroides. Cuscuta campestris. Syringodium isoetifolium. Bassia muricata. Cyperus articulatus. Bassia eriophora. Juncus bufonius. Alcea striata . Gossypium hirsutum. Astragalus tribuloides. Dinebra retroflexa. Cystopteris fragilis. Chenopodium ambrosioides. Amaranthus virdis. Lemna gibba. Atriplex glauca. Chloris gayana. Halodule uninervis. Thesium humile. Leontodon laciniatus. Corchorus trilocularis. Datura ferox. Jatropha curcas. Leptadenia pyrotechnica. Polycarpaea robbairea. Lasiurus scindicus. Senecio glaucus. Periploca aphylla. Lavandula coronopifolia. Biregional elements in the flora of Saudi Arabia include SaharoArabian and Sudanian elements. Aerva javanica. Paracaryum rugulosum. Calligonum comosum. Taraxacum cyprium. Apium graveolens. Capsella bursapastoris. Hordeum distichon. Allium sindjarensis. Schismus arabicus. Heliotropium bacciferum. Digitaria sanguinalis. Lotononis platycarpa. Achyranthes aspera. Cyperus rotundus. Anethum graveolens. Lavandula pubescens. Atriplex leucoclada. Argemone mexicana. 15. Telephium sphaerospermum. Chenopodium album. 1997). Stipagrostis plumosa. Apium nodiflorum. Deccan plateau. Biregional elements Biregional elements are species which are centered and distributed across two plant regions which are not separated by major climatic or topographic barriers. Leptochloa fusca. Ephedra pachyclada . Tropical families such as Acanthaceae. Vulpia muralis. Digera muricata. Stipagrostis hirtigluma. Rostraria pumila. Cyperus alopecuroides. Erodium oxyrhynchum. Enneapogon desvauxii. Panicum antidotale.Examples found in Saudi Arabia include: Abutilon grandifolium. Peganum harmala. Euphorbia hirta. Tamarix passerinoides. Carduus pycnocephalus. Tolpis virgata. Cerastium glomeratum. Paspalidium germinatus. Hibiscus trionum. Ficus sycomorus. Lactuca serriola. Atriplex dimorphostegia. Cynodon dactylon. 2008 kotschyi. Cladium mariscus. Andropogon distachyos. The presence of these plant species from different regions is an indication on the relationship between the Arabian Peninsula and these regions. and South Africa are represented in the Saudi Arabian flora. Koelpinia linearis. Oldenlandia capensis. Reaumuria hirtella. Arnebia linearifolia. Grewia villosa. Tropical elements in the flora of Saudi Arabia include: Abutilon pannosum. Lablab purpureus. Pulicaria undulata. Some of these species might reached the Arabian Peninsula as a result of the climate changes which the world witnessed a long time ago. Bassia indica. Aegilops Saudi Journal of Biological Sciences Vol. Pluriregional elements: Pluriregional species that grow in many regions and widely spread all over the world such as: Himalayan Mountain. Ammania baccifera. Centropodia forskalii. Digitaria ciliaris. Kickxia petiolata.Phytogeography of Saudi Arabia asper. Gastidium phleoides. 171 . Vitex agnus-castus. or as a result of the distribution by human and animals specially birds which carry seeds for a long distances . Rumex cyprius. No (1) June. Namib desert. Panicum maximum. Brassica juncea. Datura innoxia. Astragalus eremophilus. Lactuca saligna. Dactyloctenium aegyptium. He noticed after many years of observation in the Arabian Peninsula that large wadi system beds and their tributaries that cut through central and northern Saudi Arabia. No (1) June. Typha domingensis. Plantago major. Ricinus communis. Najas graminea. These Sudanian species are believed to be relict from the more humid and cooler pluvial period of the Late Pleistocene and early Holocene. the boundaries of these two regions in Saudi Arabia are still debatable. Phalaris minor. as a key indicator of the Sudanian conditions and marking its northern boundaries. Opuntia ficusindica. Plantago lanceolata . the sand dunes of the research area.Oxalis corniculata. Poa annua. 1. rather than systematic or quantitative analysis. This appears to be based on climate as well as vegetation physiognomy. running mainly from the western high lands to the east. This argument is based on the following Fig 6. Xanthium pungens. Xanthium strumarium. Xanthium brasilicum. Raphanus sativus. Vernonia cinerea. 2. The Saharo-Arabian and Sudanian floristic regions in Saudi Arabia as proposed schematically by Zohary (1973) and Mandaville (1984). Potamogeton panormitanus. still support some Acacia-dominated plant associations. Mandaville applied the strategy of Engler (1910) in considering the presence of Acacia-dominated plant associations. 2008 . but are too dry to support such vegetation. Phragmites australis. hot and vegetationless deserts such as Ar Rub Al Khali. Malva neglecta.The Saharo-Arabian (or Sindian) region. Schoenus nigricans. Boundaries of Saharo-Arabian and Sudanian regions in Saudi Arabia As was mentioned earlier. proposed a new alignment of the frontier for the Saharo-Arabian and Sudanian regions in Saudi Arabia (Figure 6). Sonchus tenerrimus. The delimitation of the frontier between the two regions in the Arabian Peninsula in its southern part has always created some difficulties for a few biogeographers as well as phytogeographers who have studied the region. Nicotiana glauca. with the exception of An Nafud. Phyllanthus rotundifolius. 172 Saudi Journal of Biological Sciences Vol. Setaria verticillata. it is argued here that the presence of Acacia spp. Polygonum aviculare. and Zannichellia palustris. Al-Nafie Lythrum hyssopifolium. Phyla nodiflora. Vicia sativa. ill-defined and very difficult to delimit. Mandaville (1984). and other associates in major wadis and their tributaries that cut through the region can not be considered sufficient reason to include a large part of Saudi Arabia within the Sudanian frontiers. Eig and Zohary extended the southern territories of the Saharo-Arabian region to below the tropic of Cancer. According to Mandaville’s new boundaries. Saponaria hispanica. Portulaca oleracea. Veronica anagallisaquatica. Potamogeton nodosus. as well as the much more vegetated sand dunes of the Ad Dahna and the Great Nafud.Abdulatif H. Potamogeton pectinatus. Suaeda maritima. As shown in (Figures. Polygonum argyrocoleum. Setaria pumila. Nasturtium officinale. the Arabian Peninsula comprises parts of two phytogeographical regions that cover some parts of the Middle East and North Africa. Salsola kali. These difficulties arise from the fact that the southern parts of the Saharo-Arabian region are occupied by very dry. Opuntia dillenii. 2 and 6). will fall completely within the Sudanian region. However. Oxalis pes-caprae. Myriophyllum spicatum. Stellaria media. Schismus barbatus. Xanthium spinosum. Samolus valerandi.The Sudanian (Sudano-Zambezian) region. Zohary (1973) suggested that these deserts could support Sudanian species as a result of their hot climate. Unfortunately. 1. 15. Ruppia maritima. where the Mediterranean and Irano-Turanian species prevail. mountains and oceans or wide seas. continuous destruction of trees for wood burning and lowering the water table through cultivation must also have restricted the presence of these species to very remote wadi beds in places. and according to the environmental setting of Saudi Arabia. The movement of people and their livestock in the region over millennia has contributed to the wide distribution and presence of these species outside their original territories. On the other hand. (1966). is not an adequate means of delimiting such realms or subregions. that can support a wide range of Sudanian species. when the climate in the northern part of the peninsula resembled the climate prevailing now in the Sudanian territories. 6-Dissimilarity between two regions is usually a result of separation by barriers such as very arid desert. This further makes the delimitation of the Sudanian boundaries to the southern part of the Great Nafud questionable. 1991). Zohary (1962. 1973). Other factors such as the history of the flora of the region. 4-Wadi systems. The Sudanian type of climate. 1991) stretches in Saudi Conclusion 173 . or the Arabian regional subzone within Saharo-Sindian regional zone (White and Leonard. these species might be relict from the Late Pleistocene and Early Holocene. since species of each region penetrate to the other. penetration of one species from one of these realms or subregions into another is possible and expected. and thus the interior part can not be included in this category. Ar Rub Al Khali is a very dry sand dune area that is not hospitable to most of the Sudanian species. After reviewing Floras and checklists of Saudi Arabia. topography and the presence or absence of other plant associations should also be taken into consideration. Guest (1966) attributed the absence of some Acacia spp. No (1) June. as well as water courses in the area. so that their absence or presence can not be explained entirely by past or present environmental conditions. as effective barriers to dispersal of further Sudanian species to the interior part of Saudi Arabia from Africa. Guest. 1973) or Nubo-Sindian local centre of endemism belonging to the Saharo-Sindian regional zone (White and Leonard. the climatic conditions. This very dry desert might have acted. from southern Iraq to the extermination of these trees by the Bedouin population who use them for fuel. All this further confuses the question as to whether Acacia-dominated plant associations can legitimately be regarded as an indicator of the Sudanian region vegetation boundary. This further emphasises the fact that drawing a single solid line on the basis of the distribution of one species alone. Some parts of Saudi Arabia have a type of climate that is favourable for the growth of Saharo-Arabian species. especially if this should prove to be a relict distribution. should be included in the East SaharoArabian Subregion which is a part of the Saharo-Arabian (Zohary. and Kurschner. -The greater part of Saudi Arabia. this description can fit only the climate of the south-western highlands of the Arabian Peninsula. The wadis currently present form niches. especially its interior and northern parts. where the environmental factors are similar. 3. (1986) have reported the presence of Acacia spp. 15.It is one of the basic assumptions in biogeography that neighbouring regions might have similarity in flora if the two regions have a similar environment. 1973). and based on the presence and absence of representative plant elements of different plant regions presented in its Flora as well as literature on the phytogeography of the Middle East. form a vegetation enclave for the survival of Acaciadominated plant associations.This area is a transitional area between two floristic realms and three sub-regions. Halevy and Orshan (1972). which can be regarded as ancient thermophilous vegetation. and in southwestern Iraq is attributable to this reason. in addition to the high mountains that surround the Red Sea and extend in the direction of dispersal. The result of the absence of these barriers can be noticed in the strong similarity between the Saharo-Arabian and the Mediterranean and Irano-Turanian deserts. As was mentioned earlier.Phytogeography of Saudi Arabia factors: 1. and other associates further north in several localities in the Jordan valley and south-western Iraq and Sinai. humid vegetation. are almost absent. where the microclimate supports such species. In Saudi Arabia. barriers between the interior part of Saudi Arabia and more northerly areas. 5-Human intervention in recent times must also have had great effect on the distribution of Acacia species. Mandaville Saudi Journal of Biological Sciences Vol. -The Nubo-Sindian Province which is a part of the Sudanian Region (Zohary. (Figure 7). 2. it can be concluded that: -It is almost impossible to draw a solid line between different plant regions in Saudi Arabia. As a result. is characterised by summer rains and high temperature in winter. 2008 (1984) suggested that the presence of these species along the stations of Darb Zobaydah pilgrim track. that are considered as the northernmost boundaries.The presence or absence of Acacia species and other associates should not be the only indicator for the delimitation of such boundaries. On the other hand. Leipzig. Riyadh Chapman. 1999.. “Soils and Summary of Meteorological Data”. Republic of Iraq. Blackwell. In: Chaudhary... Oxford. W. Geomorphology of Arabian Peniusula.. ed.Abdulatif H. Jado and J. Plant Geography of Saudi Arabia. 21. 1. Alfarhan.A. 1. Phytogeographical regions in Saudi Arabia. Flora of the Sultanate of Oman. and Coker. Ministry of Agriculture and Water. 1999. A.Verlag New York.(eds). T. -Some wetter slopes and peaks of As Sarawat mountains over 2800m above sea level form isolated small islands of very impoverished Afroalpine plants with a very low number of species so that it can be included in Afromontane archipelago-like regional centre of endemism. 275-296. Arabia over a narrow strip along the Red Sea coast north of Makkah as well as along Arabian Gulf coast. 1991) stretches over a narrow strip along the Red Sea coast south of Makkah (Tihama plain) and below 1800m on As Sarawat Mountains (Alfarhan. 1995.). Eig. 2 pts. Goudie A. Kent. more phytogeographical and environmental studies to delimit boundaries of different plant regions in Saudi Arabia precisely and accurately are called for. Beih. Distribution of Acacia raddiana. D.6.29. 1968. 2004.... pp. References Ahti. The Chorology of Old World species of Gramineae. G.. R. Ali.).. J.).. Zotl. Sussex. 1910. (4th. The Geography of the Flowering Plants. Vol. 197-208. (eds.” In: A. A.. London. Clayton. 1999. Aan. 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E-Mail: AALNAFIE@Gmail.وتمثل الحدود بين األقاليم النباتي َّة السعودي وشمال شرق أفريقيا ،وموقع المملكة العربي صعوبات للدارسين لها نظراً للتداخل الكبير فيما بينها ،وقلة المعلومات الالزمة لتحديدها. 176 Saudi Journal of Biological Sciences Vol. 2008 .بِّ 7560 .com ملخص َّة في َّ تتناول هذه ِّ الشرق األوسط ،وهي تناقش ما نشر َّباتي َّة ،وموقعها من األقاليم الن َّة السعودي َّة للمملكة العربي الدراسة الجغرافيا النباتي َّ َّة لجنوب غرب آسيا، َّة النباتي َّة حول األقاليم الجغرافي َّة ،والجغرافيا الحيوي َّباتي في هذا الموضوع بوساطة المتخصصين في الجغرافيا الن َّة في المملكة خاصة في جنوب غربها َّة منها .َّة َّة السعودي َّة للمملكة العربي الجغرافيا النباتي عبداللطيف بن حمود َّ النافع َّة َّة ،جامعة اإلمام محمد بن سعود اإلسالمي قسم الجغرافيا ،كلية العلوم االجتماعي الرياض 11432 ص .