Phytogeography of Saudi Arabia

Saudi Journal of Biological Sciences Vol. 15, No (1) June, 2008 159

Phytogeography of Saudi Arabia

Abdulatif H. Al-NafieDepartment of Geography, Faculty of Social Sciences, Imam Muhammad Ibn Saud Islamic University,

P.O. Box 5760, Riyadh, 11432, Saudi Arabia, E-Mail: [email protected]

AbstractThis 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.

Saudi Journal of Biological Sciences 15 (1) 159-176 June, 2008 ISSN 1319-562 XThe Official Journal of the Saudi Biological Society htt:www.saudibiosoc.com

IntroductionSaudi 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,

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 Shield2- 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

Saudi Journal of Biological Sciences Vol. 15, No (1) June, 2008160

Abdulatif H. Al-Nafie

slopes very gently toward the north, north east, and east, under a thin sedimentary core in the eastern part of the Arabian Peninsula, (Powers et al., 1966; Chapman, 1978, Al-Nafie, 1995).

The Arabian Shelf extends to the east of the Arabian Shield. It is made of a sequence of continental and shallow-water marine sedimentary rocks that range in age from Cambrian to Pliocene. 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. Sand accumulations represent the most recent and distinct geomorphological feature of Saudi Arabia. They cover about 37% of the entire land surface of the country. Only Nafud Al Urayq and a few smaller sand dune fields are scattered over the Arabian Shield.

Despite its reputation as a sandy desert, Saudi Arabia comprises several distinct physiographical regions. Eastward from the narrow coastal plains extend along the Red Sea, the western mountain range rises steeply from the sea level, averaging 2,130 metres north west of Al- Madinah Al-Munawarah. Its highest peak, 3,015 metres, is at Jabal Saudah near Abha in the south-western part of the country. These mountains are characterised by steep slopes towards the Red Sea and very gentle slopes towards the interior of the country.

Eastwards of the mountains is the central plateau region including the Pediplain Najd, Hisma Plateau, and Hijaz-Asir Plateau, which together form a high land terrain that composed of local mesas, buttes and lava fields. These plateaus are transected by large wadis and their tributaries, such as Wadi As Sirhan, Wadi Ar Rumah, Wadi Ranyah, Wadi Bishah, Wadi Tathlith and Wadi Ad Dawasir. These wadis flow from the higher mountains in the west, eastward to the plains of Najd, responding to the general slope of the land. These wadis are not continuous, and they are at times covered and buried by the central sand dunes. After passing the sand dunes, some of these wadis emerge under new names. For example, Wadi Ar Rumah emerges as Wadi Al Batin. The presence of these large wadis reflects the wetter climate that existed in the region in the past (Chapman, 1978, Al-Nafie, 1995).

The cuesta region, that dominates the topography of central Saudi Arabia, is made up of a nearly parallel sequence of several prominent crescent-shaped north-south escarpments. 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. The Tuwayq Escarpment (or as it is locally called, Tuwayq Mountain), which is mainly of hard marine limestone capped with upper Jurassic limestone,

extends for about 1200 km. Its average elevation is 840 metres above sea level and 240 metres above the nearby plains. The Al Aramah Escarpment, which is capped with upper Cretaceous limestone, extends 250 km northeast of Ar Riyadh, with a maximum elevation of about 540 metres above sea level and 120 metres above the nearby plains (Chapman, 1978, Al-Nafie, 1995).

Sand seas extend over about 36.9% of the interior part of Saudi Arabia, constituting one of its most prominent physiographic features. These sand dunes are:

1- Ar Rub Al Khali (The Empty Quarter) including Nafud Al Jafurah.2- The Great Nafud3- Ad Dahna4- A group of sand dune bodies that extend mainly along the western side of Tuwayq Escarpment.

Ar Rub Al Khali extends over 486245 sq. km or about 24% of the total area of Saudi Arabia. The Great Nafud is the second largest body of sand, covering about 65504 sq. km. Ar Rub Al Khali and the Great Nafud are connected by the Ad Dahna sand belts, which extend approximately 1,200 kilometres from south east of the Great Nafud to the northern areas of Ar Rub Al Khali and covers 35466 sq. km. These sand dunes and others that intersperse the interior part of the Saudi Arabia, have a variety of complex shapes, including transverse, longitudinal, star and sand mountains. Some of these magnificent red coloured sand dunes might reach heights of 300 metres above the general land surface.

The hard rock plains of As Summan Plateau lie between the Ad Dahna sand dunes and the Arabian Gulf coastal region. This coastal strip is an irregular land surface covered with marshes, salt flats and narrow sandy plains (Al-Nafie, 2004).

As a result of its location, 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. The overall climate of the country, except for Asir Province, is classified as an “arid province” within Thornthwaite’s global climatic classification, and as “dry climates” in Koppen’s classification. According to Meigs’s classification of arid and semi-arid homoclimates, most of Saudi Arabia except Asir Province and Ar Rub’ Al Khali is within an “arid climate.” (Schyfsma, 1978). Most of the country is characterised by hot and almost rainless summers. On the other hand, winter is mainly cold with limited scattered atmospheric precipitation. The mountainous south-western part of the country enjoys a



pleasant summer temperature, with more rain and cold dry winters.

Floristic Composition of the Flora of Saudi ArabiaSpecies, Genus, and Family RepresentationOne of the main characteristics of the vegetation cover of Saudi Arabia is its low floristic diversity. The number of plant species that recorded in the country is 2172 species, many of which are in the wetter areas of its south-western part. These species belong to 840 genera and 149 families (Al-Nafie, 2004). The number of species might be increased to 2250 by adding subspecies, extinct and species that have not been identified yet (Collenette, 1999). Numbers of families, genera, and species are very low compared to Saudi Arabia’s vast land area, which is probably, the result of the harsh environmental conditions that prevail in the Saharo-Arabian region which covers vast area of the country.

Plant families account only for 29% of the families in the world. An outstanding feature of floristic composition of the flora of Saudi Arabia is that a few families are of importance floristically. Among 149 families, 68 families or 45.6% of the total number of families are represented by single genus per family. 36 families, or 24.2% of the total number of families, are represented by one species per family, (Examples: Neuradaceae, Araceae, Aristolochiaceae and Barbeyaceae). The number of genera appears to be very high compared to the number of species. The number of species per genus in Saudi Arabia averages about 2.6 (840/2172). This is a common feature of desert flora. 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.

As in most tropical and subtropical deserts, most plant species of Saudi Arabia belong to a limited number of plant families, for example, 1586 species belong to 23 families or 15.4% of the total families. These plant species represent 73% of the total species in the Kingdom.

Gramineae (262 species), Compositae (233 species), and Papilionaceae (165 species) are represented by 660 species or 30.4% of the total plant species in the Kingdom.Number of families, genera, and species differs in its geographic distribution from one part of the Kingdom to another according to natural and human factors.

The greatest plant diversity, approximately 74% of the total plant species of Saudi Arabia, is found in the mountainous western area due mainly to a greater rainfall. 46 families or 30% of the families in the country such as Aloaceae, Celastraceae, Commelinaceae, and Burseraceae are found only in As Sarawat mountains (AL-Nafie, 2004).Central, eastern, and northern parts of Saudi Arabia have only 26% of the total number of plant species in the Kingdom. The low number of plant species in these areas is due mainly to harsh environmental conditions, as well as the presence of vast areas of sand dunes.

Most of Saudi Arabia’s land area does not seem to be an important centre of diversity. Number of endemic plant species in the Kingdom is 107 species or 4.9% of the total species reported. Endemic taxa in the interior part of the country are not common, 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. Most of endemic species are found in As Sarawat, Al Hijaz and Madian mountains as well as their surroundings in western Saudi Arabia which have 88 endemic species or 82.2% of the total endemic species in the Kingdom (Al-Nafie, 2004).

Phytogeographical BackgroundPhytogeography, which is also known as plant

geography, has been the subject of a number of definitions and different interpretations by different phytogeographers, ecologists and biogeographers in order to stress their particular approach to plant distribution on a global scale (Wickens, 1977a).

To botanists, phytogeography or plant geography, as Good (1974) indicated: “is that branch of botany that deals

Table 1. Number of families, Genera and Species forming flora of Saudi Arabia.

%Species%Genera%FamiliesGroup

1.3272.1189.414Pteridophyta

98.7214597.982290.6135Spermatophyta

1002172100840100149Total

(Al-Nafie, 2004)



with the spatial relationships of plants both in the present and the past. Its aim is to record and then if possible to explain, the distribution of plants over the world’s surface”To geographers, the term biogeography “has meant the study of the biosphere and of human effects on plants and animals. Some parts of the study have been spatial-synoptic (so overlapping with phytogeography and zoogeography), while other parts have been orientated towards processes such as Quaternary ecology and human impact, hence overlapping with ecology”( Goudie, et al., 1994).

It is clear that some overlapping between the three allied disciplines such as phytogeography, ecology and biogeography are inevitable. 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, genetic shift, continental drift, etc., 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”.

Ecology, in the Continental European school of phytosociology, is also concerned with describing and mapping plant communities, explaining their relationships with the environment, and examining the interactions between their constituent species. However, this school has been less concerned with the processes at work than with the precise composition of communities, and with their arrangement in a hierarchical classification according to the degree of floristic similarity between them (Clayton and Cope, 1980). It should be indicated that, although the community approach, which put more emphasis largely at the community level, continued to dominate the philosophy of vegetation description and analysis, the individualistic approach, which investigate vegetation at lower level and in more detail at individual plant and plant species levels, has become widely used and accepted in plant ecology since 1975 (Kent and Coker, 1992).

Chorology, which might be also confused with phytogeography, is concerned with the presence or absence as shown by distribution records rather than with the association between species or their relative abundance (Wickens, 1977a).

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. These natural chorological units or phytochoria were delineated by some authors, such as Schouw (1823), on a purely floristic classification,

namely in accordance with the distribution, presence and absence of certain families, genera and species that are endemic to a particular region. Other botanists and plant geographers, such as Grisebach (1884) and Schimper (1903), applied the physiognomic or structural approach, where the classification is based on external morphology, life-form, 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.

White (1983) proposed the replacement of the traditional phytogeographical hierarchy (Floral Empires, Regions, Provinces, Domains, Sectors, Districts, etc.) by a new system which attempts to establish phytochoria based chiefly on the richness of their endemic flora at the species level. In his study about Africa phytogeography, White (1983) subdivided the continent into regional centres of endemism separated by regional transition zones and by regional mosaics. A regional centre of endemism should have a total of more than 1000 endemic species. In addition, it must have more than 50 percent of its species confined to it. Following White’s system, Leonard (1988-1989) and White and Leonard (1991) sorted out the main phytochoria covering south west Asia and part of Africa.

Zohary (1973) indicated that in delineating a biogeographical region, reliable boundary lines must be based initially on the climatic zones than the taxa dominating these zones. Zohary (1973) has also discussed the diagnostic markers that delineate and characterise plant-geographical regions. According to him:

1-Each region should have a large number of endemics.

2-Each delineated region should have floristic stock where the proportion of endemics to the total number of species is high.

3-Phytogeographical regions must also be speciation areas and centres of certain groups of taxa (centre of diversity).

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.

5-Regions might differ from one another in respect of being recipients or donors. Some regions can be regarded as a recipients (e.g. Saharo-Arabian) since they have collected their species from other regions (e.g. Irano-Turanian) that can be called donors.

6-Horizontal-vegetation units endemic to each region also play a greater role in drawing boundaries between neighbouring regions. Plant communities differ markedly from one region to another although they might display



many common species.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, it should be indicated that the altitudinal zonation complex of each region is peculiar and is another characteristic of the region.

Ahti et al., (1968) suggested that phytogeographical regions can be classified according to one of three approaches: bioclimatic, edaphic-topographic or floristic.The bioclimatic approach can be used to delimit the world into broad climatic zones such as Arctic, Boreal, Temperate zones, etc. For determining smaller phytogeographical units that can not be determined by climatic data alone, other factors such as soil must also be considered. This approach is very useful for understanding the past plant distribution from fossil evidence.

The edaphic-topographic approach is dependent on ecological similarities and can be, therefore, considered as ecological classification.

The floristic approach, as indicated earlier, relies on the distribution pattern of significant groups of families, genera and species. The effect of both historical and ecological factors should also be considered (Wickens, 1977b).

Good (Floristics), (1974), Walter (Floristics + physiognomy) (1979), and Takhtajan (Floristics + endemics) (1986) have suggested six floristic realms in the world, as follows: The Holarctic (Boreal), The Palaeotropical, The Neotropic, The Australian, The Capensis (South African) and The Antarctic.

Each of the six major floristic realms is divided into several sub-regions, provinces and other smaller units. It can be noted that with the exception of the Australian and the Central Asian deserts, all other desert regions in these classifications are situated between two or more floristic realms or regions. Such desert regions, accordingly, create difficulties in the delimitation and the alignment of the frontier of these realms. As an example, the Arabian Peninsula, where Saudi Arabia is located, falls between the Palaeotropic and Holarctic realms. Some phytogeographers have classified it as falling exclusively in the Palaeotropic realm, while others have classified the northern parts of the Arabian Peninsula as being within the Holarctic realm rather than the Palaeotropic, due to its being a floristic part of the Saharo-Arabian region, which shows clear relationships with the Holarctic (Walter, 1979; Zohary, 1973).

Phytogeographic Sub-divisions of the Arabian PeninsulaFor most authorities such as Eig (1931-1933), Gruenberg-

Fertig (1954), Zohary, (1973) White, (1983), Konig, (1988), 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.

1-The Saharo-Sindian Region. (Eig, 1931-1932) or Saharo-Arabian Region (Zohary, 1973) and Saharo-Sindian-Regional Zone (White, 1983).

2-The Sudanian Region (Gruenberg-Fertig 1954) or Sudano-Zambezian Region (Takhtajan, 1986) or Sudano-Zambezo-Sindian Region (Konig, 1988). White (1983), 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. Areas above 1800 m are called Afromontane archipelago-like regional center of endemism (Leonard, 1988-1989, White and Leonard 1991).

Alfarhan (1999) identified three chorological units in Saudi Arabia based on the richness of endemism. Thes are Saharo-Sindian regional zone, Somalia-Masai regional centre of endemism and Afromontane archipelago-like regional centre of endemism. The Saharo-Sindian regional zone is divided into tree subzones; of which; Nubo-Sindian local centre of endemism and the Arabian regional subzone are in Saudi Arabia. He extended the northern boundary of Nubo-Sindian local centre of endemism towards the eastern region to cover most of western Najd plateau.

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, the general approach and terminology of Zohary (1973) for the Saharo-Arabian and Sudanian regions, which are well known, will be used to identify these regions in the forth coming discussion in this paper.

1- Saharo-Arabian1-1 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. This region is bordered by the Mediterranean and Irano-Turanian regions in the north and the Sudano-Deccanian region in the south. He divided the Saharo-Sindian region into three sub-divisions:

1-The Western Saharo-Sindian sub-region.2-The Middle Saharo-Sindian sub-region.3-The Eastern Saharo-Sindian sub-region (Figure 1).According to Eig’s classification, Saudi Arabia falls

within the Middle Saharo-Sindian sub-region, which includes the northern part of the Arabian Peninsula, besides



Egypt, Sinai, parts of lower Palestine, southern and central Jordan and lower Iraq. After Guest, 1966.

Zohary (1973), in later investigations, renamed this broad region and redefined its limits. He excluded the East Saharo-Sindian sub-region from the Saharo-Sindian, and denoted this region as the Saharo-Arabian (Figure 2). Zohary indicated that what Eig had defined as the East Saharo-Sindian is more Palaeotropic than Holarctic, and should not be included with the Saharo-Arabian region.

According to Zohary (1973), the East Saharo-Arabian Subregion of Saharo-Arabian Region covers the interior parts of Saudi Arabia.

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). This region extends from the Atlantic Ocean to the Sind desert and is divided into three main chorological parts: a Sahara regional subzone, an Arabian regional subzone and a Nubo-Sindian local centre of endemism. The Arabian regional subzone covers the interior parts of Saudi Arabia, and Nubo-Sindian local centre of endemism stretches in Saudi Arabia over a narrow a strip around the Red Sea and Arabian Gulf coasts.

1-2 Environmental Characteristics of the Saharo-Arabian RegionThe Saharo-Arabian region is climatically characterised by very high temperatures during the summer and very

low temperatures in winter, especially in the higher parts. High and extreme fluctuations in the diurnal and seasonal temperatures are also characteristic of this climate. Precipitation and atmospheric humidity are normally very low. The annual rainfall ranges from 0 mm to 100 mm, with an average of 40 mm to 150 mm in most parts. This region gets most of its rainfall during the short winter and spring seasons, mainly through the Mediterranean cyclones that pass through the Middle East during this time. On the other hand, the summer season is very dry and long. Evaporation is very high, resulting from the very intensive sunshine and very dry air.

The soils of this region are mainly classified as Torripsamments / Calcatric Arenosols in sandy deserts such as Ar Rub Al-Khali, the Great Nafud as well as Ad Dahna and other small sand dune bodies. Soil in hills, pediments and wadis are also distributed all over the region, some of them are sandy loams, others are gravely sands. They can be classified as Torriorthents / Lithic and Eutric Leptosols in hills and pediments, and Torrifluvents Calcaric Fluvsols in wadis, ravines, and low depressions (Ali, 1999). Soils are dry in most of the year and have very low organic matter and nutrient levels due to the sparse vegetation cover.

Fig 1. Phytogeographical regions of the Northern Hemisphere of the old world, mainly based on Eig`s (1931) and Zohary`s (1950) conceptions. After Guest, 1966.

Fig 2. Phytogeographical regions concerning the Middle East based on Zohary`s (1973) conception.



Perennial species that have adapted to this harsh habitat in several ways amount to 40-50% of the total species number. Species that may behave as annuals, biennials or short-lived perennials, depending on the availability of moisture as well as other ecological conditions, constitute only a small number which dose not exceed 3%. The presence of perennial species, which are mainly shrubs and dwarf shrubs is due to the ability of these species to endure high temperature, movement and burial by sand, as well as the lack of moisture and nutrition. Water conditions in rawdhat, wadis, 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.Perennial species are the main feature of the Saharo-Arabian vegetation for their presence year round, their relatively large size, and their beneficial uses.

1-3 Floral Characteristics of the Saharo-Arabian RegionThe Saharo-Arabian region is characterized by the dominance of shrubs and shrublets (Table 2). No endemic families are recorded and only a few really endemic genera such as Tribulus, Stipagrostis, Fagonia and Farsetia are found. As a result, this region is less clearly defined floristically than other areas (Takhtajan, 1969; Mandaville, 1984).

The Saharo-Arabian region is very poor in species (Takhtajan, 1969). Eig (1931-1932) estimated the number at 1500 species, of which about 1200 were steppe or desert plants and approximately 310 species are endemic (Takhtajan, 1969). Ozenda reported 1200 species for the entire Sahara in Africa (Zohary, 1973). Most of the countries which are occupied largely by the Saharo-Arabian region, such as Iraq, Egypt and the countries in Arabian Peninsula have a low number of species. Sand dunes that cover vast areas in central and northern Saudi Arabia such as the Great Nafud, Ad-Dahna as well as small sand dune bodies scattered in the central part of Saudi Arabia have only 165 species (Al-Nafie, 1995) which consist about 7.6% of the total number of plant species recorded from Saudi Arabia (Al-Nafie, 2004).

This region is very weak in endemic flora at all taxonomic levels. It is also characterised by a very low density of individuals particularly in the sand dunes and surrounding deserts, and one might travel for some distance to come across with few species.

In Ar Rub Al Khali (the Empty Quarter) which covers about 486245 km. sq. only about 20 plant species were recorded. Perennial species that have adapted to this harsh

Plant species in this region show the usual ways of adaptation to aridity and very high temperature in such a harsh environment. Vegetation cover is mainly concentrated in depressions, lower ground areas such as rawdhat, wadis and ravines, where water usually accumulated. Vegetation can also be found over stable sand dunes as well as hollows between them and sand sheets that cover hamadas and stony plains.

Annual plants grow following winter and spring precipitation. Therophytes (annuals and biennials) not surprisingly account for 50-60% of the total species of the region. They usually bloom and form luxurious growth in rawdhat, wadis and at the base of stable dunes, where moisture accumulates after sufficient rain. These annuals are mainly part of the very shallow-rooting herbaceous vegetation. They complete their life cycle within the cool season, when moisture is sufficient and temperature is appropriate. At the beginning of summer, when moisture in the top layer of the soil is almost completely exhausted, these species dry out very quickly. In some years, when there is not enough rain, such annuals might not appear, so that the area witnesses a complete or partial absence of vegetation. Usually, it might requires two periods of rain with an interval of some days to allow these annuals to flourish; if no more rain falls, vegetation cover might then dry up.

Fig 3. 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. After White and Leonard, 199. After White and Leonard, 1991.



Table 2. Saharo-Arabian species presented in Saudi ArabiaSpeciesSpecies

Iphiona mucronataAaronsohnia factorovskyiIphiona scabra Agathophora

alopecuroidesIsatis lusitanicaAizoon hispanicumJuncus punctoriusAllium dictyoprasumKickxia acerbianaAllium sinaiticumLappula sinaicaAlthaea ludwigiiLasiopogon muscoides Anabasis articulataLaunaea angustifoliaAnabasis setiferaLaunaea capitataAnastatica hierochunticaLaunaea massauensisAnchusa aegyptiacaLimonium lobatum Anisosciadium

isosciadiumLinaria haelavaAnthemis desertiiLinaria tenuisAnticharis glandulosaLotus lanuginosusAnvillea garciniiMalva aegyptiaArnebia tinctoriaMaresia pygmaeaArtemisia judaicaMatthiola arabicaArtemisia monospermaMedicago laciniataAsphodelus refractusMoltkiopsis ciliataAsphodelus viscidulusMoricandia sinaicaAsteriscus graveolensNeurada procumbensAstragalus annularisNitraria retusaAstragalus bombycinusNotoceros bicorneAstragalus caprinusOchradenus baccatusAstragalus collenettiaeOgastemma pusillumAstragalus dactylocarpusPancratium sickenbergeriAstragalus fruticosusParietaria alsinifoliaAstragalus intercedensParonychia arabicaAstragalus kahiricusPennisetum divisumAstragalus schimperiPhagnalon barbeyanumAstragalus sieberiPhoenix dactyliferaAstragalus sparsusPicris asplenioidesAtractylis carduusPicris babylonica Atractylis mernephtae Picris cyanocarpaBassia arabicaPlantago amplexicaulisBlepharis ciliarisPlantago ciliataBromus hausknechtiiPlantago cylindricaCalendula tripterocarpaPlantago maris-mortuiCalotropis proceraPolycarpon succulentumCarduus getulusPolygala negevensisCarrichtera annuaPteranthus dichotomousCarthamus nitidusPulicaria arabicaCentaurea ammocyanusPulicaria inuloidesCentaurea scopariaPycnocycla saxatilisCentaurea sinaicaReseda arabicaCitrullus colocynthisReseda muricataColchicum schimperiRetama raetamConvolvulus fatmensisRhamnus dispermaConvolvulus spicatusRhazya strictaConyza stricta

SpeciesSpecies

Rumex vesicariusCornulaca monacanthaSalsola cyclophyllaCornulaca spp.Salsola incanescans Crucianella membranaceaSalsola inermisCucumis prophetarumSalsola jordanicolaCymbopogon commutatusSalsola schweinfurthiiCyperus macrorrhizusSalsola tetrandraDeverra tortuosaSalvia aegyptiacaDeverra triradiataSalvia desertiDichanthium annulatumSavignya parvifloraDipcadi erythraeumSchimpera arabicaDiplotaxis acrisSclerocephalus arabicusDiplotaxis harraScrophularia desertiEchinops glaberrimusScropularia hypericifolia Echium rauwolfiiSeidlitzia rosmarinusEphedra alataSenecio flavus Ephedra aphyllaSilene arabicaEremobium aegyptiacumSilene hussoniiErodium glaucophyllumSilene villosaErodium neuradifoliumSilene vivianiiErodium touchyanumSpergula fallaxEuphorbia grossheimiiStachys aegyptiaca Euphorbia retusaStipagrostis ciliataFagonia indicaStipagrostis drariiGlobularia arabicaStipagrostis raddianaGomphocarpus sinaicusSuaeda aegyptiacaGymnarrhena micranthaSuaeda vermiculataGymnocarpos decandrumTamarix niloticaHaplophyllum

tuberculatumTeucrium leucocladumHelianthemum kahiricumThymus boveiHelianthemum sancti-

antoniiTraganum nudatumHelianthemum

sessiliflorumTrichodesma africanumHelianthemum stipulatumTrichodesma boissieriHeliotropium arbainenseVerbascum decaisneanumHeliotropium digynumVerbascum schimperianum

Herniaria hemistemon

Volutaria crupinioidesHippocrepis constrictaVolutaria lippiiHorwoodia dicksoniaeZilla spinosaHyoscyamus desertorum Zygophyllum albumHyoscyamus muticus Zygophyllum coccineumIfloga ruepellii

Ifloga spicata

habitat amount to 89.2% of the total species number. Species that may behave as annuals, biennials or short-lived perennials, depend on the availability of moisture as well as other ecological conditions for their survival, constitute



only a small percentage, 10,8%. Salt flats and gravel floors between the sand dunes are almost devoid of vegetation. Annual species, as well as tree forms, are almost absent from the inner zones, but might occur in the adjacent areas (Mandaville 1986).

Many of the Saharo-Arabian elements are derived and developed from the neighbouring regions, mainly the Sudanian region in the south, and the Mediterranean and Irano-Turanian regions in the north and north-west. They are developed gradually but discontinuously under the increase of aridity since the Middle Miocene, which is believed to be a transitional period climatically between the humid early Tertiary and the arid Late Tertiary and Quaternary environments (Anton, 1984).

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, including Gramineae, Cyperaceae, Compositae, and Caryophylceae. In addition, tropical families such as the Meliaceae, Sapotaceae, Combetaceae, Myrtaceae, Ceratopteridaceae and Palmae were also present (Mandaville, 1990).

Aridity has accelerated during the Holocene, especially in the last 3000 to 2000 years. The vegetation cover of the Saharo-Arabian region might have taken its present form since that time. Human populations inhabited the central and northern parts of Saudi Arabia as early as the early Pleistocene, 1.5 M.Y. B. P. ( Masry, 1977), there activities and action such as overgrazing and wood cutting are accountable for some environmental destruction and pressure over this period.

2- The Sudanian Region2-1 Region names and boundariesThe Sudanian region extends to the south of the Saharo-Arabian region occupies in the northern hemisphere, part of tropical Africa, north of the equatorial rain forest region, as well as the south-western corner of the Arabian Peninsula, a wide strip along the Arabian Gulf, the Oman Gulf and the Arabian Sea, southern Iran and Baluchistan (Figure 2).

This region was known as Sudano-Deccanian (Eig, 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- Zambezian Subregion a- Zambezian Province2- Sahelo-Sudanian Subregion a- Sahelian Province b- Sudanian Province3- Eritreo-Arabian Subregion Province a- Somalo-Ethiopian

b- South Arabian Province c- Socotran ProvinceSubregion 4-Omano-Sindian a- Province of Oman b- South Iranian Province c- Sindian Province(Takhtajan, 1986) Konig (1988) named this region as Sudano-Zambezo-Sindian and divided it into six domains as follows: 1- Sahelian Domain Domain 2- Sudanian 3- South Arabian Domain Domain 4- Afro-oriental Domain 5- Zambezian Domain (Figure 5). 6- 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

Fig 4. Phytogeographical regions concerning the Middle East, Africa, and neighbouring regions as proposed by Takhtajan, 1986.



Red Sea coast south of Makkah and below 1800m on As Sarawat Mountains. 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.

In the same investigations mentioned above, White (1983), Leonard (1988-1989) and White and Leonard (1991) suggested the abandonment and break-up of the Sudanian region, as a result of its ambiguity. They replace the region by various African phytochoria of which Somalia-Masai regional center of endemism, Nubo-Sindian local center of endemism, Afromontane archipelago-like regional centre of endemism and Afroalpine archipelago-like region covers parts of western and south-western Saudi Arabia (Figure 7).

2-2 Environmental Characteristics of the Sudanian RegionThe Sudanian region is characterised, climatically by a tropical climate with very high temperatures in summer

and warm temperatures throughout the rest of the year. Precipitation and atmospheric humidity are normally high, with an average rainfall exceeding 400 mm per year. Rain is evenly distributed through the rainy season and takes place mainly during winter, spring and summer months. High mountains in the area receive rainfall throughout the year but rainfall reaches its peak in the winter, spring and summer months.

Soils of Sudanian territories differ according to many natural and human factors, Tihama plain has deep loamy soils, which are Camborthids/Calcaric Cambisols. They are mostly loamy and occasionally gravel. In high mountains, where summers are mild, the soil moisture regime is considered to be “xeric”. The deep loamy soils in terraced slopes are Xerorthents / Calcaric Regosols. In most wadis and their flood plains where alluvial fans are formed, soils are deep, dominantly loamy and show stratification in most of their volume. Sometimes they are formed of gravel. These soils are Torriorthents / Calcaric Fluvisols, as they are devoid of any genetic horizons. Soils in lava fields are shallow to very shallow over bedrock. They are non-saline to strongly saline and have gravely, sandy to loamy texture. These soils which occur on gentle to steep slopes are actively eroded, and are classified as Torriorthents / Lithic and Eutric Leptosols (Ali, 1999).

The Sudanian territories in Saudi Arabia are mainly characterised by tropical vegetation where temperatures are high enough to support it, but rainfall is too low to maintain typical savanna or any other tropical arboreal vegetation. Areas where rain is scarce, support Sudanian species, when the rain factor can be compensated by underground moisture, or moisture stored in the soil at the end of the rainy period. Examples of these areas can be seen along the watercourses of some parts of Saudi Arabia.

At higher altitude Juniperus woodlands, semi-evergreen bushland and succulent communities are the dominant vegetation cover over the slopes of As Sarawat Mountains in this region. Forest and woodland are scattered and still survive in suitable areas all over the region. In the desert plains and lower slopes of the mountains pseudo-savanna comprising species such as Acacia spp. is the dominant vegetation cover.

2-3 Floral Characteristics of the Sudanian RegionIn contrast to the Saharo-Arabian region, The Sudanian region is characterised by hundreds of genera, numerous species and plant communities. The main vegetation types of this region are open woodland, savanna, and grassland. The Eritreao-Arabian subregion, South Arabian Province, in the south and south-western of the Arabian Peninsula are

Fig 5. Phytogeographical regions concerning the Middle East, Africa, and neighbouring regions as proposed by Konig, 1988.



Table 3. The most important Sudano-Zambezian species presented in Saudi Arabia.

SpeciesSpecies

Grewia tembensisAbutilon fruticosum Grewia velutinaAbutilon hirtum Hibiscus deflersiiAcacia asakHyphaene thebaicaAcacia ehrenbergiana Hypoestes forsskaliiAcacia etbaicaIndigofera oblongifolia Acacia gerrardiiIndigofera spinosa Acacia hamulosa

Jatropha pelargonifoliaAcacia laetaLotus glinoidesAcacia melliferaMaerua crassifoliaAcacia oerfotaMaytenus parvifloraAcacia raddiana Mimusops laurifoliaAcacia seyalMorettia parvifloraAcacia tortilisMoringa peregrinaAdenia venenataPennisetum unisetumAizoon canariensePentatropis nivalisAndrachne asperaPergularia tomentosaBalanites aegyptiacaPlicosepalus acaciaeBarleria acanthoidesPolycarpaea repens Breonadia salicinaPremna resinosa Cadaba longifoliaRhynchelytrum repens Cadaba rotundifoliaRhynchosia minimaCapparis deciduaSalvadora persicaCaylusea hexagynaSarcostemma viminaleChloris barbataSenna alexandrinaChrozophora oblongifoliaSenna italicaCissus quadrangularis Senna obtusifoliaCissus rotundifolia Sida ovata Clematis hirsutaSolanum incanumCleome arabicaSolenostemma argelCleome droserifoliaSuaeda fruticosaCocculus pendulusSuaeda monoicaCombretum molle Tamarix aphyllaCometes abyssinicaTeclea nobilis Commicarpus grandiflorusTephrosia nubicaCommicarpus plumbagineus

Tephrosia pumila Commicarpus sinuatusTerminalia brownii Commiphora gileadensis Tribulus macropterusCommiphora habessinica Tribulus pentandrusCommiphora myrrha Trichilia emetica Cordia africanaTrichodesma ehrenbergiiCordia sinensisTrigonella hamosaCrotalaria microphyllaZaleya pentandra Dobera glabraZiziphus mucronata Euphorbia cactusZiziphus nummulariaEuphorbia granulataZiziphus spina-christiFicus palmata Zygophyllum simplex Ficus vasta

the richest and the most complex with its latitudinal zonation and species diversity containing about 225 endemic species (Takhtajan, 1986; Zohary, 1973).

The most dominant genera in this region in Saudi Arabia are: Acacia, Aloe, Crotalaria, and Indigofera, which tolerate grazing, to which the grasslands are subjected (Wood, 1997). The most characteristic plant species of this region in Saudi Arabia are: Anticharis linearis, Brachiaria deflexa, Commicarpus helenae, Cordia sinensis, Grewia tenax, Grewia villosa, Hibiscus micranthum, Melhania ovata, Sida ovata, Solanum incanum, Tephrosia uniflora, and Vernonia cinerascen (Table 3 ).

In lower areas of this region in Saudi Arabia Acacia spp. and Commiphora spp.are the most dominant. In higher areas evergreen trees, shrubs and shrublets which grow also in Africa are dominant but with higher density, specially, under the zone of Juniperus forest. Riparian forest and woodlands that grow along temporary water courses are bigger in size than those growing in Africa. In As Sarawat mountains the most dominant species are: Juniperus procera, Acacia abyssinica, Acacia gerrardi, Acacia origena, Buddleja polystachya, Carissa edulis, Erica arborea, Hypericum revolutum, Myrsine africana, Olea europaea, Rosa abyssinica, Telea nobilis, and Nuxia congesta.

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, the southern part of the Arabian Peninsula can be named South Arabian Domain and considered as a separate domain under the Sudanian region. The most characteristic species of this region are: Abrus bottae, Aloe vacillans, Aloe sabaea, Anisotes trisulcus, Barleria bispinosa, Commiphora kataf, Cichorium bottae, Cyanotis nyctitropa, Crinum album, Caralluma subulata, Dianthus uniflorus, Euphorbia fruticosa, Euphorbia ammak, Euphorbia inarticulate, and Maytenus forsskaoliana.

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, 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. However, this area is the richest part of the Arabian Peninsula in terms of endemic species.

In addition to plant species that belong to the Sudanian region, the south-western part of the Arabian Peninsula has a number of Saharo-Arabian species which grow in



the desert areas and Tihama plain, such as: Haplophyllum tuberculatum, Ochradenus baccatus, Oligomeris linifolia, Pulicaria arabica, Rhazya stricta, Stipagrostis ciliatea, and Althaea ludwigii.

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, Diplotaxis erucoides and Sisymbrium irio (Wood, 1997). The most important Mediterranean woody species found in Al-Hijaz Mountains are: Lonicera etrusca, Pistacia terebinthus, Juniperus phoenicea and Anagyris foetida.

Most of the plant species presented in the south-western highlands of Saudi Arabia today, are derived from the palaeo-African vegetation that occupied the area throughout the Middle-Late Eocene and Oligocene (c. 50-24 M yrs BP) before the formation of the Great Rift Valley. This palaeotropical vegetation cover was the precursor of the Sudanian Region elements dominating the area (Ghazanfar, 2003).

Uniregional, biregional, plurirgional elementsIn addition to plant species that belong to the Saharo-Arabian and the Sudanian regions presented in Saudi Arabia. It has a large number of plant species which dominate in one (uniregional), two (bioregional) or more (plurirgional) regions.

Uniregional elementsIrano-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, which is not separated from the Irano-Turanian region by major climatic or topographic barriers. These elements include:

Acantholepis orientalis, Aethionema carneum, Ajuga chamaepitys, Alyssum desertorum, Alyssum linifolium, Alyssum marginatum, Anthemis edumea, Anthemis haussknechtii, Anthemis hyaline, Anthemis rascheyana, Argyrolobium crotalarioides, Artemisia sieberi, Astragalus spinosus, Atraphaxis spinosa, Boissiera squarrosa, Bromus danthoniae, Carthamus persicus, Centaurea eryngioides, Chorispora purpurascens, Clypeola aspera, Colutea istria, Convolvulus pilosellifolius, Crucianella ciliata, Cuscuta pedicellata, Cymbolaena griffithii, Eremopyrum bonaepartis, Eremopyrum distans, Ferula sinaica, Gagea commutata, Gagea reticulata, Gypsophila capillaris, Gypsophila pilosa, Gypsophila viscosa, Halothamnus

lancifolius, Haloxylon persicum, Holosteum glutinosum, Hyoscyamus pusillus, Hypericum sinaicum, Ixiolirion tataricum, Lepidium aucheri, Leptaleum filifolium, Linaria albifrons, Lolium persicum, Lycium depressum, Minuartia meyeri, Minuartia picta, Neotorularia torulosa, Noaea mucronata, Orobanche aegyptiaca, Papaver decaisnei, Paronychia sinaica, Phagnalon sinaicum, Phlomis brachyodon, Picris longirostris, Pimpinella eriocarpa,Pisum sativum, Poa sinaica, Prosopis farcta, Psylliostachys spicata, Pterocephalus pulverulentus, Rheum palaestinum, Rhus tripartita, Roemeria hybrida, Salvia spinosa, Scandix stellata, Scorzonera papposa, Scorzonera pusilla, Silene coniflora, Sisymbrium septulatum, Stipa hohenackeriana, Stipa parviflora, Tripleurospermum auriculatum, Typha elephantina, Valerianella dufresnia, Valerianella oxyrrhyncha, Valerianella sclerocarpa, Valerianella szovitsiana, Vulpia persica, Zoegea leptaurea, Zosima absinthifolia, and Zygophyllum fabago.

Mediterranean elementsPlants whose distribution is mainly around the Mediterranean Sea are well represented in the flora of Northern and North Western of Saudi Arabia. It can be stated that these parts of Saudi Arabia is moving closer to the Mediterranean type of vegetation. Typical Mediterranean elements found in flora of Saudi Arabia include:

Allium pseudocalyptratum, Ambrosia maritima, Ammi majus, Ammi visnaga, Anarrhinum forsskalii, Antirrhimim orontium, Arenaria deflexa, Atractylis cancellata, Avena barbata, Ballota undulata, Bellevalia flexuosa, Bifora testiculata, Bromus diandrus, Bromus fasciculatus, Bromus sericeus, Capparis spinosa, Catapodium rigidum, Chrysanthemum segetum, Crassula alata, Crepis aspera, Crucianella aegyptiaca, Daphne linearifolia, Dianthus strictus, Diplotaxis erucoides, Echium angustifolium, Echium judaeum, Echinops viscosus, Emex spinosa, Enarthrocarpus lyratus, Ephedra foeminea, Erodium laciniatum, Erucaria hispanica, Eryngium glomeratum, Ferula communis, Filago pyramidata, Galium canum, Hedypnois rhagadioloidea, Hippocrepis unisiliquosa, Hyoscyamus albus, Hypericum hircinum, Juniperus phoenicea, Kickxia elatine, Lagurus ovatus, Launaea fragilis, Lavandula dentata, Lens culinare, Linum bienne, Linum usitatissimum Lolium multiflorum, Lonicera etrusca, Lotus halophilus, Malcolmia chia, Malcolmia crenulata, Medicago truncatula, Myrtus communis, Ononis natrix, Orobanche pubescens, Pimpinella cretica, Pimpinella peregrina, Rhamnus lycioides, Ridolfia segetum, Rumex pictus, Ruta chalepensis, Scleranthus orientalis, Scorpiurus muricatus, Sedum caespitosum, Sinapis arvensis, Sonchus



asper, Taraxacum cyprium, Thesium humile, Tolpis virgata, Vulpia muralis, Vitex agnus-castus, and Viscum cruciatum.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, Tropical families such as Acanthaceae, Asclepiadaceae and Euphorbiaceae are well represented (wood, 1997). Tropical elements in the flora of Saudi Arabia include:

Abutilon pannosum, Achyranthes aspera, Aerva javanica, Chloris pycnothrix, Corchorus olitorius, Corchorus trilocularis, Cotula anthemoides, Cymodocea rotundata, Cyperus alopecuroides, Cyperus articulatus, Digera muricata, Dinebra retroflexa, Ficus sycomorus, Grewia villosa, Halophila ovalis, Halodule uninervis, Hibiscus micranthus, Hibiscus trionum, Ipomoea pes-caprae, Leptochloa fusca, Momordica balsamina, Oldenlandia capensis, Panicum antidotale, Panicum maximum, Paspalidium germinatus, Syringodium isoetifolium, and Thalassodendron ciliatum.

Biregional elementsBiregional elements are species which are centered and distributed across two plant regions which are not separated by major climatic or topographic barriers. Biregional elements in the flora of Saudi Arabia include Saharo-Arabian and Sudanian elements, and Saharo-Arabian and Irano-Turanian.

Saharo-Arabian and Sudanian elementsThe most dominant Saharo-Arabian and Sudanian elements in the flora of Saudi Arabia include:Arnebia hispidissima, Asphodelus tenuifolius, Astragalus eremophilus, Leptadenia pyrotechnica, Bassia eriophora, Leysera leyseroides, Capparis decidua, Cleome amblyocarpa, Dichanthium foveolatum, Erodium oxyrhynchum, Forsskaolea tenacissima, Heliotropium bacciferum, Helianthemum lippii, Lasiurus scindicus, Lavandula coronopifolia, Lavandula pubescens, Lotononis platycarpa, Lycium shawii, Monsonia nivea, Panicum turgidum, Periploca aphylla, Polycarpaea robbairea, Pterogaillonia calycoptera, Pulicaria incisa, Pulicaria undulata, Stipagrostis hirtigluma, Stipagrostis obtusa, Tamarix passerinoides, Telephium sphaerospermum, and Tetrapogon villosus.

Saharo-Arabian and Irano-Turanian:The most dominant Saharo-Arabian and Irano-Turanian elements in the flora of Saudi Arabia include:

Achillea fragrantissima, Adonis dentata, Aegilops

kotschyi, Aeluropus lagopoides, Allium sindjarensis, Anchusa milleri, Arnebia decumbens, Arnebia linearifolia, Astragalus tribuloides, Atriplex dimorphostegia, Atriplex glauca, Atriplex leucoclada, Bassia indica, Bassia muricata, Calligonum comosum, Callipeltis cucullaris, Centropodia forskalii, Cutandia memphitica, Enneapogon desvauxii, Ephedra pachyclada , Euphorbia chamaepeplus, Filago desertorum, Gastrocotyle hispida, Kickxia petiolata, Koelpinia linearis, Lappula spinocarpos, Leontodon laciniatus, Malcolmia africana, Onopordum ambiguum, Onobrychis ptolemaica, Paracaryum rugulosum, Peganum harmala, Plantago ovata, Reaumuria hirtella, Rostraria pumila, Rumex cyprius, Satureja nabateorum, Schismus arabicus, Senecio glaucus, Stipa capensis, Stipagrostis plumosa, and Zoegea purpurea.

Pluriregional elements:Pluriregional species that grow in many regions and widely spread all over the world such as: Himalayan Mountain, Deccan plateau, Canary Islands, Namib desert, and South Africa are represented in the Saudi Arabian flora. The presence of these plant species from different regions is an indication on the relationship between the Arabian Peninsula and these regions. Some of these species might reached the Arabian Peninsula as a result of the climate changes which the world witnessed a long time ago, or as a result of the distribution by human and animals specially birds which carry seeds for a long distances .Examples found in Saudi Arabia include: Abutilon grandifolium, Adiantum capillus-veneris, Alcea striata , Amaranthus graecizans, Amaranthus virdis, Ammania auriculata, Ammania baccifera, Anagallis arvensis, Andropogon distachyos, Anethum graveolens, Anogramma leptophylla, Apium graveolens, Apium nodiflorum, Argemone mexicana, Brassica juncea, Brassica rapa, Capsella bursa-pastoris, Cardamine hirsute, Cardaria draba, Carduus pycnocephalus, Cerastium glomeratum, Ceratophyllum demersum, Chenopodium album, Chenopodium ambrosioides, Chenopodium murale, Chloris gayana, Cladium mariscus, Convolvulus arvensis, Coronopus didymus, Cuscuta campestris, Cynodon dactylon, Cyperus rotundus, Cystopteris fragilis, Dactyloctenium aegyptium, Datura ferox, Datura innoxia, Datura stramonium, Digitaria ciliaris, Digitaria sanguinalis, Equisetum ramoisissimum, Eragrostis cilianensis, Eragrostis pilosa, Eruca sativa, Euphorbia hirta, Gastidium phleoides, Gnaphalium luteo-album, Gossypium hirsutum, Hordeum distichon, Ipomoea aquatica, Ipomoea nil, Jatropha curcas, Juncus bufonius, Lablab purpureus, Lactuca saligna, Lactuca serriola, Lemna gibba, Lepidium sativum, Limosella aquatica,



Lythrum hyssopifolium, Malva neglecta, Myriophyllum spicatum, Najas graminea, Nasturtium officinale, Nicotiana glauca, Opuntia dillenii, Opuntia ficus-indica,Oxalis corniculata, Oxalis pes-caprae, Phalaris minor, Phragmites australis, Phyla nodiflora, Phyllanthus rotundifolius, Plantago lanceolata , Plantago major, Poa annua, Polygonum argyrocoleum, Polygonum aviculare, Portulaca oleracea, Potamogeton nodosus, Potamogeton panormitanus, Potamogeton pectinatus, Raphanus sativus, Ricinus communis, Ruppia maritima, Salsola kali, Samolus valerandi, Saponaria hispanica, Schismus barbatus, Schoenus nigricans, Setaria pumila, Setaria verticillata, Sonchus tenerrimus, Stellaria media, Suaeda maritima, Typha domingensis, Vernonia cinerea, Veronica anagallis-aquatica, Vicia sativa, Xanthium brasilicum, Xanthium pungens, Xanthium spinosum, Xanthium strumarium, and Zannichellia palustris.

Boundaries of Saharo-Arabian and Sudanian regions in Saudi ArabiaAs was mentioned earlier, the Arabian Peninsula comprises parts of two phytogeographical regions that cover some parts of the Middle East and North Africa.

1- The Saharo-Arabian (or Sindian) region.2- The Sudanian (Sudano-Zambezian) region.Unfortunately, the boundaries of these two regions in

Saudi Arabia are still debatable, ill-defined and very difficult to delimit. 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. These difficulties arise from the fact that the southern parts

of the Saharo-Arabian region are occupied by very dry, hot and vegetationless deserts such as Ar Rub Al Khali, as well as the much more vegetated sand dunes of the Ad Dahna and the Great Nafud. Zohary (1973) suggested that these deserts could support Sudanian species as a result of their hot climate, but are too dry to support such vegetation. As shown in (Figures, 1, 2 and 6), Eig and Zohary extended the southern territories of the Saharo-Arabian region to below the tropic of Cancer. This appears to be based on climate as well as vegetation physiognomy, rather than systematic or quantitative analysis. Mandaville (1984), proposed a new alignment of the frontier for the Saharo-Arabian and Sudanian regions in Saudi Arabia (Figure 6). Mandaville applied the strategy of Engler (1910) in considering the presence of Acacia-dominated plant associations. as a key indicator of the Sudanian conditions and marking its northern boundaries. 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, running mainly from the western high lands to the east, still support some Acacia-dominated plant associations. These Sudanian species are believed to be relict from the more humid and cooler pluvial period of the Late Pleistocene and early Holocene. According to Mandaville’s new boundaries, the sand dunes of the research area, with the exception of An Nafud, will fall completely within the Sudanian region.

However, it is argued here that the presence of Acacia spp. 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. This argument is based on the following

Fig 6. The Saharo-Arabian and Sudanian floristic regions in Saudi Arabia as proposed schematically by Zohary (1973) and Mandaville (1984).



factors:1- The presence or absence of Acacia species and

other associates should not be the only indicator for the delimitation of such boundaries. Other factors such as the history of the flora of the region, the climatic conditions, topography and the presence or absence of other plant associations should also be taken into consideration.

2- This area is a transitional area between two floristic realms and three sub-regions. As a result, penetration of one species from one of these realms or subregions into another is possible and expected. This further emphasises the fact that drawing a single solid line on the basis of the distribution of one species alone, especially if this should prove to be a relict distribution, is not an adequate means of delimiting such realms or subregions.

3- 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. Some parts of Saudi Arabia have a type of climate that is favourable for the growth of Saharo-Arabian species. The Sudanian type of climate, that can support a wide range of Sudanian species, is characterised by summer rains and high temperature in winter. In Saudi Arabia, this description can fit only the climate of the south-western highlands of the Arabian Peninsula, and thus the interior part can not be included in this category.

4-Wadi systems, as well as water courses in the area, form a vegetation enclave for the survival of Acacia-dominated plant associations, which can be regarded as ancient thermophilous vegetation. As was mentioned earlier, these species might be relict from the Late Pleistocene and Early Holocene, when the climate in the northern part of the peninsula resembled the climate prevailing now in the Sudanian territories. The wadis currently present form niches, where the microclimate supports such species. Zohary (1962, 1973), Guest, (1966), Halevy and Orshan (1972), and Kurschner, (1986) have reported the presence of Acacia spp. and other associates further north in several localities in the Jordan valley and south-western Iraq and Sinai, that are considered as the northernmost boundaries. This further makes the delimitation of the Sudanian boundaries to the southern part of the Great Nafud questionable.

5-Human intervention in recent times must also have had great effect on the distribution of Acacia species, so that their absence or presence can not be explained entirely by past or present environmental conditions. 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. Mandaville

(1984) suggested that the presence of these species along the stations of Darb Zobaydah pilgrim track, and in south-western Iraq is attributable to this reason. On the other hand, 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. Guest (1966) attributed the absence of some Acacia spp. 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.

6-Dissimilarity between two regions is usually a result of separation by barriers such as very arid desert, humid vegetation, mountains and oceans or wide seas. Ar Rub Al Khali is a very dry sand dune area that is not hospitable to most of the Sudanian species. This very dry desert might have acted, in addition to the high mountains that surround the Red Sea and extend in the direction of dispersal, as effective barriers to dispersal of further Sudanian species to the interior part of Saudi Arabia from Africa. On the other hand, barriers between the interior part of Saudi Arabia and more northerly areas, where the Mediterranean and Irano-Turanian species prevail, are almost absent. 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.

ConclusionAfter reviewing Floras and checklists of Saudi Arabia, 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, and according to the environmental setting of Saudi Arabia, it can be concluded that:

-It is almost impossible to draw a solid line between different plant regions in Saudi Arabia, since species of each region penetrate to the other, where the environmental factors are similar.

-The greater part of Saudi Arabia, especially its interior and northern parts, should be included in the East Saharo-Arabian Subregion which is a part of the Saharo-Arabian (Zohary, 1973), or the Arabian regional subzone within Saharo-Sindian regional zone (White and Leonard, 1991), (Figure 7).

-The Nubo-Sindian Province which is a part of the Sudanian Region (Zohary, 1973) or Nubo-Sindian local centre of endemism belonging to the Saharo-Sindian regional zone (White and Leonard, 1991) stretches in Saudi



Arabia over a narrow strip along the Red Sea coast north of Makkah as well as along Arabian Gulf coast.

-The Eritreo-Arabian Province which is also a part of the Sudanian Region (Zohary, 1973) or the Somalia-Masai regional centre of endemism (White and Leonard, 1991) stretches over a narrow strip along the Red Sea coast south of Makkah (Tihama plain) and below 1800m on As Sarawat Mountains (Alfarhan, 1999).

-Mountains higher than 1800m above sea level should be considered as an impoverished outlier of Afromontane archipelago-like regional centre of endemism (White and Leonard, 1991). The dominant species in this area are Juniperus procera and several evergreen shrubs.

-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.

Finally, more phytogeographical and environmental studies to delimit boundaries of different plant regions in Saudi Arabia precisely and accurately are called for.

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الجغرافيا النباتيَّة للمملكة العربيَّة السعوديَّة

عبداللطيف بن حمود النَّافعقسم الجغرافيا، كلية العلوم االجتماعيَّة، جامعة اإلمام محمد بن سعود اإلسالميَّة

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Phytogeography of Saudi Arabia