Phytogeography of the tropical north-east African mountains

The tropical north-east African mountains are tentatively divided into four phytochoria, the formal rank of which is not defined. The division is based on patterns of distribution and endemism in the region. The recognition of a distinct Afromontane phytochorion is now well established (Chapman & White, 1970; Werger, 1978; White, 1978). However, there is still very little information on the phytogeography of the individual mountains or mountain systems. This study hopes to fill a little of the gap by analysing distribution patterns and patterns of endemism in the flora of the tropical north-east African mountains. The north-east African mountain system is the largest in tropical Africa (see e.g. map in White, 1978). At the core of this system is the large Ethiopian massif, around which are located various mountains and mountain chains. These include the Red Sea Hills in the Sudan, the mountain chain in northern Somalia, the south-west Arabian mountains, and the Imatong mountains of south-east Sudan. The latter are often referred to the East African mountain system (White, 1978) but. as I will point out later, they also have a close connection with the south-west highlands of Ethiopia. The paper presents some results of my study of the mountain flora of tropical north-east Africa, particularly the forest species. Where no source is indicated, the data are from my own unpublished studies.

The paper presents some results of my study of the mountain flora of tropical north-east Africa, particularly the forest species.Where no source is indicated, the data are from my own unpublished studies.

RESUME LA PHYT O GÉO GRA PH IE DES MONTAGNES TROPICALES D E L 'A F R IQ U E D U NORD-EST
Les montagnes tropicales du nord-est africain sont divisées á titre d'essai en quatre phytochories, dont le rang nest pas défini.La division est basée sur les modes de distribution et d 'endémisme dans la région.

PATTERNS OF ENDEMISM
The endemism of the tropical north-east African mountains can be divided into four geographical groups: 1. Endemics o f the south-western part o f the Ethiopian highlands.A few endemics on the high mountains are found also on one other isolated mountain, for example: Also on Jebel Marra.

Endemics o f the Ethiopian plateau
Species in this group seem to be confined to the plateau of northern and central Ethiopia, and occur in comparatively dry vegetation types, e.g. on slopes, rocky outcrops and stony, well-drained soil.Species in this group are more or less confined to the eastern escarpment of the Ethiopian plateau, the mountain chain in northern Somalia, and the south-west Arabian mountains.Some species in this group may also occur on Socotra and on Jebel Akhdar in Oman.The species occur in upland dry evergreen forest or in associated types of evergreen bushland.(These endemics should not be confused with the even more important group of endemics in the deciduous bushlands of the Ogaden and the Somalian lowlands.)The most notable example in this group is Barbeya oleoides Schweinf., represent ing a monotypic family (Fig. 12).Other examples are: Mimusops laurifolia (Forssk.)Friis Fig. 13; Friis, 1981.

Species o f the Ethiopian plateau
Examples of this distribution may be found in the genus Acacia, but the taxonomy and distribution of the species of this genus in Ethiopia have not yet been published.

Species o f the eastern Ethiopian escarpment and adjacent mountains
Examples are: Berberis holstii Engl.White, 1970), from his analysis of the Afromontane tree flora of Malawi, that vicariism scarcely exists in that area.
A moderate degree of vicariism seems to exist between the Afromontane flora of tropical north east Africa and that of other Afromontane areas.Of the endemics mentioned, only the following are not replaced by vicarious species either in the Mediter ranean or temperate regions or on other African mountains: Endemic genera: Hypagophytum, Spiniluma, Barbeya and Poskea.Endemic section: Scadoxus sect.Gamolepis; only species, S. nutans.
A case of local vicariism is found in Linum L.: the Mediterranean species L. strictum L. and L. corymbulosum Reichb.f. occur in northern and central Ethiopia, but seem in southern Ethiopia to be replaced by the Afromontane endemic L. volkensii Engl., which also occurs in East Africa (Fig. 15).

OUTLIERS
According to Gillett (1955) the flora of threevegetation types in south Ethiopia is more closely related to the flora of the East African mountains than to the flora of northern Ethiopia.It would appear from this that elements of the northern and southern floras of Ethiopia might have a closer affinity to the floras to the north and south of them respectively.That this is correct, at least to some degree, is shown by a number of species or genera which reach their southernmost limit in Ethiopia and Somalia, but which occupy their main range in the temperate or Mediterranean areas, or in the Himalayas.Similarly, there are a number of species and genera which reach their northernmost limit in Ethiopia.

Southern limit species
Examples are: Rosularia (DC.) Stapf.c. 25 species; distributed from the eastern Mediterranean to central Asia.The only African species is R. semiensis (Fig. 10), which reaches its southern limit in the Semien mountains.Saxifraga L. c. 300 species; occurring in the temperate region and in the Himalayas.The only African species, S. hederifolia, reaches its southern limit in the Arussi mountains (Rampi, 1973).Debregeasia Gaud.c. 17 species; a genus of tropical and subtropical Asia, with many species in the Himalayas.The only African species, D. bicolor (Roxb.)Wedd., is distributed from the western Himalayas to Harar and Shoa (Fig. 16).Rosa L. c. 150 species; a widespread genus in the north temperate area.Only one African species, R. abyssinica Lindl., which is closely related to the R. moschata group of the Himalayas and western China.It occurs in the Yemen and Ethiopia as far south as Bale (Fig. 17).Primula L. c. 500 species; a widespread genus in the north temperate region and the Himalayas.The only African species, P. verticillata Forssk.occurs in the south-west Arabian mountains and in Ethiopia, with a southern limit just south of Addis Ababa (Bizzari, 1970).

Northern limit species
There is a large number of southern Afromontane genera and species which reach their northern limit in Ethiopia and Somalia.A few examples taken from the Afromontane shrubs and trees are: Schefflera J. R. & G. Forster.Three species of this genus are important in the forests of Ethiopia.Two of them, S. volkensii (Engl.)Harms and S. myriantha (Bak.)Drake, reach their northern limit at the latitude of Addis Ababa.The third species, S. abyssinica (A.Rich.)Harms, has its northern limit in Tigre (Bamps, 1974).Psychotria orophila Petit.A common forest species with the same northern limit as Schefflera volkensii and S. myriantha (Petit, 1972).Hypericum L. Examples similar to those of Schefflera can be found among the woody species of Hypericum (Bamps, 1971).

CONCLUSION
As seen from the distribution maps and data presented above, there seems to be reasonably good coincidence between the patterns of distribution revealed.It is therefore tempting to suggest that these patterns represent some kind of division of the tropical north-east African mountains into four phytochoria (Fig. 24).
Classical hierarchical biogeographical classifica tions have been less widely accepted in the recent years.White (1978) now prefers a less formal system of centres of endemism and transition zones and regional mosaics, rather than classifying the phyto- choria in provinces, regions and domains.I tentatively suggest that the four phytochoria on Fig. 24 represent four local centres of endemism, with the Ethiopian plateau as the least marked one.
Croizat, Nelson & Rosen (1974) have suggested the term 'generalized track' for persistently repeated distribution patterns.Tracks indicated by a fraction of the total number of species are termed 'coincident individual tracks'.As seen from the data presented for non-endemics and outliers, the four phytochoria on Fig. 24 can also be taken to represent such coincident individual tracks.
been suggested that vicariism is extremely important in the Afromontane flora, and that species replace each other on different mountains.White, on the other hand, concludes (Chapman & VICARIISMIt has