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is shortest in the north and longest in the south. Most of the rain is convectional with a marked afternoon and evening maximum. Dry Winter weather is very stable but intense thunderstorms occur in summer. In the semiarid north, early rainy season winds associated with thunderstorms bring dust storms or hoboobs.
The Red Sea coast and the eastern slopes of the Red Sea Hills, influenced by the Red Sea, have northerlies throughout the year. These bring rain and clouds, rain falling chiefly in winter. The higher relative humidity of this narrow coastal strip markedly influences the flora and fauna of the area.
The vegetation of the Sudan (Andrews 1948) is divided into seven principal Districts from north to south (Figure 326):
Isolated areas unrelated to their immediate surroundings are the Red Sea Hills (Erkowit) and Gebel Elba (southeastern Egypt adjacent to the Sudan frontier and administered by the Sudan Government).
The following brief abstract provides a generalized picture
of the chief floral aspects of the Sudan. For further details, see Andrews (1948) and various chapters in Tothill (1948).
l. Desert District
The Libyan and Nubian Deserts receive from zero to two inches of unequally distributed rainfall annually. The southern margin of this District west of the Nile is about 16°N. Towards the Nile and eastwards, this margin swings gradually northward from a point immediately south of Damer to just north of Port Sudan. Except for the Nile area, vegetation in these areas is confined to a few depressions and rare watercourses harboring some runoff moisture. Almost no woody plants occur. Towards the southern boundaries, on broadly undulating plains of loose red sand, a very few clumps of plants exist and towards the Nile scant stands of samm acacias appear. In the stark, rocky mountain masses of Kassala Province slight vegetation persists in valleys.
In the Nile Valley there are a greater variety of plants, especially date and dompalms, and some shrubs, herbs, and grasses, besides four species of acacia trees. Cultivation of the seluka type, confined to Nile basin areas, utilizes silt deposited by Nile floods. Some waterwheels, saqiyas, are used. Even this will disappear with the advent of the lake behind the proposed High Dam just south of Aswan in Egypt.
2. Acacia Desert Scrub District
This and the following semidesert areas are nowhere so rich as semidesert in the American sense of the word. The Acacia Desert Scrub District extends as a sandy, rocky two hundred mile wide belt, sometimes rolling and with dunes, bordering the northern desert area. Two to twelve inches of annual rainfall is distributed through the four winter months. Many areas are entirely treeless; where vegetation does occur Acacia trees and some shrubs, or a few shrubs, short grasses, and no trees are found. A slightly greater variety of trees exist near the Nile. The Red Sea Hills at the eastern periphery of this District support a separate flora characterized by the dragon's blood tree and various droughtresistant herbs. Here, too, valleys and plains contain dompalms and samr acacia trees. The seacoast supports a separate flora, among which mangrove is noteworthy.
3. Acacia Short Grass Scrub District
A rather narrow, short grass belt fringes the Desert Scrub District to the south. Rainfall increases from twelve to twenty inches annually and falls six to eight months a year, thus maintaining an open woodland type of country with short grasses and herbs. The soil is more water-retaining than that of deserts and supports a greater variety of trees, of which Acacia species dominate. The Butana area (Figure 3) is partially treeless desert and elsewhere open grass plain with thorn scrub. Taller grasses, and denser shrubs and trees grow near the Atbara River and dompalm forests exist at the River's upper reaches.
The southern limit of the distribution of camels in the Sudan coincides with the southern margin of this District.
The intensively cultivated and irrigated Gezira area between the Blue and White Niles is a special feature of this region. Resources from this area produce most of the revenue for the Sudan Government and it is one of the most densely settled areas of the Sudan.
For purposes of the present study, special attention should be called to the outlier of the Acacia Short Grass Scrub District in eastern Equatoria Province, from near Kapoeta eastwards, and in southeastern Upper Nile Province.
4. Acacia Tall Grass Forest District
Extending from the previous District to the Bahr El Arab and thence southwards to the borders of Equatoria Province, the Acacia Tall Grass Forest District is the largest single vegetational unit of the Sudan. Much of it is frequently called the Central Rainlands, on which vast herds of cattle graze and some effort devoted to cotton, peanuts, melons, various legumes, dura, and teff grass is undertaken. Three highland areas, the Gebel Marra group, the Nuba Mountains, and the Ingassana Hills, break the monotonous plains. The vast sudd or papyrus swamps of the Nile and the seasonally inundated "toich" areas are special features of this area. Rainfall ranges from twenty to forty inches annually and soil types vary from dark heavy clays to light sandy
loams. While acacia trees are still outstanding, Combretaceous and other broad leaf trees are scattered among them, in some places quite thickly and with ground cover not seen to the north. The species of Acacia also differ in part from those northwards. A large number of floral associations are described by Anderson (loc. cit.) from this District. This is the area of vast migrations of domestic animals from May to September during the rainy SeaSOrle
5. Broad-leaved Woodland and Forest District
This red ironstone area, with forty to sixty inches of rainfall annually, supports grassy woodlands of varying characteristics, swamps, toich, and luxuriant tropical forests. From Yel westward we find the most extensively forested area of the Sudan. In this area of high elephant grass woodlands, depression and gallery forests are interspersed with forested grasslands and low mountains and hills. Northwards from the Congo watershed the trees become shorter but much varied in species and density. Extensive swamps and important dense, highland forest areas occupy part of the east bank. Gradually, from Yei to near Kapoeta, the grassy plains lose their forest aspect and become more open. Thorny acacias are scattered on the plains and broad leaf trees are confined to patches surrounding termite mounds and fringes bordering streams.
From Kapoeta eastwards less rain falls and an outlier of the Acacia Short Grass Scrub District takes over.
6. Forest Districts
A. Gallery Forests fringe larger streams in the previous District and are most highly developed in the Yambio and Yei areas. In eastern Equatoria these forests are restricted to streams at the base of mountains and to mountain ravines. On smaller streams the forest is only a single ranked fringe; downstream it consists of heavier gallery forest trees in wider ranks that form a close canopy and provide rain forest conditions.