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E OUATORIAL AFRICA
100 50 X
N Kajo Kaj'
Ň i B DAN
100 200 Miles
0 Zemio Tambun
1. 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 in mediately south of Damer to just north of Port Sudan. Except for the Nile area, vegetation in these areas is confined to a few de pressions 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 samr 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.
This and the following semi desert 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 hmdred mile wide belt, sometimes rolling and with dunes, bordering the north ern desert area. Two to twelve inches of annual rainfall is dis tributed 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 slight ly 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 drought resistant 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 main taining 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 des ert 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.
Extending from the previous District to the Bahr El Arab and thence southwards to the borders of Equatoria Province, the Aca cia Tall Grass Forest District is the largest single vegetational unit of the Sudan. Much of it is frequently called the Central Painlands, 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 an nually 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 migra tions of domestic animals from May to September during the rainy season.
5. Broadleaved Woodland and Forest District
This red ironstone area, with forty to sixty inches of rain fall annually, supports grassy woodlands of varying characteris tics, swamps, toich, and luxuriant tropical forests. From Yei 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 condi. tions.
B. Bowl or Depression Forests occur where there is sufficient runoff of water from surrounding slopes. These are the Azza Forest in Meridi District, and the Lotti and Laboni Forests of the Acholi Hills. These approach the true climatic rain forests of the Congo.
c. Cloud Forests of the Sudan are limited to the higher reaches of the Dongotona and Imatong Mountains.
7. Swamps and Grassland Districts
A. Permanent Swamps are chiefly the vast Sudd area of the White Nile and others on smaller rivers. These are of lesser importance for the present study.
B. Seasonally Inundated Land, or toich, along the White Nile and its tributaries, supports the vast cattle herds of the Dinka and Nuers. Tree growth is inhibited and vast grass meadows stretch to the horizon. A foot or so of water covers these meadows during the rains. Other seasonally inundated land along the Blue Nile supports sunt forest.
c. Grasslands as such are negligible in the Sudan except for the toich and deforested areas or recently abandoned cultivation. Short turf occurs on rocky hills, plateau, and ironstone pans.
Mountain meadows on shallow soil high in the Imatong and Dongotona Mountains contain grasses and herbs growing to a height of three feet.