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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°14. Towards the Nile and eastwards, this margin swings gradually northward from a point im. mediately 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 clups 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 ome 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 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 hudred 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 periphey 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 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 attehtion 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 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 nnnotonous 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 diffe 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_ tibia Efpdomestic animals-from May to September during the rainy season.
This red ironstone area, with forty to sixty inches of rain. fell 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 swams and important dense, highland forest areas oc_ cupy 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
From Kapoeta eastwards less rain falls and an outlier of the Acacia Short Grass Scrub District takes over.
A. Gallery Forests fringe larger streams in the previous strict an are most Highly developed in the Yambio and Yei eas. 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_
B. Bowl 2 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. Seasonall Inundated Land, or toich, along the White Nile and its tri u aries, 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.
0. Grasslands as such are negligible in the Sudan except for the toich EH deforested areas or recently abandoned cultivation. Short turf occurs on rocky hills, plateau, and ironstone pans.
D. Mountain meadows on shallow soil high in the Imatong and Dongotona Mountains contain grasses and herbs growing to a. height of three feet.
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