TheImprovement ofTropical and Subtropical Rangelands

218 IMPROVEMENT OF TROPIOAL AND SUBTROPIOAL RANGELANDS
Vegetation flourishes during the rainy summer season, is adequate
during the cool, dry season, and is thoroughly dried out
during the hot, dry period, when water-or, more accurately, lack of
water-is the crucial concern, and a decided concentration of all nomadic
groups around permanent eourees of water is the rule. In the
eouth, the vegetation is composed of a savanna complex of various
acacias (Acacia nilotica, A. ,enegtJl, A. ftatJa, and A. tortw.) with an
understory of grasses and herbs, particularly cram-cram (CenchrtUI
6iftortUl).1 To the north, in keeping with the declining rainfall totals,
the occurrence of acacias becomes less frequent, and the understory
diminishes to scattered clumps of had. (Cornulaca monacantha) and
86at (A""tid.a pungen,). In the far north, acacias are restricted to
the beds of wadis (usually dry intermittent atreams) or especially
favored locations in the uplands where they form a major part of the
diet ofcamelsj had., shat, awaraj (Calligonum como,um), and various
halophytes form the preferred forage of camels and sheep.
North of the Adrar, the major climatic influence is derived from
the Atlantic, rather than from the tropics, and the regime is distinctly
Mediterranean in character. Unlike eouthem Mauritania, precipitation
in the north occurs at the very end ofthe summer and during the
early months ofwinter.· Although this rainfall may be locally heavy,
totals throughout the northern areas are small, and the distribution
is usually sporadic and highly localized. While permanent water supplies
can almost always be located in wadi beds and beneath sand
dunes at no great distance from the surface, most of the rocky expanses,
hamad.4B and regs, are devoid ofdependable water reeources.
Here the winter rains are of crucial importance, for they cause a rich
ephemeral vegetation to spring up wherever they occur, while the
rainpools that collect in isolated hollows assure adequate water for
man and beast during the winter grazing season, thus permitting the
exploitation of what would otherwise be barren and unusable areas.
MIGRATION CYCLE
The B.lgalbat Lqacem
The Rigaibat, the most powerful nomadic group in upper and
middle Mauritania, form the basis ofthis case study. They are a tribe
of remarkable eolidarity, with a social structure that has attracted a
great deal ofinterest. G They are grouped into two major divisions, the
Lqacem (or eastern) and the Sahel (or western) Rigaibats. It is the