Tropical Soil-Vegetation Catenas and Mosaics: A Study in the South-Western Part of the Anglo-Egyptian Sudan

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Abstract
During an expedition to the south-western part of the Anglo-Egyptian Sudan in the dry season of 1938-9, data were collected with a view to elucidating the confused soil and vegetation types characteristic of African savanna-woodland country. Despite the seemingly chaotic mixture in which the subordinate types occur, it was found that they could be distinguished, their mutual relationships recognized, and apparent correlations established with factors operating. This was made possible through the adoption of the following general thesis and its close application to the minor as well as major soil-vegetation units seen in the field. This thesis puts in a precise form the relation between rainfall, topography and the soil-vegetation complex--a relation well known to exist and most easily seen in the tropics, but seldom worked out in terms of detailed, localized phenomena of soil and vegetation. Basically the thesis maintains (subject to certain exceptions) that the development of the soil on any site is mainly determined by the local topography through its effect on water movement. Soils are thus divided primarily into three complexes: High-level (eluvial), slope (colluvial) and low-level (illuvial). Broadly speaking, eluvial soils lose material; colluvials gain and lose concurrently; illuvials gain material and are flooded. The resultant differences are, naturally, of major importance. Such a division involves the concept of a catena, or topographically determined sequence of soils, following the usage of Milne (1935). Recognition of this catenary arrangement, complicated by the mosaic form imposed by micro-relief, is combined with the observation that over extensive regions the recurring units of a particular soil type repeatedly bear plant communities with the same distinctive characteristics, and with the same or related floristic composition. This constant relationship is shown even by the most closely defined soil-type units (phases) and their vegetation, often on very small areas of ground. The concept of the catena hence embraces soils and vegetation together. Thus is enabled the formulation of a comprehensive classification of minor and major soil-vegetation units and their relatively easy recognition in the field. Superimposed are the modifying effects of different steepness and length of slopes and of different rainfall, producing catena-variants with their own peculiarities of soil detail, floristic content and sometimes dominant species. Such a method of approach to synecological problems, wherever local differences in the water conditions are not obviously subordinate in influence to some other factor or factors, seems capable of wider application than it has received. It can be used with advantage in the allocation of land for agriculture and forestry, notably in the seasonally dry tropics. The south-western sudan (map, p. 9) consists of a dissected peneplain with a gentle dip from south-west to north-east. The soils have a uniform geological origin, being derived in situ or by water carriage from basement gneiss; blown sand is, however, sometimes present in the northernmost part of the province visited. The vegetation over most of the area is savanna woodland, dominated by deciduous trees. The area as a whole is divided into five natural regions distinguished primarily by topography and rainfall: (1) the jebel country in the immediate neighbourhood of rocky hills; (2) the watershed region on the south-western border, with narrow valleys and perennial streams; (3) the main tributary-river region with comparatively wide, shallow valleys in rolling country; (4) the almost level flood-plain region; (5) the permanent swamp or Sudd region. The account which follows deals only with regions (3) and (4), which are considered jointly; region (3) is predominantly one of eluvial and colluvial soils and region (4) is very largely illuvial. Emphasis is laid on the control of soil and vegetation by the alternation of marked dry and wet seasons, causing desiccation and local flooding, and on the profound effects of almost ubiquitous yearly grass fires, cultivation, erosion and termite activity. The main features of the soil-vegetation catena and complexes, as shown in regions (3) and (4) at large, are described on the basis of widespread observations and four sets of detailed records. The `composite catena' thus derived is figured in a profile drawing (Fig. 2, p. 13). The characteristics of each of the three soil-vegetation complexes, their phases and phase communities, as they were observed in the main tributary-river and flood-plain regions, are described in the sequence shown by the following table. The phases are designated by symbols which express the subdivisions of E (eluvial), Col (colluvial), and Il (illuvial). The eluvial phases are classified mainly on the basis of depth, the colluvials on texture, the illuvials on drainage conditions. It should be understood that a grass layer (whose specific composition is often indeterminable in the dry season) is practically ubiquitous, forming the major part of the vegetation cover on sites where scattered trees are quoted as the vegetation `dominants'. The eluvial complex, whose phases occur mainly in the form of a mosaic, comprises the hard `ironstone' of the higher ground and the soils and vegetation associated with it. This complex has the shallowest and coarset soils. The deepest of them (apart from termite mounds) is the eluvial deep phase (E/Dp); this is apparently stable under present conditions and is dominated by a mixture of deciduous trees, with or without a small proportion of hard-leaved evergreens, over an understorey of smaller deciduous trees and shrubs and a herb layer dominated by tall perennial grasses. The specially favourable habitats produced by the superposition of termite mounds (E/Dp+) on this phase give rise to islands of vegetation including a higher proportion of evergreen trees, which shelter a dense...