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

C. G. T. Morison, A. C. Hoyle and J. F. Hope-Simpson
Journal of Ecology
Vol. 36, No. 1 (Jul., 1948), pp. 1-84
DOI: 10.2307/2256646
Stable URL: http://www.jstor.org/stable/2256646
Page Count: 108
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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 thicket of smaller evergreen and deciduous trees, lianes, shrubs and herbs, grasses being almost eliminated. Parts of the mosaic where the A soil-horizon is less deep (E/S) bear a lower, more open, and entirely deciduous tree canopy, with deciduous shrubs, herbs and a dense layer of tall perennial grass. Mixing and redistribution of shallower variants of E/S seem the cause of the phase called E/S.R, whose vegetation is reduced mainly to annual grasses. The shallow, eroded phase (E/S.Er), in which the B1 pea-iron horizon is exposed, bears a rather open thicket of shrubs, with annual and perennial grasses. The final result of erosion is an outcrop of the B2 ironstone horizon; this phase (E/O) bears no vegetation at all except in cracks and pockets. With sufficient slope and rapidity of erosion there is an accumulation of ironstone detritus between the edge of the eluvial complex and the upper colluvial zone; this E-Col transition zone is a particularly favourable habitat for certain evergreen and broad-leaved deciduous trees. The soils and vegetation of the eluvial complex are summarized in Table 1 (p. 17). The colluvial soils, deeper and finer than the eluvials, are produced by the sorting of material mainly derived from the erosion of the eluvial mosaic. They form a zoned complex, or sub-catena, grading from coarse-grained shallow soils at the top of the slope (Col/C) to fine-grained deep ones at the bottom (Col/F), although the differentiation of separate colluvial zones varies according to conditions of slope and rainfall. By far the most common and characteristic colluvial zone is the fine-grained, red soil (Col/F1) bearing a vegetation dominated by species of Combretum. This zone is a favourite site of cultivation; it is also subject to fierce fires. The latter circumstance alone may be sufficient to account for the absence of well-grown mixed woodland which, there is reason to suppose, could otherwise occur. Under existing conditions the colluvial complex is poor in characteristic species. A transition zone (Col-Il) usually occurs between Col/F1 and the illuvial complex, and may be very extensive where the slope is scarcely perceptible. This transition zone, situated at about the limit of normal flooding, is also widely cultivated and, with its groves of Borassus and other less gregarious, but conspicuous, preserved or planted trees, constitutes a main feature of the country, especially in and around the flood-plain region. The soils and vegetation of the colluvial sub-catena are summarized in Table 2 (p. 22). The illuvial complex comprises those soils which occupy the seasonally flooded low-level ground. They are the deepest and generally much the heaviest soils. They are composed of colluvial material washed from adjacent higher ground by the rain-flood, with the addition (except in some minor valleys and small basin sites) of alluvial material carried from a distance by a general valley or plain flood. Micro-relief determines whether the illuvial phases are zoned or form a mosaic; its undulations induce the latter arrangement most obviously where the general slope is very gentle. Also correlated with micro-relief is the nature of the drainage. By this latter property two main groups of phases are distinguished, those with free and those with impeded drainage. This difference does not necessarily involve the occupation of a regularly higher level in the catena by one group than by the other. The illuvial drained phases (Il/Dr) occur where there is nothing to prevent flood-water draining away to an effective drainage-channel; the surface is even, though usually sloping very gently. Such terrain results in zonation rather than mosaics. The distinctive upper phase of this group (Il/Dr1) bears an association characterized by spaced Terminalia trees; the lower phase (Il/Dr2) bears the open Hyparrhenia grassland of the `toiches', which are often extensive and provided such a familiar feature of the broader valleys and flood plains. The illuvial impeded phases (Il/Id) show signs of drainage-impedance. They are associated with uneven ground, which promotes mosaics of phases. From Il/Id1 to Il/Id3 there are, progressively, a greater degree of impedance, finer texture, and darker colour, culminating in the black soil of the completely undrained `basin' site (Il/U). The tree vegetation of these impeded phases shows single-species dominance, which is especially characteristic of illuvial sites; in Il/Id2, however, the trees (Mitragyna) are often far apart, with intervening grassland of `toich' character though composed largely of Vetiveria nigritana instead of Hyparrhenia. The occurrence of termite mounds, providing habitats raised above flood level and suitable for tree growth, is a characteristic feature of the two less markedly impeded phases. The soils and vegetation of the illuvial complex are summarized in Table 3 (p. 27). As already explained, the generalized picture given by the foregoing account of the composite catena varies with conditions of topography and rainfall. Where these conditions are intermediate between the extremes encountered, as they are east of Wau, not only are all three complexes of the catena clearly represented, but also there is the clearest development of the E complex--the higher ground being broad enough for its micro-relief to vary repeatedly (cf. Halima variant, below) but not so flat as to make such variation relatively insignificant (cf. Aweil variant). Here, too, there is a relative abundance of evergreen trees, which have a marked influence on subordinate vegetation; evergreens are much rarer on the eluvial complex where the rainfall is either higher or lower. The colluvial complex is best developed where a somewhat higher rainfall associated with narrower and steeper ridges (Halima variant) causes more downward movement of eroded material; provided the lower slopes are long enough and not too steep, the colluvial material is then sorted out into two, or rarely three, distinct colluvial zones. Below them the illuvial complex is poorly represented in this variant owing to the absence of wide valleys. The illuvial complex is most elaborate in the flood-plain region, where it predominates. Here, with very gentle slopes (e.g. north of Aweil), the duration and mode of drainage of the seasonal flood have their greatest influence, while the elevation of the higher ground is so slight that between it and the flooded levels there is often no typical colluvial slope. The catina-variants so produced form a series which may be summarized as follows: These variants differ in their characteristic plant species, which are listed in Table 4 (p. 35). The account given of the catena, its complexes, phases and variants, is based on observations made in many places, but detail was added by the study of three transects representing the three catena-variants distinguished. The pedological, vegetational and floristic records from these transects are given in the last part of the paper. They are included in order to provide practical illustration of the fundamental soil-vegetation units, whose recognition is essential if the catena/mosaic interpretation of the soils and plant communities is to be successfully applied. Transect I (Wau) shows a comparatively simple and distinct zonation which presents a clear picture of the catena sequence; it did, in fact, provide the key to the whole soil-vegetation problem. Associated with this first transect was a detailed study of the eluvial mosaic, especially of termite mounds. It was confirmed that the soil of the mounds contains a high proportion of clay, which seems to be washed out, especially near the surface, as the mounds become disused and collapse. This process is apparently accompanied by a change from an alkaline to a slightly acid reaction comparable with that in the soil of the surrounding level ground. Many of the characteristic plants, which become established on the favourable site provided by the mounds, evidently fail to persist for long after the mound is obliterated. The mounds nevertheless play an important role in the development of the eluvial woodland as a whole, in general through interaction with the fire-factor and in particular because they encourage the establishment of Khaya senegalensis. This important evergreen canopy-tree persists as the characteristic dominant of the dense patches of vegetation on comparatively level sites probably occupied formerly by mounds. Transect II (Halima), because of its interrupted slope, is composed of two catenas, its sequence of complex being E→ Col→ E→ Col→ Il. The upper catena shows local dominance by Isoberlinia, one of the chief contributors to the vegetational appearance of the Halima catena-variant. The lower catena displays a particularly clear example of colluvial zonation, combined with the poor representation of illuvial ground which is a further characteristic of this variant. Transect III (Aweil) shows a lack of clarity in zonation, due to a combination of very slight general slope with undulating micro-relief. A comparatively simple eluvial mosaic and a very extensive and rather confused but interesting illuvial complex are separated by ill-defined colluvial and colluvial-illuvial transition zones. In addition to irregularities of terrain, termite mounds make a very important contribution to the complexity of the whole transect. Single samples of the various soils on these transects give analysis results corresponding well with those obtained by Greene (1939 and unpublished MS.) from similar soils in the Sudan. The following general points are derived from the figures. (i) Finer texture in lower horizons. (ii) Progressively finer texture from eluvial to colluvial and thence to illuvial soil. (iii) Coarser eluvial soils where steeper slopes and higher rainfall promote greater eluviation. (iv) All the soils (except those from termite mounds) are more or less acid. (v) With greater depth the acidity increases in eluvial and decreases in illuvial soils, while colluvials vary in this respect. (vi) Exchangeable calcium, determined for one transect, was most plentiful near the surface in soils from all three complexes. (vii) The silica/sesquioxide ratios of comparable eluvial samples, one from each transect, show a gradation of magnitude inverse to that of rainfall. (viii) In general this ratio is highest in illuvial soils, which are subject to resilication, and lowest in colluvials, whose material can on this account be considered more thoroughly weathered than that of the eluvials. (ix) The silica/ferric oxide ratio, controlled as it is by weathering, resilication and loss of ferrous iron, does not show a regular progression according to topographical level, but it seems likely to prove useful in characterizing the pedogenic conditions of the various phases. Two features of a general kind may be noted in the tables recording the vegetation of the transects. (i) The `drift' of species from the upper to the lower sections of each transect, the differences between the sections being most obvious in Transect I and least so in Transect III. (ii) The eluvial complex has by far the richest flora, although in Transect III the fact is obscured by the quantity of characteristic termite-mound species at lower levels. From each of the vegetation tables for the more complicated transects (II and III) closer study brings out a number of detailed points, several of which are noted as typifying the catena-variant concerned.

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