Soil and Vegetation Catenas on the Forest-Savanna Boundary in Ghana

Abstract

A study was carried out on two similar soil catenas and their associated vegetation from a locality on the forest-savanna boundary in west-central Ghana. Soil samples were analyzed for major plant nutrients and the values obtained related to the vegetation using principal components analysis and analysis of foliar nutrient levels. Vegetation types were found to occupy distinct catenary positions and to be associated with characteristic soil types. Levels of total phosphorus, exchangeable calcium and potassium, water content, and pH in the topsoil were all found to be higher under forest than under savanna, but total nitrogen was not different. The change from savanna woodland to open-tree savanna coincided with decreasing levels of nitrogen and potassium in foliage samples and of potassium in the topsoil. Differences in soil fertility are, however, probably secondary, and it is concluded that the primary factors separating vegetation types relate to effective soil depth, soil texture, and water supply. EXTENSIVE AREAS of Guinea Savanna vegetation bordering the forest zones in West Africa have been classically interpreted as representing a biotic climax, established and maintained by the periodic burning associated with traditional agriculture. Recent, more detailed studies have, however, indicated the importance of edaphic factors in determining the pattern of vegetation on the forest-savanna boundary; in particular, attention has been drawn to the relation of vegetation patterns to the catenary development of soils (Spichiger 1975), to soil texture as it affects soil moisture and nutrients (Askew, Moffatt, Montgomery, and Searl 1970, 1971), and to soil nutrients directly (de Rham 1970, Goodland and Pollard 1973). This paper reports the study of two soil catenas and their associated vegetation from a locality on the forest-savanna boundary near Degedege in westcentral Ghana (latitude 7?48.5'N, longitude 2?26.5' W). Field work was carried out in July and August 1976, during the short, summer dry season. A description and classification of the vegetation at Degedege has already been published (Swaine, Hall, and Lock 1977), and so in this study attention was concentrated on the soil catenas and levels of major nutrients in the topsoils. Floristic and physiognomic data were collected to distinguish the main vegetation units and to relate them to existing classifications. GEOGRAPHICAL CONTEXT In this region of Ghana, forest and savanna form a mosaic, with forest in narrow bands along streams and in patches on summits, and savanna on the slopes between (fig. 1). The description of 'forest-interfluve' has been proposed for this pattern (Swaine et al. 1977), contrasting it with the 'savanna-interfluve' pattern, more widespread in West Africa, in which forest is confined to the valleys. Climatic data are not available for the immediate vicinity of Degedege, but from those of the nearest stations annual rainfall can be estimated at ca. 1200 mm. This is distributed in a two-peak pattern with maxima in May-June and September, separated by a comparatively cool, dry period; the main dry season from November to April includes at least five 'arid' months (sensu Mohr, i.e., less than 10 cm in a month). As noted by Swaine et al. (1977), the forest savanna boundary in this region of Ghana runs approximately east-west, apparently cutting across the local trend in annual rainfall. The underlying rocks are mainly metamorphics of the Lower Birrimian formation, part of the Precambian basement. Country rock was not exposed in the study area, but fragments of phyllite and quartz (the latter from intruded veins) were found in soil pits. Topography is very gentle, with slopes of less than 3? in the study area, resulting from dissection of a Tertiary peneplain surface (Brash 1962). Hilltop cappings of hardened ironpan seem widespread, though seldom exposed at the surface. Cultivation in the study area, approximately 1.5 km from the nearest village, appears to be sporadic, rather than on a regular rotation and is entirely confined to the hilltop forests and their margins. Savanna trees in the area showed moderate to severe fire damage, suggesting irregular burning; one of the forest areas sampled was in the process of regeneration after clearing. The area is not subject to animal grazing, but termites are universally abunlPresent address: International Institute of Tropical Agriculture, PMB 5320, Ibadan, Nigeria. 224 BIOTROPICA 11 (3): 224-234 1979 This content downloaded from 157.55.39.149 on Sat, 09 Jul 2016 05:56:45 UTC All use subject to http://about.jstor.org/terms dant and are probably an important factor in the ecology of the area.