Forest gradients in West Africa : a spatial gradient analysis

The tropical rain forests of West Africa, west of the Dahomey interval, once covered some 40 million ha. Being on the western fringe of the African continent, they receive abundant rainfall from the SW monsoon. Further inland, rainfall gradually decreases and the forests give way to savanna and ultimately to the Sahara desert.This Upper Guinea forest block used to cover most of Liberia and parts of Côte d'Ivoire and Ghana. Here, deforestation rates are among the fastest in the world. Humans have reduced the forest cover by some 80 %. Most of the forest has been converted to agricultural land. Fire and heavy timber mining have left the remaining forest in a poor state. Sustainable forest management has not yet been attained. A key prerequisite for achieving such management is more and better knowledge of the ecology of these complex and highly diverse ecosystems and of their species.Gradients are gradual changes in space e.g. of species composition in the ecosystems. In this book forest gradients are studied at two levels of scale: the regional forest gradient from the coast to the forest- savanna boundary, and local gradients along slopes in the landscape. The species composition of the large forest trees with a diameter exceeding 70 cm was studied; this entailed adapting the existing methods used in vegetation science to cope with these huge subjects. In West African forest exploitation 70 cm diameter is a common limit for selective cutting of trees.At the regional scale, the <strong><em>spatial gradient analysis</em></strong><em></em> consisted of a three-step approach: 1. ordination of forest areas and species; 2. spatial interpolation of the ordination scores of the forest areas; 3. relating spatial trends in species composition to trends in rainfall and patterns in lithology and relief.Tree species composition in a forest area was determined by using forest inventory data from the pre- logging era in SE Liberia and SW Côte d'lvoire. These data were ordinated together with data from three sample plots of 20 ha each established in Taï National Park. The old and new sample plots together covered 21 640 ha. Of the largest tree species, 53 were retained for the ordination.The forest gradient was mapped by interpolating lines of equal ordination scores and plotting these on the map. The ordination table allowed these ordination scores to be translated into the corresponding species composition. Each tree species has an individual position along the gradient, given by its ordination score. The ecological range of the species is indicated by the range of sample scores in which it occurred. The gradient map with isoscore lines provides a valid alternative to mapping by types or classes and overcomes the problem of transitional types.Over 400 km a pronounced regional forest gradient was found from the Liberian coast towards the forest- savanna boundary in Côte d'Ivoire. This gradient correlated well with the SW-oriented rainfall gradient. On part of the map the forest gradient showed an anomaly. The forests on a band of sericite-chlorite schist from Taï National Park towards the NE were ranked 'wetter' than expected from their position on the rainfall gradient. Apparently, the rainfall effect was compensated by the greater moisture content of the soils derived from sericite-chlorite schist. Furthermore, the rain shadow of Putu range (753 m above sea level) was reflected in the forest gradient by a zone of fast compositional change. These results are comparable with those of other studies on the regional forest gradients in Ghana, Côte d'Ivoire and Liberia.Forest gradients along slopes spanning a 20 to 40 m elevation interval were studied in Taï National Park in sample plots at three locations: near Zagné in the drier north-west (plot size 23 ha), near Taï in the middle (25 ha) and near Para in the wetter south-west of the Park (22 ha).All trees in these plots with a diameter larger than 70 cm were recorded and mapped. Swamp areas were excluded from these plots. A digital terrain model and a physiographic soil map were prepared for each site.The approach used for the analysis of the slope gradient contained the same elements as for the regional gradient, but in reverse order: 1. the soil survey revealed that the spatial trend in environmental variation was related to local elevation; 2. thus, <em><strong>contour samples</strong></em> of trees growing within an elevation interval were composed. A series of 11 or 12 consecutive intervals was calculated at each site in such a way that each sample covered 2 ha. 3. Tree species composition was determined per sample and all samples of the three sites were ordinated together using Detrended Correspondence Analysis (DCA). The sample scores on the first ordination axis were plotted against elevation to check the hypothesis of elevational gradients. Tree density, species richness, basal area and stem biomass per contour sample were analysed in a similar way for elevational trends.The <strong><em>contour sampling technique</em></strong> proved to be appropriate for the analysis of floristic slope gradients in large trees. The spatial autocorrelation of the contiguous samples is assumed to counterbalance the small number of trees per sample. The species order resulting from this ordination was similar to the one obtained in the regional gradient and hence, could be interpreted as a wet-dry axis. On the three sites species composition changed from "drier" species upsIope to "wetter" downslope. The lower slope on a drier site was similar in species composition to the upper slopes of its wetter neighbour. Thus, slope gradients are sliding gradients on the regional forest gradient. The regional gradient was related to rainfall and lithology. These factors are largely expressed in soil moisture availability. Hence, gradients in moisture availability probably explain the forest gradients along slopes.Tree density and species richness of the large trees decreased towards the wet end of the gradient, both between sites, e.g. from Zagné to Para, and within the Taï site, from upper slope to lower slope. This trend in species richness of the large trees contradicts one of the tenets of the Pleistocene forest refugia theory, namely that species richness increases towards the core area of a refugium. A Pleistocene refugium is hypothesized to have existed in the Grabo hills to the southwest of Taï National Park.Basal area curves showed a peak in middle slope positions and declined towards both the upper and lower end of the catena. A general trend of increasing wood density of the large tree species was found towards the wet end of the gradient.Some implications of the continuous variability model are evaluated as a scientific basis for forest management and conservation of biodiversity. At the regional scale, forest management should be adapted to the individual position of each forest area along the gradient. The tree species chosen and the forest land evaluation should both reflect this position. Catena plots are suggested as more appropriate for forest inventory than random sampling in areas with pronounced slope gradients.Any conceivable strategy for conservation of biodiversity in West Africa must aim at protecting forests along the entire gradient, because at any point along the gradient different species attain their optimum. A "Green Sickle" is advocated which, if adopted and implemented, would link National Parks and forest reserves from the savanna down to the Atlantic coast.In western Côte d'lvoire. there are two promising areas where conservation efforts could still produce worthwhile results: 1. the hills of Grabo, a hypothetical Pleistocene forest refugium with a high degree of endemism in Côte d'Ivoire, which merits the status of national park; 2. the semi- deciduous forests, which contain many species not found in wetter forests.Regional and local gradients result in a great diversity of forests in West Africa. The satellite image on the front cover confirms this broad variation. Forests are not just hectares, trees not just cubic metres. Management of the most species-rich ecosystems on earth is a challenge for present and future generations. It will need international support and the efforts of all those fascinated by this pearl of our blue planet.