William D. Hawthorne

Biodiversity of West African Forests: An Ecological Atlas of Woody Plant Species

The rain forests of West Africa have been designated as one of the worlds hotspots of biodiversity. They extend from Ghana to Senegal and are referred to as the Upper Guinean forests. Because of their isolated position, they harbor a large number of rare and endemic animal and plant species.This book focuses on the biodiversity and ecology of these forests. It analyzes the factors that give rise to biodiversity and structure tropical plant communities. It also includes an atlas with ecological profiles of rare plant species and large timber species.

The intermediate disturbance hypothesis applies to tropical forests, but disturbance contributes little to tree diversity

The intermediate disturbance hypothesis (IDH) predicts local species diversity to be maximal at an intermediate level of disturbance. Developed to explain species maintenance and diversity patterns in species-rich ecosystems such as tropical forests, tests of IDH in tropical forest remain scarce, small-scale and contentious. We use an unprecedented large-scale dataset (2504 one-hectare plots and 331,567 trees) to examine whether IDH explains tree diversity variation within wet, moist and dry tropical forests, and we analyse the underlying mechanism by determining responses within functional species groups. We find that disturbance explains more variation in diversity of dry than wet tropical forests. Pioneer species numbers increase with disturbance, shade-tolerant species decrease and intermediate species are indifferent. While diversity indeed peaks at intermediate disturbance levels little variation is explained outside dry forests, and disturbance is less important for species richness patterns in wet tropical rain forests than previously thought.

How important are forest elephants to the survival of woody plant species in Upper Guinean forests

Elephant populations have declined greatly in the rain forests of Upper Guinea (Africa, west of the Dahomey Gap). Elephants have a number of well-known influences on vegetation, both detrimental and beneficial to trees. They are dispersers of a large number of woody forest species, giving rise to con- cerns that without elephants the plant diversity of Upper Guinean forest plant communities will not be maintained. This prospect was examined with respect to four sources of inventory and research data from Ghana, covering nearly all (more than 2000) species of forest plant. Evidence supporting the hypothesis that plant populations are collapsing without elephants is conspicuously absent in these data- sets, although Balanites wilsoniana is likely to suffer dramatically on a centennial scale in the absence of forest elephants. A few other species are likely to decline, although at an even slower rate. In the context of other processes current in these forests, loss of elephants is an insignificant concern for plant biodiversity. Elephant damage of forests can be very significant in Africa, but loss of this influence is more than compensated for by human disturbance. Elephants have played a signi- ficant part in the shaping of West African rain forest vegetation. However, it is the conservation of elephants that should be of primary concern. Tree populations should be managed to promote them, rather than vice versa.

Trees and farming in the dry zone of southern Honduras II: the potential for tree diversity conservation

The potential of the dry zone agro-ecosystem of southern Honduras to contribute to the conservation of Mesoamerican dry forest tree diversity is evaluated. Four rural communities containing eight land uses were surveyed using rapid botanical sampling resulting in the identification of 241 tree and shrub species. As a result of ordination analysis, it is concluded that the land uses are relatively similar in their species composition, particularly maize fields (milpas), fallows, pastures and woodlots, because of the predominance of natural regeneration. Therefore all land uses might contribute to local tree diversity conservation. Those land uses in which planting also contributes to diversity, home gardens (solares) and orchards, are more distinct; however the tree species found there are widespread and often exotics and thus not the usual focus of conservation measures. Across the landscape the total complement of species considered a global priority for biodiversity conservation is very low and therefore this agro-ecosystem does not represent a good place in which to implement dry forest tree diversity conservation programmes. Instead its value is likely to be in the contribution that tree diversity makes to rural livelihoods. Particular consideration is given to Swietenia humilis Zucc. (small leaved mahogany) and its status as a threatened species is questioned because of its abundance within this landscape and its wide distribution.

The forests of Upper Guinea: gradients in large species composition

The forest belt in Upper Guinea stretches over 2000 km from Senegal in the west to Togo in the east. In this chapter a vegetation map is presented for the whole upper region

Rainfall and temperature affect tree species distributions in Ghana

We evaluated the relative importance of annual rainfall, temperature and their seasonality to tree species distribution in Ghana. We used species presence/absence data from 2505 1-ha plots systematically distributed over Ghanas forests. Logistic regression was used to determine species responses to four climatic variables generated from the Worldclim database. The distribution of 95% of 20 species was significantly associated with annual rainfall, 60% with rainfall seasonality, 45% with isothermality and 40% with temperature seasonality. Annual rainfall explained on average most of the variation (17%, range = 0.5–52%) in species distribution, followed by rainfall seasonality 5% (range = 0.5–27%), isothermality 4% (range = 0.8–24%) and temperature seasonality 1% (range = 0.4–4.5%). Our results suggest that, out of the climatic variables investigated, rainfall is the main factor determining tree species distribution in Ghana; temperature also influences the distribution of a number of species, although it explains much less of the variation. The reduction in annual rainfall that prevailing climate-change scenarios predict for the region will result in a shift in the distribution of most species, whereas the predicted increase in temperature variation is likely to have little effect.

Six new Pavetta (Rubiaceae), including three ‘litter-bin’ species from the evergreen forests of Western Africa

Summary. Six new species of Pavetta L. are described from the Wet Evergreen forests of Ghana, Ivory Coast and Liberia: Pavetta sonjae, P. bomiensis, P. abujuamii, P. ankasensis, P. quasidigita and P. sapoensis, the first of these with ornamental potential. The last three are considered ‘litter-bin’ species, a morphological syndrome whereby forest understorey plants accumulate litter in their leaf axils. Certain venation patterns are defined precisely and used to delimit species. The six new species are illustrated, compared with similar species and their conservation status is evaluated.