Evaluation of some indigenous trees for urban landscape design: a case study of the Kumasi Metropolis in the Ashanti Region of Ghana

Trees found in the local environment can be classified by their uses, water requirements, families and even origin. In terms of origin, there are basically indigenous and exotic species. Indigenous trees as those that arrived at their present environmental setting by natural means, for example water, animal, wind dispersal, without assistance of man. On the other hand, exotic trees are those that people brought from a place. Endemic trees, which are subsets of indigenous trees, are they are those that evolved in a certain location and are found only there. In practice, exotic and indigenous trees are planted in landscapes, but the indigenous, in general, are most appropriated for local environments, as compared to the exotics, because they have the advantage of being climatically suited and live in some degree of equilibrium with pest organisms such as insects and fungi. Exotic trees such as Cassia siamea (Siamese Cassia), Acacia auriculiformis and Polyalthia longifolia (Weeping Willow) dominate the Ghanaian urban landscape. Looking at their form, colour and rate of growth, these plants from South East Asia, are high ornamental performance trees. Some of them are rightfully located, while others are not. Ghana has indigenous trees that are believed to comparatively measure to the standard of these exotic ornamental trees, but no conscious attempt has been made to select them for landscape applications as it has been the case with evaluation of indigenous trees for timber. This study sought to systematically select some indigenous trees of Ghana to fit into the urban setting by using Kumasi as the case study area. The methodology consisted of a baseline study of tree species populations in a purposively selected neighbourhood to establish the extent of dominance posed by exotic trees. Tree selection criteria were set for various urban landscape areas namely: street, residential, reserved, parklands and campuses. A database of the indigenous and exotic trees was prepared and sieved through the criteria to pick candidate trees for the various urban landscapes. Factors that were used to build the criteria were form, height, wood density, vegetation zone, conservation status, growth rate, stem base type and leaf fall behaviour. The results were that some indigenous trees were selected for specific urban landscape areas in the Kumasi metropolis. For example, Spathodea campanulata and Ekebergia capensis were selected as potential trees for street use. Among the exotic trees captured from the baseline studies, some were found to be mislocated, even as others were accepted and retained for their present use. For example, Cassis siamea was rejected as nature reserve tree on the basis of conservation status, but retained as street and parkland tree,whilst Cedrella odorata was outrightly not accepted for any use because of its low wood density. Recommendations for the use of indigenous trees for landscape applications were made, for instance collection of data on young trees (similar to what is done for mature trees) to enable their use in comparatively short – lived projects such as strreets. Another vital recommendation set aside reserved lands for the preservation of threatened tree species. In addition, topics needing further research were presented, for example, shrubs that mature into small trees have to be investigated to fill tight street locations under utility lines. Further work on the selected trees concerning how they are going to adapt to urban conditions was also suggested. Lastly, additional work was proposed to test the atmospheric carbon dioxide reduction capacity of the selected tress for the purpose of reducing global warming.

Source: KNUST