Urban tree composition, diversity and structural characteristics in North-western Nigeria

Urban trees composition and diversity are gaining more relevance for academic, urban planning and sustainability purposes. Despite their free ecosystem services, little is documented on urban trees especially in developing countries such as Nigeria. This study assessed the composition, diversity, structural characteristics and provenance of trees in two cities of North-western Nigeria. Landsat images for 2015 were used to classify each city into five broad land cover classes to facilitate stratified sampling. Data on tree species, height, Diameter at Breast Height (DBH) and provenance was collected from a total of 373 plots of 30 x 30 m dimension. A total of 1558 trees belonging to 56 species in 46 genera and 22 families were assessed with generally more categories of native than exotic species but higher populations of exotic stems. Azadirachta indica and Mangifera indica were the most dominant species in both cities while Meliaceae, Anacardiaceae, Fabaceae, Moraceae and Combretaceae were the dominant families. Lower diversity (H' = 1.8), lower evenness (J = 0.53) and lower mean density (42.4 trees/ha) were recorded in Sokoto compared to Zaria (H' = 3.30, J = 0.84 and density = 50.48 trees/ha). A one-way ANOVA revealed significant difference in these parameters across the land cover types within the cities except mean height which was not significantly different in Sokoto. An independent samples t-test also revealed significantly different mean height, diameter and Basal Area (BA) but similar plot densities between the cities. Urban tree composition, diversity and structural characteristics vary significantly between these two cities in North-western Nigeria and though exotic species have fewer categories, their stems are gradually dominating the cities. This may cause alterations in tree species composition and diversity and possibly, biotic homogenization. This calls for expanded tree planting with more emphasis on the native species.