Diversity and structure of urban forests of Sahel cities in Niger

Urbanization in the Sahel is constantly competing with and greatly affecting the woody flora in urban areas. Urbanization can replace the species mix, leading to changes in plant community composition; however, there are limited studies that assess these attributes in Sahelian cities. This study assessed urban forests in Niamey and Maradi, two important cities of Niger, West Africa for differences between the cities and across LULC types in terms of species diversity and stand structure. Woody species were inventoried in 357 plots in urban forests located across seven land use/land cover types. Stem diameter at breast height of trees and shrubs ≥2.5 cm, total height and crown diameter were measured. Eighty-six species belonging to 69 genera with 33 families were inventoried in Niamey, while in Maradi 91 species belonging to 70 genera with 30 families were enumerated. Fabaceae was the dominant family in both cities. Azadirachta indica accounted for 41 and 54% of all stems in Niamey and Maradi. Residential areas had a higher species richness than other land uses. The majority of the species documented in the two cities were exotics representing 52% of all species encountered. Mean Shannon diversity index (H′) and standard deviation was 2.31 ± 0.43 for Niamey and 2.14 ± 0.74 for Maradi. The similarity index of the two cities was 70%. Urban forests of Maradi had significantly higher levels of species richness and evenness as well as tree density and canopy cover than urban forests of Niamey. Across LULC types, canopy cover varied significantly (p < 0.05) but basal area had a marginal variation whilst, stem density was not significantly different. There were significant positive relationships (p < 0.05) between species richness, basal area and canopy cover across the LULC types in both cities. The study shows that land use/land cover types influence the characteristics of urban forests and differences exist in diversity and stand structural characteristics of urban forests in the two cities. Future development of urban forests in the two cities must increase the planting of native species to improve their value for biodiversity conservation.