Stand Structure, Woody Species Diversity and Carbon Stocks in Urban Forests of a West African Sahelian Zone: A Case of Niamey and Maradi Cities, Niger

Abstract

Urbanization is a driver of deforestation, which results in the loss of biodiversity and ecosystem services. However, through the development of urban forests some of these losses can be regained in cities. Many studies report of the role urban forests play in biodiversity conservation and climate change mitigation. Nevertheless, there is paucity of information about the role of urban forests in West African Sahel cities in biodiversity conservation and carbon storage in the literature. This study determined the woody species diversity, stand structure, and carbon stocks in urban forests of Niamey, and Maradi (two important cities in Niger) across seven-land use/land cover (LULC) types. Data was collected through an inventory of 30.70 ha and 20.25 ha of urban forests in Niamey and Maradi respectively based on a stratified random sampling approach. Biodiversity indices and structural parameters were used to determine the woody species diversity and stand structure of the forests across the different LULC types. Allometric equations were used to estimate the aboveground biomass (AGB) and root shoot ratio for belowground biomass. A total of 4689 trees and shrubs belonging to 115 species in 35 families was found in the two cities. 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. Mean Shannon diversity index (H’) for Niamey was (H’= 2.31 ± 0.43) and in Maradi (H’= 2.14 ± 0.74). The similarity index of the two urban forests was 70% suggesting close resemblance of the cities’ urban forests. Fabaceae was the dominant family in both cities. Exotic species accounted for 62 and 77% of all trees in Niamey and Maradi respectively. Azadirachta indica accounted for 41 and 54% of all trees respectively in Niamey and Maradi. The species diversity and structural characteristics varied among the LULC types (p < 0.05) with residential areas having a higher species richness than other land uses. Urban forests in built areas of the cities had higher species richness than peri-urban forests. There were more trees than shrubs in both cities. The three most dominant species in terms of AGB were Azadirachta indica (47%), Faidherbia albida (16%), and Khaya senegalensis (12%) in Niamey. In Maradi, the equivalents were Azadirachta indica (61%), Terminalia mantaly (7%), and Faidherbia albida (6%). Exotic species represented 60 and 78% of the total carbon stocks in Niamey and Maradi respectively. The overall urban forests carbon stocks were 31.63 (15.63, 47.64) t/ha in Niamey and 58.30 (13.10, 103.50) t/ha in Maradi and were not significantly different. However, carbon stocks varied significantly across the LULC types in each city. The neem tree alone constituted 54% and 61% of carbon stocks in urban forests of Niamey and Maradi respectively. The conversion of peri-urban forests into urban forests led to carbon gain. This study showed that urban forests in Niamey and Maradi are important source of woody species diversity and carbon sinks and may therefore be important in climate change mitigation and other ecosystem services. The study recommends the promotion of the use of native species in urban forests in Niamey and Maradi for a dual benefit of carbon and native species conservation that is at present lacking in the two cities. Also carbon stocks in urban forests in Niger should be integrated into the national carbon stock accounting for sustainable urbanization.