Abstract
Loss of native tree species constitutes a major socio-economic and ecological problem. Nevertheless, there is scanty data on the role of Sahelian cities in native woody species conservation and climate change mitigation. This study tried to determine the native woody diversity, structure, and carbon stocks in Niamey, and Maradi, Niger. The study used the plot method to inventory dendrometric variables based on a stratified sampling approach from peri-urban areas to the built environment. Biodiversity indices and allometric models were used for the data analysis. A total of 63 native woody species representing 47 genera and 24 families were recorded in the overall urban forests in two cities. In Niamey, 46 species belonging to 35 genera and 19 families were recorded while in Maradi, 50 species, 41 genera, and 19 families were inventoried. Fabaceae was the dominant families in both cities with equal richness (13 species).
Top1. Introduction
Native tree species are under threat due to human activities and climate change. For instance, 1.8% of all biodiversity hotspot areas will be taken up by spatial urban expansion by 2030 (Seto et al., 2012). In addition to that, 90% of the protected areas are likely to be modified by urbanization mostly in developing countries (Mcdonald et al., 2008). Urbanization destroys the natural environment (Elmqvist et al., 2013a) with the introduction of exotic species which also leads to biotic homogenisation (Mckinney, 2006). According to Dunn and Heneghan, (2011) native species richness decreases and exotic species increases with urbanization gradient. Furthermore, Pysek, (1998) reported that the native biodiversity decreases from villages to the urban areas and cities have a greater number of exotic species with respect to the total floristic list. Urbanization influences negatively vegetation cover (Seto et al., 2012) leading to a great reduction in biomass (Arroyo-Rodríguez et al., 2017).The depletion of native species due to urbanization leads to the reduction of biodiversity in urban ecosystems (Aronson et al., 2014; McKinney, 2002; McPherson and Simpson, 2003), and ecosystems services such as carbon biomass (Arroyo-Rodríguez et al., 2017) therefore it has biogeochemical cycles (Wang et al., 2017) and regional and global climate (Seto and Shepherd, 2009).
Although urban areas expansion continue towards the natural vegetation, urban areas play a crucial role in the biodiversity conservation (Elmqvist et al., 2013b; McKinney, 2002). In addition that urban areas uptake considerable amount of atmospheric carbon dioxide through urban green areas (Chen, 2015; Nowak et al., 2013a; Reynolds et al., 2017) such as indigenous woody species in urban areas (Stoffberg et al., 2010).
Empirical studies across the world pointed out cities can serve as native woody biodiversity refugia. In urban areas in Asia (Yan and Yang, 2017) found an important proportion of native woody species in Beijing. Similar results found that cities are home of native woody plant such as in Central European cities as reported by (Pysek, 1998), in Hannover (Matthies et al., 2017). Cities in African also have been reported to be a habitat of native woody species such as in cities of the Eastern Cape, South Africa (Kuruneri-Chitepo and Shackleton, 2011), in Lomé in Togo (Raoufou et al., 2011), Abuja in Nigeria (Agbelade et al., 2017), Kumasi in Ghana (Nero et al., 2017) and Nairobi in Kenya (Nyambane et al., 2016). These studies reported on woody plant species (native and exotic) used in urban green spaces, their systematic composition and growth characteristics. These studies do not however, provide information on the extent to which urban areas can be key especially to native woody species conservation and the role of native woody flora to climate change mitigation. In addition, lacking in the literature is a comparison between peri-urban native woody vegetation and different land use and land cover (LULC) types found in built areas. Besides, there are no study in the international literature on the City Biodiversity Index (CBI) such as native biodiversity in the city and ecosystems services provided by biodiversity in the city from West Africa Sahel cities. This impedes effort to generalise native urban woody vegetation characteristics across different climatic zones as well as regions with contrasting socio-economic, cultural and environmental characteristics like West Africa Sahel.
Niamey and Maradi are two cities in Niger, a country that lies right in the centre of the Sahel. With an annual population growth rate of 3.9% and mass migration of human population from rural to urban centres (INS, 2016), about 19% of the total population of Niger are urbanites (INS, 2016). Niger is facing serious urbanization issues (Issaka, 2013) such as depletion of public urban green areas and local biodiversity depletion. For instance green belt of Niamey has initial land mass of 2500 hectares which is reduced to 2336 hectares due to the construction in the green belt of Niamey (Ministère de l’Environnement du Niger, 2010). In addition to that, 90% of urban areas in Niger used wood as source of domestic energy (Ministère de l’Environnement du Niger, 2010). The spatial expansion of Niamey and Maradi cities has led to the depletion of natural vegetation and loss of emblematic local woody species (Communication Personal, Saadou, Mahamane, 2018) to Niamey and Maradi regions. The urbanization model of such cities is posited to modify native woody vegetation composition, resulting in either fragmentation or loss of biodiversity (Blair, 1999; Mckinney, 2006) or conservation of indigenous woody biodiversity in urban areas (Yan and Yang, 2017).
Key Terms in this Chapter
Native Woody Species: Local or indigenous woody plants.
Native Woody Species Conservation: Any act or practice to protect and preserve a variety of indigenous woody species in a given area of the planet.
Urban Forests: A collection of woody species found in urban and peri-urban areas.
Carbon Sequestration Potential: Ability of a green plant to remove carbon dioxide from the atmosphere via photosynthesis process.
Urban Woody Species Values: different benefits or services such as carbon dioxide reduction, fodder, oxygen, fruit and leaves for human consumption, etc. provided by urban woody flora.
City Biodiversity Index: Number of native woody species and the carbon stock are examples of City biodiversity Index.
Urban Forest Structure: Biophysical aspects of the urban forests such as the name of species, stem density, tree cover, etc.