Land cover change impacts on the tree density and carbon stock in Sokoto Metropolis, North-Western Nigeria

Urban trees and forests provide an array of ecosystem services to urban dwellers; the most important of which is the storage and sequestration of atmospheric carbon. However, these resources are being increasingly affected by land cover changes caused by accelerated and unregulated growth in most urban areas of the world. The objective of this study was to assess land cover change impacts on the density of trees and carbon stock in Sokoto metropolis, North-western Nigeria. Landsat images for 1990, 1999 and 2015 were processed for land cover classification and change detection using the Maximum Likelihood Classification and Post Classification Comparison techniques. Data on tree species and their origin was collected from field survey of 189 sample plots while the density and distribution of carbon stock in the metropolis was quantified and predicted with the InVEST model. The classification revealed five broad land cover classes which include the Built-up Area, Farmland, Green Area, Open Space and Wetland/Water. The Built-up and Green areas continuously increased from 25.4% and 13.7% of the total area respectively in 1990 to 59.03% and 16.91% respectively in 2040. Conversely, the Farmland and Open Space continuously decreased from 38.2% and 20.9% of the total area respectively in 1990 to 12.96% and 9.94% of the total area respectively in 2040. The Wetland/Water on the other hand fluctuated in size throughout the study period, with a net reduction of approximately 60 hectares. The pattern of expansion was generally radial but more evident in the north and eastern parts of the metropolis. Overall classification accuracy was 91.7%, 92.1% and 90.3% for 1990, 1999 and 2015 respectively. The study recorded 722 tree stems belonging to 30 species in 17 genera and 14 families. Majority of the species (21) are native to the area while 9 are exotic. However, the exotic species contributed more tree stems (73.3%) than the native species (26.7%). About 62% of these stems were recorded in the Built-up Area but the highest stem density of 78.4 ha-1 was recorded in the Green Area. The most dominant species were Azadirachta indica, Mangifera indica, Adansonia digitata, Ficus polita and Terminalia catappa while the dominant families were Meliaceae, Fabaceae, Moraceae, Combretaceae and Anacardiacea. There was significant difference in mean trees stem diameter (f = 5.79, p < 0.001) and basal area (f = 5.21, p < 0.001) across the land cover classes but no significant difference in mean trees stem height (f = 1.82, p > 0.123). Based on NDVI and NDBI differencing, a simple linear correlation revealed significant positive relationship between urban expansion and tree density in the metropolis for 1990 (r = 0.980, p = 0.001), 1999 (r = 0.986, p = 0.001) and 2015 (r = 0.972, p = 0.001). Carbon density, distribution and sequestration in the metropolis were also strongly influenced by the type and extent of land cover, with majority of the carbon contributed by the soil. The total carbon was estimated at 697,563.49 Mg, 717,972.27 Mg and 731,465.96 Mg, corresponding to mean carbon of 73.9 Mg/ha-1, 76.0 Mg/ha-1 and 77.5 Mg/ha-1 in 2015, 2030 and 2040 respectively and valued at USD 34,874,625.00 (NGN 12,624,614.00), USD 35,894,975.00 (NGN 12,993,980,950.00) and USD 43,883,814.00 (NGN 15,885,940,668.00) in the respective years. The total sequestered carbon on the other hand, was estimated at 2,0407Mg between the year 2015 and 2030 and 1,3497Mg between the year 2030 and 2040 and valued at USD 734,399.00 (NGN 265,852,438.00) and USD 548,091.00 (NGN 191,831,850.00) between the respective years. The Built-up and Green Areas contributed more carbon than the other land cover classes while trees species with the highest carbon storage were Tamarindus indica, Azadirachta indica, Parkia biglobosa, Delonix regia, Mangifera indica and Sclerocarya birrea. These species are thus recommended to be widely planted in the metropolis. Urban afforestation and climate change mitigation programmes in Sokoto metropolis should thus place emphasis on higher carbon storing species and especially the native species which are declining in population.

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