SPECIES DIVERSITY, SOCIO-ECONOMIC AND ENVIRONMENTAL IMPORTANCE OF TREES IN URBAN AND PERI-URBAN CENTRES IN THREE VEGETATION ZONES, NIGERIA

Abstract

Urban and peri-urban trees provide numerous benefits that can improve human livelihood, human health and environmental quality in and around urban and peri-urban areas. Some of these benefits include: fruits and nuts, vegetables, improvements in air and water quality, building materials, energy provision, cooler air temperatures, reductions in ultraviolet radiation, social benefits, etc. This research was carried out to assess the level of species diversity and examine socio-economic and environmental importance of trees in urban and peri-urban areas in three vegetation zones of Nigeria, which are essential to formulate policies for management of urban and peri-urban forests. The study will create database for urban and peri-urban forest tree species in rainforest, derived savanna and guinea savanna vegetation zones of Nigeria. Multistage sampling technique was used for the selection of three (3) vegetation zones, two (2) cities in each vegetation zone and two (2) forest types (one urban and one peri-urban) from each city. Data were collected from 20% of the built-up area within each city. ArcGIS was used to estimate the 20% of the built-up area in both urban and peri-urban centres where tree growth data were collected and questionnaire administered. The coordinates of selected locations in the city were obtained using hand held Global Positioning System (GPS) unit. Tree growth data was collected from urban and peri-urban centres of the selected cities in the three vegetation zones. All trees with diameter at breast height (dbh) ≥ 10 cm were identified, dbh measured and their frequencies of occurrence taken. Socio-economic data were obtained using semi-structured questionnaire. Twenty respondents were selected from urban and peri-urban centers in each city for the administration of questionnaire for social and environmental data. Thus, two hundred and forty (240) questionnaires were administered in this study (i.e. 2 centres in each city (1 urban and 1 peri-urban) * 2 cities in each vegetation zone * 3 vegetation zones * 20 respondents)). Data were analyzed using one-way analysis of variance, descriptive statistics and student t-test. ArcGIS 10.3 software package was used to produce Normalized Diference Vegetation Index (NDVI) map for this study. The data obtained were used to compute the tree growth and yield parameters, tree species diversity indices, determine socio-economic and environmental importance of trees in urban and peri-urban centres of the study areas. The results revealed that a total of one hundred and twenty eight tree species (54, 19 in urban and peri-urban areas of Ibadan, respectively; 56, 16 in urban and peri-urban areas of Port Harcourt, respectively; 69, 20 in urban and peri-urban areas of Abuja, respectively; 36, 13 in urban and peri-urban areas of Minna, respectively; 62, 31 in urban and peri-urban areas of Kabba, respectively; as well as 72, 15 in Omu-Aran, respectively) were identified across urban and peri-urban areas in the three vegetation zones. Statistically, the number of tree species in Abuja (urban and peri-urban) and Omu-Aran (urban and peri-urban) were significantly different but those of the urban and periurban centres of the other cities were not significantly different. Also, findings from this research indicated that a total of thirty-nine (39) families [(25, 16 in urban and peri-urban areas of Ibadan), (27, 12 in urban and peri-urban areas of Port Harcourt), (27, 12 in urban and peri-urban areas of Abuja), (17, 9 in urban and peri-urban areas of Minna), (25, 19 in urban and peri-urban areas of Kabba) and (30, 14 in urban and peri-urban areas of Omu-Aran)] were encountered in the study areas. Mean total volumes (m3) of all individual trees encountered in the urban and peri-urban areas of each city are: 684.3 and 281.2 m3 (Ibadan); 907.2 and 321.5 m3 (Port Harcourt); 752.8 and 191.0 m3 (Abuja); 428.5 and 291.7 m3 (Minna); 134.9 and 63.7 m3 (Kabba) as well as 267.3 and 117.03 m3 (Omu-Aran). The results of tree growth parameter revealed that basal area and volume were significantly different across vegetation zones but other tree growth parameters were not significantly different. However, the results showed that significant differences existed between species diversity indices and growth parameter between the two forest types (urban and peri-urban) forests in all the cities. This study revealed that the benefits derived by the people from urban and peri-urban trees (landscape beautification, park services, nutritional supplement, etc) are higher in the guinea savanna zone (75 to 90%) than in the rainforest (25 to 75%) and derived savanna 30 to 60% zones, while the use of the trees and/or their products for animal fodder is higher in guinea savanna zone (15 to 85%) and derived savanna zone (40 to 85%) than in the rainforest zone (10 to 50%). The result of Normalized Difference Vegetation Index (NDVI) map for the cities in this study revealed that Port Harcourt has the highest NDVI values followed by Minna, Omu-Aran, Kabba, Abuja and Ibadan. The result of NDVI maps indicated that derived savanna had the highest green coverage, followed by guinea savanna, with rainforest having the least green coverage area. This study also provided baseline information for policy formulation for the management of tree species urban and periurban centres as well as the contributions of urban and peri-urban forests to tree species conservation in Nigeria. Management options should focus on increasing tree species diversity in urban and peri-urban area through avenue trees, gardens, urban paks, woodlots, etc. All stakeholders should also be involved in the management, planning, planting, maintenance and protection of urban and peri-urban forests for healthy trees species conservation