Evaluating urban tree diversity and the “10-20-30” rule of urban forestry in Ilorin metropolis, Nigeria

Abstract

Urban forests comprising trees in streets, parks, and other urban settings, provide essential ecological, economic, and social benefits. The “10-20-30” rule of urban forestry is a guideline to promote tree species diversity and enhance the resilience of urban forests. This study therefore evaluates urban tree diversity in Ilorin metropolis, Nigeria, with a focus on adherence to the "10-20-30" rule. This research employed a systematic survey of urban tree species across various central areas in Ilorin covering about 20% of Ilorin's landmass. Tree species were identified to species level. Data were analyzed using descriptive statistics, Shannon-Wiener's index, and Margalef’s index to assess diversity and species richness. The results revealed high species diversity, with the Fabaceae family being the most represented. Notable species include Albizia lebbeck, Ficus macrocarpa, and Polyalthia longifolia. The Shannon-Wiener index of 3.88 and Margalef's index of 10.5 indicate significant species diversity and richness. The study confirmed compliance with the “10-20-30” rule, as no single species (Polyalthia longifolia = 8.40%, Azadirachta indica = 6.36%, Ficus macrocarpa = 5.80%), genus (Ficus = 14.85%, Terminalia = 8.84%, Polyalthia = 8.40%) or family (Fabaceae = 21.71%, Moraceae = 15.01%, Annonaceae = 11.35%) exceeded the thresholds. These findings highlight the importance of diverse urban forests in enhancing urban resilience and sustainability, offering valuable insights for policymakers and urban planners to improve urban forestry management in developing cities. To enhance urban resilience and sustainable forestry in Ilorin metropolis, it is crucial to maintain adherence to the “10-20-30” rule by fostering species, genus, and family diversity. Conservation efforts should prioritize diverse species like Albizia lebbeck, Ficus macrocarpa, and Polyalthia longifolia, while policymakers and urban planners should integrate these findings into actionable strategies.