Spatio-temporal analysis of vegetation dynamics in derived savannah, Ogun State Nigeria from 2002 to 2023

Abstract

Vegetation covers is a significant component of biogeochemical cycles. Derived savannah of Ogun State has been affected by vegetation loss and climate change in recent times. There is lack of information on the rate/extent of vegetation loss in the last two decades. This study assessed changes in vegetation cover in derived savanna ecosystem of Ogun State from 2002 to 2023. Landsat images were downloaded from the repository of the United States Geological Survey (USGS). Composites of red, green, blue and near-infra-red spectral bands of study period were obtained and classified using Maximum Likelihood (ML) algorithm into Land Use Land Cover (LULC) categories as follows: bare soil; built-up areas; forest; and grassland. Change in area extent and rate of change of area of classified images were determined for the study period. Confusion matrix of classified images were generated and compared with Google Earth satellite image with accuracy assessed using kappa coefficients. Overall accuracy of the classified images ranged between 79% and 88% with kappa coefficients of between 0.71 and 0.83. Results showed that built-up area increased from 10.3% cover in 2002 to 35.9% cover in 2023. However, there was a significant decline in forest cover from 31.5% to 13.7% for the same period. Significant increase at 4.2 km2 per year in area extent was observed for built-up LULC class while a decline of 2.0 km2 per year in forest cover was recorded for Forest LULC category from 2002 to 2023. The study revealed that urbanization increased as extent of initial forest cover were degraded and replaced with physical infrastructure. Therefore, there is urgent need for policies that promote conservation and sustainable management of forests and grasslands, as well as measures to promote green infrastructure and urban greening initiatives to address the decline in vegetation cover in the derived savanna ecosystem of Ogun State.

Keywords

• Land use land cover (LULC)
• Derived savannah ecosystem
• Vegetation loss
• Maximum likelihood classification

Ethical Statement

Not applicable

Bitki örtüsü dinamiklerinin 2002-2023 yılları arasında Ogun Eyaleti Nijerya'da seçilen savanada zamansal-mekansal analizi

Abstract

Bitki örtüsü biyojeokimyasal döngülerin önemli bir bileşenidir. Ogun Eyaleti'nin savanları, son zamanlarda bitki örtüsü kaybından ve iklim değişikliğinden etkilenmiştir. Son yirmi yılda bitki örtüsü kaybının oranı/derecesi hakkında bilgi eksikliği bulunmaktadır. Bu çalışma, 2002'den 2023'e kadar Ogun Eyaleti'nin savan ekosistemindeki bitki örtüsündeki değişiklikleri değerlendirmiştir. Landsat görüntüleri Amerika Birleşik Devletleri Jeolojik Araştırma Kurumu'nun (USGS) web sitesinden indirilmiştir. Çalışma dönemine ait kırmızı, yeşil, mavi ve kızıl ötesi spektral bantların kompozitleri elde edilmiş ve Maksimum Olasılık (ML) algoritması kullanılarak Arazi Kullanımı/Arazi Örtüsü (LULC) kategorilerine göre şu şekilde sınıflandırılmıştır: çıplak toprak, yerleşim alanları, orman ve otlak. Çalışma dönemi için sınıflandırılmış görüntülerin alan büyüklüğündeki değişim ve alan değişim oranları belirlenmiştir. Sınıflandırılmış görüntülerin hata matrisi oluşturulmuş ve kappa katsayıları kullanılarak değerlendirilen doğrulukla Google Earth uydu görüntüsüyle karşılaştırılmıştır. Sınıflandırılan görüntülerin genel doğruluğu, 0,71 ile 0,83 arasında, kappa katsayıları ise %79 ile %88 arasında değişmektedir. Sonuçlar, yerleşim alanının 2002'deki %10,3 kapalılıktan 2023'te %35,9'a yükseldiğini göstermiştir. Ancak aynı dönemde orman örtüsünde %31,5'ten %13,7'ye önemli bir düşüş göze çarpmaktadır. 2002'den 2023'e kadar, yerleşim LULC sınıfı için alan büyüklüğünde yılda 4,2 km2'lik önemli bir artış gözlemlenirken, orman LULC kategorisi için yılda 2,0 km2'lik bir azalma kaydedilmiştir. Çalışma, başlangıçtaki orman örtüsü azaldıkça ve yerini fiziksel altyapıya bıraktıkça kentleşmenin arttığını ortaya çıkarmıştır. Bu nedenle, ormanların ve otlakların korunmasını ve sürdürülebilir yönetimini destekleyen politikaların yanı sıra, Ogun Eyaleti'nin savan ekosistemindeki bitki örtüsündeki azalmayı ele almak için yeşil altyapıyı ve kentsel yeşillendirme girişimlerini teşvik edecek önlemlerin alınmasına acil ihtiyaç bulunmaktadır.

Keywords

• Arazi kullanımı/arazi örtüsü (LULC)
• Savana ekosistemi
• Bitki örtüsü kaybı
• Maksimum olasılık sınıflandırması

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