Geospatial modelling of vegetation dynamics and carbon sequestration capacity in Ekiti State, Nigeria, using MODIS data and CASA models

The accurate estimation of terrestrial carbon sequestration capacity is fundamental to national climate mitigation efforts and achieving Sustainable Development Goals (SDGs). This study assessed vegetation dynamics and modelled Net Primary Productivity (NPP), a proxy for carbon sequestration capacity in Ekiti State, Nigeria, over the 11 years (2014–2024). The study deployed the seasonal and annual Carnegie Ames Standard Approach (CASA) model, driven by high-resolution Landsat 8 Enhanced Vegetation Index (EVI), derived APAR ERA5 climate data, and Terraclimate moisture data, to study the influence of seasonal climate drivers and demographic expansion on carbon stocks. Furthermore, the 30 m CASA-derived NPP outputs were evaluated against the coarse (500 m) MODIS (MOD17A3HGF) NPP product to assess the cross-sensor consistency of regional carbon estimates. The NPP estimated by the CASA model and MODIS product were 482.61 ± 38.95gC m⁻² yr⁻¹ and 474.63 ± 35.89gC m⁻² yr⁻¹, respectively. Temporally, the findings revealed a critical disparity: the MODIS NPP trend showed a statistically significant decline (t = -0.491, p = 0.041) over the decade, whereas the higher-resolution CASA NPP showed only a weak, insignificant decline (t = -0.127, p = 0.648). Seasonal analysis revealed that wet-season NPP is light-limited, while dry-season NPP is water-limited. Furthermore, spatial analysis identified a strong negative correlation between the Normalised Difference Building Index (NDBI) and NPP (Spearman’s r = -0.79, p < 0.01), indicating that demographic growth and urban expansion are primary drivers of localised carbon sink degradation. These results demonstrate that seasonally antagonistic climate factors govern local carbon sequestration and highlight the necessity of high-resolution, locally calibrated models for capturing the complex interactions between climate and human activities in ecological transition zones.