Quantifying the Urban–forest Trade-off in a Rwandan Secondary City: A 35-Year Multi-temporal Remote Sensing Assessment of Huye District (1990–2025)

Abstract

Rapid urban expansion is transforming land-cover patterns in Rwanda’s secondary cities, with potential consequences for forest resources and sustainable land-use planning. This study assessed long-term changes in built-up and forest cover associated with urban expansion in Huye District, Rwanda, from 1990 to 2025, and examined the spatial relationship between built-up expansion and forest-cover decline. A multi-temporal remote sensing approach was applied using Landsat imagery for four reference years (1990, 1999, 2018, and 2025). The imagery was pre-processed and classified using supervised maximum-likelihood classification into five land-cover classes, with classification accuracy assessed using error-matrix analysis and ground-reference data, achieving overall accuracies exceeding 85% for all years. Post-classification change detection quantified land-cover transitions and changes in built-up and forest areas, while Pearson correlation analysis assessed the association between built-up and forest areas, and a 5-km proximity buffer was used to examine the spatial concentration of forest-cover loss around expanding built-up areas. Built-up land increased from 2.80 km² (0.52% of the district) in 1990 to 49.30 km² (8.94%) in 2025, representing a 1,660.71% increase, while forest cover declined from 74.50 km² (13.79%) to 35.20 km² (6.51%), corresponding to a 52.75% reduction. Consequently, the built-up-to-forest area ratio shifted from approximately 1:26.6 in 1990 to 1.40:1 in 2025, indicating a substantial reversal in the relative extent of these land-cover types. Built-up and forest areas exhibited a strong inverse association across the four observation years (Pearson r = −0.92), although the relationship was not statistically significant at the 5% level (p = 0.078; n = 4). Spatial analysis further indicated that forest-cover loss was disproportionately concentrated within the 5-km zone surrounding expanding built-up areas. Overall, Huye District experienced substantial built-up expansion accompanied by pronounced forest-cover decline over the 35-year period. The strong inverse temporal association and concentration of forest loss near expanding built-up areas suggest that urban growth is an important spatial correlate of forest-cover change, although the limited number of temporal observations constrains statistical inference. The findings highlight the need to integrate forest conservation into secondary-city expansion planning through enforceable urban growth boundaries, protection of forested buffer zones, and targeted reforestation and restoration measures.