Mapping urban tree canopy loss as an indicator of development pressure in the Colombo metropolitan area (2004–2024)

Abstract

Urban tree canopy is essential to ecological resilience and liveability, yet increasingly threatened by urban growth. This study quantified canopy change in the Colombo Metropolitan Region from 2004 to 2024, using high-resolution Google Earth imagery and a 500m grid-based Tree Cover Mapping approach. Development pressure was modelled in ArcGIS through a composite index combining accessibility, population density, built-up intensity, and land use. Canopy cover declined from 31.45% (8,978 ha) to 23.19% (6,619 ha), with losses concentrated in high-pressure zones such as Pettah, Borella, and Battaramulla. Pearson and Spearman analyses show a moderate, statistically significant positive association between the Development Pressure and the Canopy Reduction (Pearson’s r = 0.36; Spearman’s ρ = 0.34; p < 0.001; N = 947), confirming that higher pressure is strongly linked to greater canopy loss. Spatial overlays revealed areas under high pressure show stronger canopy reduction and retain less canopy, whereas low-pressure areas particularly public and protected lands maintain or increase cover. A 30–40% canopy “transition range” in moderate-pressure areas signals a critical intervention point. These findings demonstrate that how nuanced spatial patterns, provide actionable evidence for integrating canopy targets, targeted greening, and systematic monitoring into Colombo’s planning policies to balance urban development with ecological sustainability.