Urbanisation of a growing tropical mega-city during the 21st century — landscape transformation and vegetation dynamics

Fine scale spatial and temporal patterns in land-cover dynamics arising from rapid urbanisation of tropical regions are poorly understood. We quantify changes in landcover across the Bangkok region using high-resolution aerial imagery from ∼ 2004 to ∼ 2018 and address three questions: i) does urbanisation generate temporal shifts in the form of vegetation cover-urbanisation intensity relationships?, ii) do urban expansion and densification generate different vegetation dynamics?, iii) do net changes in vegetation cover and loss vary with urbanisation intensity? The form of vegetation cover-urbanisation intensity relationships exhibited negligible temporal variation, supporting the use of space-for-time substitution approaches for predicting future landcover dynamics. During our study period impervious surface cover increased by ∼ 474 km2 and there were net losses of grassland (∼583 km2) and rice-fields (∼94 km2), and a net gain in tree cover (∼137 km2). These changes have substantial implications for urban heat islands, flood risk and biodiversity. Urban expansion contributed more to vegetation dynamics than densification, partly because expansion impacted more land. Densification minimised loss of green-space, grasslands, and agriculture (rice-fields), but generated substantial local tree cover loss, which is critical to retain in highly urbanised areas. This contrasts with increasing tree cover elsewhere, including areas experiencing urban expansion. Trade-offs thus arise between impacts on different vegetation types when meeting the demand for tropical urban development through densification or expansion. Densification benefits most vegetation types but must be accompanied with tree protection and planting schemes to balance these trade-offs and minimise detrimental impacts of densification on people, ecosystem services and biodiversity.