Abstract

Rapid urbanization in tropical megacities intensifies environmental stress on urban vegetation, particularly during the summer season characterized by high temperatures, elevated evapotranspiration, and limited moisture availability. This study examines spatiotemporal patterns of summer urban vegetation greenness and moisture in the Bangkok Metropolitan Region (BMR), Thailand, using Sentinel-2 multispectral imagery. Vegetation conditions were evaluated for the summers of 2016, 2018, 2022, and 2024 using the Normalized Difference Vegetation Index (NDVI), Green Normalized Difference Vegetation Index (GNDVI), and Normalized Difference Moisture Index (NDMI). Pearson correlation analysis was applied to investigate relationships among vegetation greenness, photosynthetic activity, and moisture status under urban environmental stress. The results show that mean summer NDVI and GNDVI values remained relatively stable throughout the study period, indicating that average vegetation greenness has been maintained despite ongoing urban expansion. However, declining maximum NDVI values and increasing spatial heterogeneity reveal a gradual loss of dense and healthy vegetation, particularly in peri-urban areas. NDMI analysis indicates persistently low to slightly positive mean values, reflecting widespread summer water stress across the BMR, with clear contrasts between moisture-rich peripheral zones and the highly urbanized core. Strong positive correlations between NDVI and GNDVI confirm their effectiveness in representing overall vegetation vigor, while moderate correlations between greenness indices and NDMI highlight a partial decoupling between visual greenness and moisture availability. These findings suggest that maintaining average vegetation greenness alone is insufficient to sustain key ecosystem services in rapidly urbanizing tropical cities and emphasize the need to integrate vegetation structure and moisture considerations into urban green infrastructure planning.