Climate variability and air pollution threaten urban tree growth in a tropical city

Urban trees provide vital ecosystem services in cities, including temperature regulation, air purification and climate adaptation. However, their growth is increasingly threatened by changing climate and air pollution. This study investigated the long-term effects of climate variability and air pollutants on radial growth of two commonly planted tropical tree species (Swietenia macrophylla and Samanea saman) in Sylhet City, a rapidly urbanizing area in northeastern Bangladesh. Using the dendrochronological approach, we developed tree-ring chronologies for both species and assessed correlations with precipitation, temperature, and air pollutants — particulate matter (PM₂.₅ and PM₁₀), Nitrogen oxide (NOₓ) and Ozone (O₃). Both species responded positively to monsoonal precipitation, highlighting the importance of seasonal water availability on tree growth. Conversely, the rising mean temperature significantly reduced radial growth in both species, particularly in April (S. macrophylla: r = -0.35, p = 0.02; S. saman: r = -0.33, p = 0.03) and June (S. macrophylla: r = -0.39, p = 0.01; S. saman: r = -0.42, p = 0.005). Among the pollutants, PM_2.5 and O₃ were associated with reduced growth in both species during February and January, respectively. These findings revealed that rising temperatures and high pollutant concentrations negatively influence tree growth in Sylhet City. Tree rings of both species can be used as a biomonitoring tool for tracking urban environmental change. Our study highlights the importance of climate-resilient and pollution-tolerant species selection to ensure sustainable urban forestry in tropical cities.