Vegetation plays a vital role in modulating climate and the carbon cycle on land through processes like photosynthesis, also known as gross primary production (GPP). The significant presence of vegetation in Hong Kong, covering over 70 % of the land area, highlights the potential for terrestrial carbon sink to contribute to achieving carbon neutrality in such a metropolitan city. Meanwhile, the terrestrial ecosystem is also influenced by climatic and environmental factors. This study investigates the historical spatiotemporal dynamics of GPP in the subtropical forests of Hong Kong and the key drivers behind its trend and interannual variability between 2002 and 2018. We used the Terrestrial Ecosystem Model in R-Hong Kong (TEMIR-HK), a localized process-based ecophysiological model, to evaluate the changes in GPP induced by changing CO<sub>2</sub> concentration, temperature, ozone (O<sub>3</sub>) concentration, and changing leaf area index (LAI) shaped by these factors as well as land use. Simulation results indicate an increasing trend of GPP, with an average annual GPP of 1.75 TgC yr<sup>−1</sup> , which is around 15 % of the annual total anthropogenic carbon emission from Hong Kong, suggesting a limited but indispensable potential of forestry to achieve city-level carbon neutrality. Model simulations of GPP show satisfactory results when spatially comparing with satellite-based GPP dataset (<em>R</em> = 0.89), with slight difference of +8.7 % on average. Factorial simulations reveal LAI changes dominate both trend (+0.0134 TgC yr<sup>−2</sup> ) and interannual variability (standard deviation: 2.77×10−2 TgC m<sup>−2</sup> yr<sup>−1</sup>) of GPP in Hong Kong. This result highlights that local-scale reforestation could influence GPP trend over the whole city and emphasizes the importance on the accuracy of LAI input in ecosystem-scale photosynthesis modelling. This work contributes to improving the scientific understanding on subtropical forest ecosystems, and highlights the potential, though limited, of Hong Kong forests to play their parts in working toward carbon neutrality targets.