Modeling Climate Regulating Service Provided by Hong Kong Vegetation and Its Decadal Climatic and Environmental Drivers

Terrestrial vegetation regulates climate by photosynthetic uptake of CO₂. The extensive coverage of vegetation in Hong Kong (more than 70% of land) highlights the potential for terrestrial carbon sink to play a major role in achieving carbon neutrality in such a metropolitan city. To quantify the potential, local measurements and allometric modeling have estimated the aboveground biomass (AGB) on plot scales in Hong Kong. However, it remains a challenge to illustrate the temporal trends and spatial distribution of carbon uptake by vegetation in the city, and to understand what factors have shaped them. Here, we aim to estimate the net primary productivity (NPP) of Hong Kong vegetation, and identify the key drivers for variability of NPP on a city scale. We use the Terrestrial Ecosystem Model in R-Hong Kong (TEMIR-HK), a localized process-based ecosystem model, to evaluate the changes in NPP trends induced by changing CO₂ concentration, temperature, ozone concentration, and changing leaf area index (LAI) shaped by these factors as well as land use. Simulation results show an increasing trend of NPP, with an average NPP of 1.53 Tg C y^-1, which is less than 10% of the annual total anthropogenic carbon emission from Hong Kong, suggesting a limited but indispensable potential of urban forestry to achieve city-level carbon neutrality. The factorial simulations show that increasing ambient CO₂ concentration is the most dominant driver of increasing NPP among all potential drivers. This suggests that the globally well-mixed CO₂ concentration is impacting NPP more than the local climate, environmental and land-use changes in Hong Kong.