The role of vegetation in the CO₂ flux from a tropical urban neighbourhood

Abstract. Urban surfaces are usually net sources of CO₂. Vegetation can potentially have an important role in reducing the CO₂ emitted by anthropogenic activities in cities, particularly when vegetation is extensive and/or evergreen. A direct and accurate estimation of carbon uptake by urban vegetation is difficult due to the particular characteristics of the urban ecosystem and high variability in tree distribution and species. Here, we investigate the role of urban vegetation in the CO₂ flux from a residential neighbourhood in Singapore using two different approaches. CO₂ fluxes measured directly by eddy covariance are compared with emissions estimated from emissions factors and activity data. The latter includes contributions from vehicular traffic, household combustion, soil respiration and human breathing. The difference between estimated emissions and measured fluxes should approximate the flux associated with the aboveground vegetation. In addition, a tree survey was conducted to estimate the annual CO₂ sequestration using allometric equations and an alternative model of the metabolic theory of ecology for tropical forests. Palm trees, banana plants and turfgrass were also included in the survey with their annual CO₂ uptake obtained from published growth rates. Both approaches agree within 2% and suggest that vegetation sequesters 8% of the total emitted CO₂ in the residential neighbourhood studied. An uptake of 1.4 ton km⁻² day⁻¹ (510 ton km⁻² yr⁻¹) was estimated as the difference between assimilation by photosynthesis minus the aboveground biomass respiration during daytime (4.0 ton km⁻² day⁻¹) and release by plant respiration at night (2.6 ton km⁻² day⁻¹). However, when soil respiration is added to the daily aboveground flux, the biogenic component becomes a net source amounting to 4% of the total CO₂ flux and represents the total contribution of urban vegetation to the carbon flux to the atmosphere.