The role of vegetation in the CO<sub>2</sub> flux from a tropical urban neighbourhood

Abstract

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.

Effects of urban density on carbon dioxide exchanges: Observations of dense urban, suburban and woodland areas of southern England

“…12 can be attributed to climatic differences, for example emissions in urban Montreal are relatively high given the vegetation fraction of the site due to fossil fuel combustion for building heating (Bergeron and Strachan, 2011), whilst the year-round warm climate in Singapore means emissions from buildings are limited to combustion for cooking (Velasco et al, 2013). The net exchange for the Swindon, Helsinki and Singapore sites are almost identical (Table 3).…”

“…In U the emissions remain high throughout the daytime, reflecting the intensity of activities. In low density residential areas, such as Swindon, rush-hour peaks are more typically observed, for example in Melbourne (Coutts et al, 2007), Montreal (Bergeron and Strachan, 2011), Helsinki (J€ arvi et al, 2012) and Mexico City (Velasco et al, 2013). Typical wintertime fluxes are approximately five times smaller at S (9 g C m À2 day À1 ) than U (50 g C m À2 day À1 ), whilst summertime fluxes are roughly ten times smaller at S (2 g C m À2 day À1 ) than U (22 g C m À2 day À1 ).…”

“…12, the vegetation fraction is a useful proxy for approximating carbon emissions from urban areas. Besides the plan area of vegetation cover, species and biomass may also be relevant (Velasco et al, 2013), for example evergreens have greater potential for carbon assimilation compared with deciduous trees (Falge et al, 2002). Variation in net ecosystem exchange among vegetated ecosystems is about 1 kg C m À2 yr À1 , ranging from small CO 2 release to CO 2 uptake approaching À0.9 kg C m À2 yr À1 (Falge et al, 2002;Soussana et al, 2007;Thomas et al, 2011;Wilkinson et al, 2012).…”

Estimated carbon dioxide exchange for three native species in an ecological reserve of Mexico City

“…It is estimated that the world's forests capture over 650 Mt of carbon, which helps to mitigate climate change and preserve biodiversity (FAO, 2010). However, not only forests sequestrate CO 2 ; it has been shown in several studies that the presence of trees in urban areas favors the reduction of some air pollutants and contributes to carbon sequestration (Yang et al, 2005;Kordowski and Kuttler, 2010;Velasco et al, 2013). There are several techniques for estimating carbon sequestration; one of them is through the use of technologies such as satellite imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS), whose purpose is to model carbon and water cycles globally.…”

“…Urban forests can contribute significantly to the reduction of air pollution by relieving the intensity of the heat island (Yang et al, 2005). Velasco et al (2013) mentioned that vegetation in an urban area contributes to the capture of CO 2 during the day and promotes local air cooling due to transpiration, serving as an alternative mitigation for climate change.…”