The role of vegetation in the CO<sub>2</sub> 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. Negative daytime CO₂ fluxes, for example have been observed during the growing season at suburban sites characterized by abundant vegetation and low population density. 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 biogenic flux. 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 captures 8% of the total emitted CO₂ in the residential neighbourhood studied. A net uptake of 1.4 ton km⁻² day⁻¹ (510 ton km⁻² yr⁻¹ ) was estimated from the difference between the daily CO₂ uptake by photosynthesis (3.95 ton km⁻² ) and release by respiration (2.55 ton km⁻²). The study shows the importance of urban vegetation at the local scale for climate change mitigation in the tropics

The role of urban green infrastructure in offsetting carbon emissions in 35 major Chinese cities: A nationwide estimate

“…The estimated sequestration rate of this study is lower than the sequestration rate (3.06 t/ha/year) of urban trees in the United States (Nowak et al, 2013), which might be mainly due to that the averages of trees, shrubs and grass were applied in this study, while only trees were considered for urban forests in the United States (trees typically sequesters carbon at higher rates than shrubs and grass). Additionally, the carbon sequestration rate of China's urban green infrastructure is lower than Gainesville in Florida (2.5 t/ha/year, according to Timilsina, Staudhammer, Escobedo, & Lawrence, 2014) and Vancouver in Canada (2.8 t/ha/year, according to Christen et al, 2011), but higher than Singapore (1.4 t/ha/year, Vesasco et al, 2013) and Seoul in Korea (0.5-0.8 t/ha/year, according to Jo, 2002), which might be associated with the differences in climatic factor, plant density and size, growing condition, and management approaches.…”

Anthropogenic and Biogenic Features of Long-Term Measured CO2 Flux in North Downtown Houston, Texas

“…11 is likely dominated by human respiration in this area presuming that (i) traffic emissions are very low at nighttime, (ii) industrial emissions are near absent during weekend nights, and (iii) ecosystem respiration drops to very low values near freezing. The fCO 2 from human exhalation as observed at the tower likely changes with human activity, building occupancy, and time of day, but an average 200 to 280 μmol s −1 person −1 during nighttime has been estimated (Nemitz et al, 2002; Velasco et al, 2013), with an up to three times higher daytime value. Under the assumption that the total population in our study domain (9 km 2 ) was equally distributed, we used the latest, zip‐code based US Census data to estimate a population density of 16,026 persons km −2 .…”

“…Flux nonstationarity was evaluated via a common stationarity test by separating the 30‐min data period into six equal 5‐min intervals (Foken and Wichura, 1996). Although a criterion of <30% difference between period and total standard deviations has typically been used to determine the stationarity over near homogeneous surfaces, a more relaxed criterion of 60% was used considering the urban heterogeneity (Velasco et al, 2013). In addition, a friction velocity threshold of u * ≥0.2 m s −1 was also applied to account for low turbulence conditions.…”

“…This approach has been applied to a number of urban areas in North America, including Chicago, IL (Grimmond et al, 2002), Baltimore, MD (Crawford et al, 2011) and Montreal, Canada (Bergeron and Strachan, 2011), and in Europe in the cities of Marseille, France (Grimmond et al, 2004), Helsinki, Finland (Vesala et al, 2008a), Edinburgh, UK (Nemitz et al, 2002), London, UK (Kotthaus and Grimmond, 2012) and Basel, Switzerland (Vogt et al, 2006). Other urban areas include Mexico City, Mexico (Velasco et al, 2005) and Singapore (Velasco et al, 2013). …”

“…They found summer time fluxes to be lower than during winter, mainly due to the greater vegetation influence in suburban areas such as Melbourne, Australia (Coutts et al, 2007), Montreal (Bergeron and Strachan, 2011), Essen, Germany (Kordowski and Kuttler, 2010), and Tokyo, Japan (Moriwaki and Kanda, 2004). There have been efforts to estimate the agreement between measured fluxes and the bottom‐up approach conducted for a residential area in Singapore (Velasco et al, 2013) and for central London (Helfter et al, 2011), with discrepancy <2% with a local scale and <3% with a regional emission inventory, respectively.…”