Urban greenery is widely seen as a vital measure to cut carbon emissions, but if the wrong trees are planted, the carbon emitted from the soil might be more than what the trees absorb, a new study has found.
Case studies of Telok Kurau and Escandon in Mexico City found that vegetation in the Singapore neighbourhood, which is made up of 64 per cent woody trees and 36 per cent palms and other plants, absorbed 8 per cent of the area’s carbon emissions. But this is outstripped by the carbon emissions from the soil there (12 per cent of the area’s emissions).
In contrast, in Escandon, which has almost entirely woody trees (97.5 per cent), the 0.6 per cent of emissions from the soil is offset by the 2 per cent of emissions absorbed by the greenery. This is despite the fact that it has fewer trees in a given area and a smaller percentage of green surface than in Telok Kurau.
In their research paper on Telok Kurau published last month, urban air-pollution expert Erik Velasco and his co-authors wrote that tree-planting programmes must “consider the growth rate and potential carbon uptake when selecting type and species”. Priority should be given to woody trees and “large trees should not be replaced by young trees and palms, as it is the tendency along secondary roads in Singapore”. The study on Telok Kurau is the first of its kind for a tropical city.
Apart from picking woody trees over palms and ornamental plants — which take in less carbon — urban planners should also consider soil respiration in efforts to mitigate greenhouse gas emissions, said Dr Velasco, a research scientist at the Singapore-MIT Alliance for Research and Technology’s Centre for Environmental Sensing and Modelling.
“We could think that if we cover everything with pavement, we are not going to have emissions from soil (through roots and microbial activity), and it’s true. But then, we are not going to have trees and this is going to have a strong impact on the total carbon cycle.” He added that cities play a crucial role in efforts to mitigate carbon emissions.
To address concerns of accidents caused by tree branches snapping, Dr Velasco said, at the sidelines of a National University of Singapore workshop on climate change challenges yesterday, that more slow-growing native species could be planted.
Telok Kurau was chosen because a low-rise neighbourhood was needed to operate equipment that measures carbon-dioxide flux in the atmosphere. The neighbourhood emitted 6,502 tonnes per square kilometre of carbon dioxide per year, with the bulk (72 per cent) coming from vehicles. Human respiration contributed 17 per cent and the rest came from households (7 per cent), and vegetation and soil (4 per cent).
The findings have yet to be shared with the authorities as the research paper has just been published, said Dr Velasco. Similar studies for other cities in the region could be done, and he is looking to continue research on soil respiration and flux in fine particulate matter and other greenhouse gases in Telok Kurau.
