Do trees really mitigate climate change? We have heard so much about reducing carbon dioxide emissions by planting more trees, but does it really work?
Singapore, like any other urban city in the world, grapples with the problem of high carbon emissions.
But are the local authorities doing the right thing to reduce emissions and combat climate change? Many trees have been planted along expressways and roads. Green spaces now dot residential estates. But do these trees actually reduce CO2 in the atmosphere and offset the emissions from our cars and industries? Those are usually the reasons we cite for planting them, after all.
A recent joint study by scientists from the Singapore-MIT Alliance for Research and Technology (SMART) and the National University of Singapore (NUS) reveals that planting trees does not necessarily reduce carbon dioxide in the atmosphere.
The amount of carbon dioxide absorbed by a tree during photosynthesis is dependent on the tree’s biomass. In other words, it all depends on the size of the tree, says Dr. Erik Velasco, who led the study on the role of vegetation in the carbon dioxide flux from residential neighborhood of Singapore.
“Larger, woody trees can sequester – or take in – more carbon dioxide than smaller trees such as palm[s],” says Dr. Velasco, adding that green spaces in Singapore tend to be planted with a number of palm trees and smaller trees for aesthetic purposes.
Sometimes, woody trees are even replaced by these smaller trees.
In an effort to beautify our landscape, have authorities overlooked the environmental value of woody trees? Are aesthetics – which attracts tourists to our beautiful island – more important than our environment?
Green spaces may be pretty, but they may also hold only superficial value, one that does not extend beyond their being aesthetically pleasing.
To make things worse, these green spaces can act as emission sources instead!
Investigations carried out from two separate approaches, each of which involved measuring the carbon dioxide emissions and the absorption of carbon dioxide by type of vegetation using a micrometeorological flux tower, suggest that trees take in only eight percent of the total carbon dioxide emitted.
But when soil respiration is taken into account, net carbon dioxide emissions become positive, causing trees to become emission sources instead of a carbon dioxide sinks – otherwise known as ‘absorbers’.
Dr. Velasco’s research also found that large woody trees take in 67.1 percent of carbon dioxide, while palm trees take in 5.2 percent and turfgrass 25.2 percent.
“It shows that the type of tree we plant is very important,” he says. “We need to replace smaller, aesthetic trees with larger, woody trees to really mitigate climate change.”
“Large trees should not be replaced by young trees and palms, as is the tendency along secondary roads in Singapore,” he wrote in the conclusion of his paper, which was published on the 16th of October, 2013. His study was completed last year.
Although his two-and-a-half-year-long study cannot be said to be representative of the state of carbon emissions for the whole of Singapore, having only been conducted in the Telok Kurau area due to the homogenous residential terraces there that makes it easier to use the micrometeorological flux tower for measurement, it provides good insight into the issues with the type of vegetation being planted in Singapore, and raises good questions about whether the large variety of trees here really serve to combat climate change.
The data collected from Telok Kurau can also be compared to data collected from the urban district of Escandon in Mexican City, where the majority of trees are also evergreen. It’s been found that although Telok Kurau has nearly 270 more trees per square kilometre than Escandon (with 15 percent of green surfaces compared to only six percent in that Mexican neighbourhood), the greenery in Escandon acts as a sink, while the greenery in Telok Kurau acts as a source of carbon dioxide.
This is because woody trees in Escandon make up 97.5 percent of vegetation, while in Telok Kurau they only account for 64 percent. 36 percent are palm trees, which have a much smaller biomass compared to large woody trees. And having a smaller biomass means that they absorb significantly less carbon dioxide during photosynthesis than their larger counterparts.
Let us not take our garden city for granted, or take so much pride in being a ‘garden city’ that we forget the real value of greenery. Trees can be used to combat climate change, a problem many urban cities are facing. Urban areas are usually the main sources of carbon dioxide emissions, with more industrial activity, vehicle traffic and household consumption releasing large amounts of greenhouse gases into the atmosphere.
Singapore has a national per capita emission of 6.8 tonnes of carbon dioxide per year. Perhaps it’s time to revise the type of green spaces we have, and protect our environment and homes from the potential disasters that accompany global warming and climate change. And with public trees island-wide, the role of the authorities in this is crucial.
It just starts with planting the right type of tree.
Report prepared by Candice
Pictures are kindly provided by Singapore-MIT Alliance for Research and Technology