Green streets

As we confront a changing global climate there is constant, and well warranted, debate as to what we can do as humanity to promote a healthy climate situation. Central to the debate is the role of trees in reducing the pollution that contributes to climate change. Every so often misleading headlines to the effect that trees increase pollution levels crop up, but trees are certainly an overall pollution fighter and new research has shown that they might be more powerful in this regard than previously thought.

Plants do many things that theoretically benefit an urban environment. It is estimated that a mature leafy tree produces enough oxygen in one season to sustain ten humans for a year. Plants also fight noise pollution, provide windbreaks, offer shade, and stabilise the soil. There is also evidence that plants reduce air pollution.

Plants trap and hold particle pollutants (dust, ash, pollen and smoke) that can damage human lungs. They absorb carbon dioxide (CO2) to the extent that one acre of trees absorb CO2 equivalent to what is produced by driving your car 42,000 kilometres. Every now and then it is suggested that trees contribute to air pollution but that is actually a misunderstanding.

What actually happens is that many trees emit reactive molecules known as volatile organic compounds (VOCs), the most common of which is called isoprene. These VOCs are pollutants but they are so reactive that they quickly get consumed in the atmosphere, and some react with nitrogen oxides emitted from combustion engines to form longer-lived more stable organic nitrate compounds.

The other issue around plants is whether the amount of particulate pollution that they absorb is significant. Previous research has suggested that trees reduce pollution from particulate matter by only about five per cent. A new study though has suggested that this is an underestimate.

The study looked at the effects of grass, vines, and trees in an urban environment. Their analysis showed that at street level these plants can reduce nitrogen dioxide (NO2) levels by as much as 40 per cent and can reduce particulate matter by 60 per cent. The authors even suggested that building “green billboards” in cities to boost plant levels would be worthwhile. Certainly a green billboard would be more edifying than another ad suggesting that beer drinking will build your circle of friends or that a triple beef burger combined with deep fried potato and a fructose sweetened fizzy drink somehow equals a “happy” dining experience.