By Brendan Moyle 05/06/2019


June 5 is the UN Environment Day with the theme this year, being air pollution. Air pollution comprises a number of gases and microscopic particulates that impact on ecosystems and human health. Both the quality of life and human longevity is impacted by these pollutants, often in urban areas. One only has to think of news stories from London or Beijing to realise human health suffers. (Header photo was taken by me in Beijing on a day when we were warned it was unsafe for anyone to be outside).

Urban Forests

One of the important tools we have to beat air pollution is Urban Forests. I’ve mentioned these before in connection to biodiversity conservation. Urban forests however, are also one way air pollution can be reduced.

An Auckland Urban Forest

Forests are more than trees. They include all the associated shrubs, ferns and wildlife. The trees however perform several important functions.

First, they filter the air. Their leaves have tiny pores (stomata) that are used for gas exchange in photosynthesis. Carbon dioxide goes in, water vapour and oxygen comes out. This flow of gases also pulls in many local pollutants- carbon monoxide, nitrous dioxide, ozone and sulphur dioxide. The scale of this is significant. For instance, in a 2017 study of Barcelona- a city not known for an extensive urban forest network- over 300 tons of these gases were removed from city’s air annually by their forests.

Second, their bark traps particulates. The particulates in the air settle on the trunks and branches of the trees.

They also sequester carbon. The same study of Barcelona estimated their forests sequestered over 5000 tonnes of carbon.

Managing Urban Forests

Managing urban forests however, is difficult. One of the big problems is the different ownership structures. Some forests in a city will be owned and managed by the city itself, often in reserves or public parks. Others can be found in schools or hospitals or universities. Then many elements of the urban forest are managed by households. This matrix of ownership type is one challenge.

The second is that urban forests are not typically managed for conservation or pollution. Trees are maintained in public parks for recreation. Households plant trees for fruit, for shade or privacy, or for aesthetic reasons. Many of the external benefits generated by these trees aren’t compensated. Households or landowners in effect, provide air purification (and similar services) for no pay. But the cost of managing these trees fall on the household. And trees also generate negatives. The pollen causes allergies. The fallen leaves clog gutters. The shade of the branches deprive rooms of sunlight. This sets up a ready conflict between public policy and households.

Another complication is that the benefits of trees vary a great deal by species. Some trees and plants are considered noxious in New Zealand and their (eventual) removal is desired. For others benefits vary. For instance, the ability to trap and hold water on the trunk of a tree (to moderate local flooding) depends partly on bark. Rough bark can capture and hold more water during heavy rain than smooth trunks. All trees are not equal.

The Way Forward

At the moment urban forests are maintained by a mixture of public provision, regulation of landowners and the goodwill of households. This means city planners do have to take into account all the benefits of the trees in the area, including pollution control. To date we have been very timid at applying economic incentives to households or others to encourage growth of our urban forests. Perhaps it is time to get bolder. And as voluntary action has very low compliance and enforcement costs, persuading households and landowners to voluntarily plant trees that are efficient at reducing pollution can be attempted.