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Trees and the Public Realm - Westminster City Council

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<strong>Trees</strong> <strong>and</strong> <strong>the</strong> <strong>Public</strong> <strong>Realm</strong> (Draft) Page 11<br />

3. THE VALUE OF TREES<br />

12. <strong>Trees</strong> in cities provide a range of tangible benefits which have considerable beneficial<br />

impacts on <strong>the</strong> lives of those who live <strong>and</strong> work in cities but do not have daily access to<br />

o<strong>the</strong>r more traditional types of open space. There are also a number of less obvious<br />

benefits that can sometimes be difficult to quantify, such as social <strong>and</strong> economic<br />

benefits. However, as is emphasised throughout this document, trees cannot be treated<br />

in isolation <strong>and</strong> should be carefully considered in <strong>the</strong>ir context.<br />

Environmental benefits<br />

13. Heavy traffic, commercial <strong>and</strong> domestic heating systems <strong>and</strong> o<strong>the</strong>r background<br />

sources all contribute to poor air quality in <strong>Westminster</strong>. Concentrations of particulate<br />

matter <strong>and</strong> nitrogen dioxide currently exceed air quality st<strong>and</strong>ards across <strong>the</strong> city. Whilst<br />

<strong>the</strong>re is still a degree of uncertainty over <strong>the</strong>ir interaction with pollution, trees absorb <strong>and</strong><br />

filter gaseous pollutants (ozone, sulphur dioxide, carbon monoxide, nitrogen dioxide)<br />

through <strong>the</strong>ir leaves. The physical properties of trees can also have a direct influence on<br />

pollution levels in <strong>the</strong> area <strong>the</strong>y are planted, for instance through acting as windbreaks<br />

<strong>and</strong> reducing dispersion 1 .<br />

14. <strong>Trees</strong> can also have a positive effect on <strong>the</strong> environment by:<br />

• Cooling <strong>the</strong> city <strong>and</strong> benefiting micro-climate <strong>and</strong> humidity. The shading<br />

properties of trees will become increasingly important in light of <strong>the</strong> trends<br />

shown in recent UK climate change predictions.<br />

• Sequestering (or temporarily holding) carbon dioxide, <strong>the</strong> main greenhouse gas<br />

(although, in terms of trees in towns, this role is limited).<br />

• Trapping dust <strong>and</strong> particulate matters on all aerial parts, which are <strong>the</strong>n washed<br />

away by rainfall. Conifers are <strong>the</strong> most effective in capturing particulates,<br />

followed by deciduous trees with coarse, hairy leaves 2 .<br />

• Producing oxygen as a bi-product of photosyn<strong>the</strong>sis.<br />

• Potentially reducing localised extremes in temperatures (cooling in <strong>the</strong> summer,<br />

warming in <strong>the</strong> winter – countering urban heat isl<strong>and</strong> effects), although in<br />

certain locations <strong>the</strong>y can actually trap heat <strong>and</strong> reduce breeze.<br />

• Reducing <strong>the</strong> effects of flash flooding.<br />

• Providing habitats for a broad range of wildlife in <strong>the</strong>ir own right, or by<br />

contributing to habitat.<br />

1 In spite of <strong>the</strong>ir absorbing/filtering effects, <strong>the</strong> presence of trees near roads can sometimes lead to<br />

higher concentrations, tipping <strong>the</strong> scales due to <strong>the</strong>ir damping of wind speed. It is <strong>the</strong>refore important<br />

not to allow too much of a ‘tunnel’ to be created by mature trees along busy roads, <strong>and</strong> to plant trees<br />

with lower foliage density in those locations.<br />

2 Although some trees, like <strong>the</strong> London Plane, do produce <strong>the</strong>ir own ‘particulates’ at times of <strong>the</strong> year,<br />

which can cause allergic or uncomfortable reactions in some people.

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