Environmental benefits

Biodiversity

Trees are an important wildlife habitat. They provide nesting sites for birds and support a wide range of insects that are an important food source for birds and other wildlife.  A single oak tree can, in fact, support over 280 different species of wildlife.  In an urban setting, linear corridors of habitat are among the most important, connecting otherwise isolated areas to each other and out to the rural surroundings. Trees and other vegetation along highways, waterways and railways are particularly important to wildlife in this respect.

Pollution interception

Research undertaken in the West Midlands by Lancaster University (Hewitt et al, undated) has established that trees can remove a number of pollutants from the atmosphere, including ozone, nitrogen dioxide and particles.  However, trees also produce volatile organic compounds, VOCs, which in combination with some man made pollutants can lead to an increase in ozone, particulates and other pollutants.  Different species of tree have different net effects on air quality.  Willows, poplars and oaks can potentially worsen air quality during hot weather, whilst ash, alder and birch have amongst the greatest beneficial effects.

Carbon sequestration

It is well known that trees, in common with all vegetation, absorb carbon dioxide (one of the principal greenhouse gases) and release oxygen during the process of photosynthesis. The carbon absorbed by trees in this process is stored in the wood. Whilst this most well known of benefits is real it seems it is often overstated. Never the less, trees do have an important role to play in reducing the effects of greenhouse gases, not only through carbon sequestration but perhaps more importantly through the effects that careful planting can have on fuel use.

Energy use

Careful tree planting can reduce the amount of fuel used on both heating and cooling buildings. A considerable amount of research has been undertaken to quantify this in the United States, but little such research has been undertaken in the UK. Clearly differences in climate mean that figures here can not be directly related to any part of the USA. Trees provide shelter and reduce wind speed, thus reducing heat loss from buildings during winter. They also provide shade in the summer, whilst the evapo-transpiration of water from the leaf surface has a general cooling effect on surrounding air. This can significantly reduce the need for air conditioning during hot weather.

Noise reduction

Trees and other vegetation can play an important role in attenuating noise through reflecting and absorbing sound energy. One estimate suggests that 7db noise reduction is achieved for every 33m of forest (Coder, 1996) whilst other reported field tests show apparent loudness reduced by 50% by wide belts of trees and soft ground (Dwyer et al, 1992).

Hydrology

Trees have a number of hydrological effects. These include reducing erosion and improving water quality through interception of pollution. Perhaps the most important effect in Britain at present, given the trend for increasing winter flooding, is the reduction in ground water runoff. One study has estimated that for every 5% increase in tree cover area, run-off is reduced by 2% (Coder, 1996).