The ‘green cities’ movement has argued that nature should be allowed to permeate the built environment, seeing nature in the city as a necessity rather than just a luxury11. This is because biodiversity can deliver important environmental, social and economic benefits, something recognised by the government’s promotion of biodiversity in community strategies.

Ecological Services
Vegetation can deliver a range of important ecological services and later in this guide we highlight examples of how this can be achieved:

• Carbon sink – Trees have a significant capacity to absorb carbon dioxide. 1 hectare of woodland can absorb emissions equivalent to 100 family cars.
  
• Pollution control – Vegetation has a significant capacity to attenuate noise and filter air pollution from motor vehicles. Street trees can remove sulphur dioxide and reduce particulates by up to 75%. Noise attenuation can be as much as 30 dB per 100 metres. Wetland ecosystems are also effective in filtering polluted run-off and sewage.
  
• Air conditioning – In urban areas the heat island effect can increase temperatures relative to open countryside by up to 5 degrees celcius. Vegetation provides natural air conditioning. A single large tree can be equivalent to five room air conditioners and will supply enough oxygen for ten people.
  
• Microclimate control – Vegetation can improve local microclimate conditions by providing shade in summer. It can also reduce wind effects created by streets and wind loads on buildings, potentially reducing heating requirements by up to 25%.
  
• Flood prevention – Vegetation can reduce excessive run-off and increase rainfall capture. This reduces the risk of flooding in low lying areas and can also recharge soil moisture and groundwater.

There is a strong case for biodiversity to be designed into new developments to offset climate change effects, thereby improving their liveability.

Quality of Life
Natural greenspaces can deliver a range of important social benefits, improving the quality of life for urban citizens and making higher density housing more attractive and liveable.

• Health and wellbeing - Since the creation of the first public parks in the 19th century planners have recognised nature’s importance in improving peoples quality of life. Accessible green space creates opportunities for recreation and exercise, and studies have shown that it increases children’s creative play, social skills and concentration span. Natural greenspaces reduce stress and encourage relaxation, providing a sense of freedom and exhilaration.
  
• Social cohesion - Natural greenspaces can encourage greater social interaction. This more active use of greenspaces, including streets and communal spaces, can contribute to a more lively public realm - a key urban design objective. Participation in the design and stewardship of green space can help strengthen communities. Nature reserves can create a focal point for life-long learning about nature (see Section 1.3).

Economic Value
Natural greenspaces can increase property values, reduce management overheads, and reduce healthcare costs.

• Property values – Street trees and views of natural landscapes and waterways can increase property values by between 6% and 18%, as well as helping to sustain values over the long-term and improving the image of difficult to develop brownfield sites, as demonstrated by Greenwich Millennium Village’s ecology park. Shoppers may also be willing to pay up to 10% more to shop in tree-lined streets.
  
• Management costs – Traditionally our greenspaces have been intensively managed, requiring significant and costly inputs of nutrients, herbicides and pesticides. Experience has shown that ecologically self-sustaining landscapes can significantly reduce the need for these inputs.