NSW residents are currently waiting for the state government to respond to the deluge of submissions opposing the new draft biodiversity and land clearing laws. The application of this legislation to Sydney’s bushland is unknown at present.

Urban Forests

Essentially there is a conflict between the need to accommodate the expectation of continuing high population growth and the desire to maintain a desirable city to live in. The high population growth is totally undesirable but we have had plenty to say about that before.

Now turning to the ideal of having liveable cities, one of the major elements is adequate tree cover. This is not impossible to achieve if there is the political will to get the planning right. Given the attitudes of the current NSW government to trees, residents will need to make it loud and clear that they will not accept a continuation of current policies.

In North Sydney Council’s Urban Forest Strategy document 2011, urban forest is defined as:

… the totality of trees and shrubs on all public and private land in and around urban areas and is measured as a canopy cover percentage of the total area. The canopy cover may vary in density depending on the vegetation type (eg almost solid cover under rainforest vegetation to more open cover under woodland or eucalypt forests) however the canopy cover percentage for urban forest measurement purely measures the percentage of land that has tree or shrub vegetation (over 3 m tall) above it, regardless of density.

Benefits of Urban Forests

Reduction in the Urban Heat Island Effect

Heat from the atmosphere is used in vegetation transpiration processes which convert water from leaves to water vapour. This has a cooling effect similar to that when humans perspire. Trees can transpire significant volumes of water and it has been estimated that a mature tree can transpire up to 150 L/day. In a hot dry location this produces a cooling effect similar to that of two air conditioners running for 20 h.

Much of the urban landscape is paved and devoid of vegetation. This means that there is usually little water available for evaporation, so most available natural energy is used to warm surfaces. Construction materials are dense, and many – particularly dark-coloured surfaces like asphalt – are good at absorbing and storing solar radiation.

Trees are a very effective means of blocking the sun’s radiation and, depending on the species and its maturity, up to 95% of the incoming radiation can be blocked. Trees can reduce a building’s temperature by directly blocking radiation through windows and cooling the surrounding air, and can also keep the soil cool thus providing a sink for heat from the building. These effects have been quantified as reducing air temperatures in built up areas by 1 to 5°C.

Reduction in Energy Demand

A typical adaptation response to the lack of external cooling is reliance on air conditioning, which generates waste heat and contributes to greenhouse gas emissions. Studies in the US have indicated that 5 to 10% of electricity demand in cities is from air conditioners used to compensate urban heat island effects. Increases in peak energy demand drive up infrastructure costs and increase the risk of power cuts during heatwaves. Air conditioner use amounts to nearly one-third of the power consumed in Perth on the hottest days in February and March.

Other Benefits

• Reduction in water run-off and erosion during storm events
• Improvement in water quality as the slowing of water run-off allows particulate matter to precipitate out and not pollute waterways
• Wind mitigation
• Improvement in air quality by absorbing gaseous pollutants and trapping particulate matter on their leaves
• Storage of carbon
• Reduction in noise pollution
• Shade for parked cars and outdoor pursuits
• Visual amenity and privacy
• Improved property values
• Attraction of birds and habitat for other wildlife

Recommended Canopy Cover

There have been many international studies of recommended canopy cover targets for urban forest. For example, according to the North Sydney report, for this climatic zone the recommended percentages for specific land-use areas are:

• 15% cover in central business districts
• 25% cover in medium and high density residential areas
• 50% cover in low density residential areas

Many cities now have urban forest targets in place, for example:

• Sydney city aims to increase urban forest cover (including private land) by 50% by 2030 and 75% by 2050
• Melbourne aims to increase canopy cover from 22 to 40% by 2040
• London aims to increase urban tree cover from the current 25% to 32% by 2065
• In 2015 New York City achieved a goal of planting one million trees to increase its urban forest by 20%
• Australian government has pledged to develop decade-by-decade goals out to 2050 for increased overall tree coverage

In 2014 the Institute for Sustainable Futures documented the tree cover in various parts of Sydney: Hornsby (59%), Ku-ring-gai (52%), Blacktown (19%), Parramatta (23%), City of Sydney (15%) and Rockdale (12%).

The figures for Hornsby and Ku-ring-gai are distorted by the inclusion of national parks that can provide some regional benefits such as improved air quality and cooling breezes if the wind is blowing in the right direction. However they do not provide the same benefits associated with trees in the immediate vicinity.

Stumbling Blocks

There are many barriers to achieving and maintaining urban forest. Generally the larger the trees the more effective are the benefits listed above. However there is a cost with managing larger trees. Trees ultimately need to be replaced when they reach their useful life expectancy. In an urban environment one cannot allow trees to reach the age where they become dangerous. There is also the need to achieve access for solar panels.

Developers will argue that the land is worth much more if it is built on. As we have seen all too often in Sydney, house blocks that had room for gardens, now have high and medium density housing with minimal, if any, tree cover remaining. SEPP65, the planning policy for high-rise is supposed to provide for deep soil planting but it does not seem to be enforced.

SEPP65, the planning policy for apartments covering all of NSW, now has a minimum deep soil requirement of only 7% of site area (Apartment Design Guide, p61). This is totally inadequate for a high rainfall area such as the North Shore where the native trees can reach heights of 30 m or more. Ku-ring-gai Council requires a minimum 50% site area for deep soil landscaping on sites over 1800 m².  This should be maintained and not over-ridden by SEPP65 Apartment Design Guide requirements.

Tiny backyards in new fringe areas and boundary-to-boundary development of new houses in existing suburbs often eliminate private land’s potential contribution to the urban forest. This puts incredible pressure on the public realm to provide the urban forest on top of needing to meet demands for infrastructure.

Compact city land-use policies and urban forest policies need to work together to ensure that cities have high-quality built environments and extensive tree cover. These policies must set the overall goals and outline strategies to deliver on them for both public and private land.