Urban forests

Development and urban intensification add to transport-related greenhouse gas emissions by increased traffic volumes and contribute to rises in urban heating and stormwater runoff by increased amounts of hard impermeable surface.

Production of oxygen

Trees use light energy from the sun to convert carbon dioxide and water into glucose and oxygen.

This process is called photosynthesis. Oxygen is released into the atmosphere through the leaves.

Reduction of air pollution and greenhouse gas emissions

Through the process of photosynthesis, trees remove carbon dioxide, nitrous oxides, sulphur dioxide, carbon monoxide and ozone from the atmosphere.

Trees remove carbon from the atmosphere and store it in their leaves, branches, roots and trunks.

Trees also act as filters to remove particulate matter from the atmosphere.

Particulates that are greater than 2.5 microns and smaller than 10 microns in diameter (PM2.5) are easily inhaled and cause damage to the heart and lungs.

Trees can remove particulate matter from the atmosphere by capturing it on their leaves, contributing to the quality of the environment and public health. 

Cooling hard surfaces and waterways

Large urban areas often have higher temperatures than their surrounding rural areas.

This is known as the urban heat island effect and is a recognised problem worldwide. Urban heat islands result from a complex built environment with a high density of human activities.

Trees cool the city and restrict unwanted weed growth in waterways by intercepting ultraviolet rays and transpiring water from their leaves. 

Stormwater and erosion control

Trees manage stormwater flows through their canopies and root systems.

The canopies intercept rainfall, and root systems act like sponges by soaking up water which is then taken up to be used by the tree during its growing processes. The bigger and healthier the root systems the more stormwater is managed.

Healthy trees help reduce the nitrogen, phosphorus and heavy metal content in stormwater. Numerous studies have been undertaken to evaluate the potential for using street trees as elements of a stormwater system.  

Trees also reduce erosion by protecting soils from the impact of heavy rain, binding soil on slopes and river and stream margins, and up-taking and processing excess ground moisture.

Habitat for flora and fauna and increased biodiversity

Trees provide shelter and food for a variety of birds and small animals through their flowers, fruits, leaves, buds, and woody parts. Bacteria and fungi invade the trees causing decay pockets which create nesting sites for some birds and small animals, including native bats.

Decomposing leaves, twigs and non-woody roots increase soil fertility and structure for soil-borne organisms including bacteria, fungi, protozoa, nematodes, arthropods and earthworms.

Trees also provide habitats for other plant life. This includes parasitic plants that live directly off the tree by feeding on it or epiphytes which use the tree as support.

Trees can also create a protective environment that allows the growth of plants that would otherwise not be there e.g. shade dwelling or frost tender plants. Conversely, some trees can also discourage other trees and plants from establishing in close proximity by creating too much shade, using up available nutrients or they may have allopathic qualities.

Trees contribute to overall plant biodiversity as well as indigenous and endemic biodiversity.

Quantifying the economic benefits that trees and other urban forest components deliver can provide sound arguments for deciding whether trees are planted and maintenance budgets are allocated to appropriately manage them.  Some of the economic benefits of an urban forest include:

A measurable return on investment

Trees:

• Store carbon
• Intercept stormwater
• Remove air pollution 
• Provide energy savings.

Extending the life of roads

Asphalt streets contain a mix of aggregate, binder, and asphalt cement, which is laid on top of compacted material.

As surface temperature rises, the binder evaporates and breaks down and the surface begins to harden. Surface hardening makes it easier for cracks to form. If not treated small cracks lead to big cracks and allow water to penetrate and weaken the compacted base material beneath.

As the cost of constructing and repairing roads increases, there is a need to protect the City’s investment. Better surface performance translates into reduced maintenance and repair costs and therefore reduces total life cycle costs.

Large shade trees reduced maintenance costs over time when compared to maintenance costs for unshaded road surfaces. Planting large growing trees to create shade also reduces planting costs by up to 50% as fewer trees are required to achieve the desired outcome.

Increased property values

Trees add beauty and character to urban areas by adding colour through their shapes, flowers and foliage, softening hard surfaces, providing shade, shelter and birdlife and generally contributing to the liveability of an area.

Healthy trees planted in streets enhance neighbourhood aesthetics and have been proven to increase property values. It is estimated that properties in tree-lined streets are valued around 30% higher than those in streets without trees.

Increased patronage and spending in retail and commercial areas

Studies overseas have demonstrated that a large and well cared for tree canopy within retail and commercial areas promotes positive consumer perceptions and behaviour.

Consumers surveyed in these studies say that they are prepared to travel greater distances to visit a well treed retail or commercial area and are willing to pay more for goods and services than they would otherwise pay at areas that do not have a mature, high-quality tree canopy.

Supporting tourism

Christchurch is the hub of tourism in the South Island. Visitors are drawn by our Garden City identity. Research shows that ‘The Garden City’ fits with Christchurch’s product and is the most recognised City identity in New Zealand.

The Garden City identity increases our ability to attract business investment as well as visitors, and increase the ‘sense of place’ for residents. Trees in Christchurch support and enhance the Garden City identity. They help promote Christchurch as a destination and in doing this support tourism.

Mental wellness

There is a large amount of evidence that now demonstrates that exposure to nature in the form of trees, grass, and flowers offers positive interactions that generate extensive physiological, social, and mental wellness benefits that can help alleviate the burden on national health systems.

In some studies it has been suggested that a view of green space, including trees, can also encourage hospital patient recoveries, reducing the amount of time spent in hospital.

Views of green environments also have an impact on workplace productivity. One study found that workers who did not have a nature view from their desks had 23% more sick leave than those workers who had views of nature.

Wood products, renewable energy

Because trees are woody biomass they provide an affordable source of renewable energy when harvested. Trees grown for production forestry are sold for export as logs, pulp or processed timber or can be used for carbon credits.

Trees also provide a source for housing materials, furniture, fences, garden structures and ornaments.

Urban trees and vegetation have many positive benefits for the community by providing and allowing daily interaction with nature. Specific benefits include:

Neighbourhood character and the Garden City identity

Trees enhance neighbourhood character and identity and the Garden City identity through seasonal colour changes, different shapes, forms, patterns, textures, flowers and seeds.

Trees contribute to the liveability of Christchurch by naturalising and humanising built environments through softening hard surfaces and harsh outlines of buildings, complementing building development, and screening unsightly and undesirable views.

Shade and shelter

Trees provide thermal comfort to pedestrians and cyclists by blocking harmful UV rays.

It is estimated that a person standing in direct sunlight takes 20 minutes to burn however when standing underneath a tree that provides 50% dappled shade the burn time increases to 50 minutes with a further increase in burn time to 100 minutes for a tree that casts full shade.

Trees also provide shelter from rain and wind.

Traffic safety

In the past trees have been viewed as traffic hazards due to their close proximity to the road and their immovability. There have been calls to only plant trees that are frangible – trees that, when mature, only grow to around 100mm in diameter so they will break if hit by a vehicle.

More recently trees and roadside landscaping have come to be seen as aiding traffic and pedestrian safety:

• psychological traffic control by reducing driver speed through defining the road edge and narrowing the road
• improving driver vision by blocking glare from the sun
• providing a barrier between traffic and pedestrians
• decrease in driver response to stress (Dixon and Wolf 2007)
• forewarning of different surroundings and identifying upcoming features e.g. pedestrian refuges.

Cultural, food production and medicinal use

Throughout the ages, trees have fulfilled a number of needs. Trees have been used as sources for food, medicines, toys, cooking utensils, carving, arts and crafts, tools, transport (ships, canoes, and wagons), paddles, dyes, weapons, building materials.

Trees have also been used as symbols or to commemorate historic occasions, special events or people.

Trees have also been used as way finders to guide travellers and as meeting places.

Trees also provide a source for housing materials, furniture, fences, garden structures and ornaments.