Street Plaza Trees

About this pattern

Protecting, maintaining and growing street trees requires a long-term commitment. With care, trees should be long-lived, healthy and provide increasing shade to enable cool streets. The attention to good growing conditions and on-going tree management that maintains shade-giving canopies suggests that installing and caring for street trees should be done in unison with urban water management. (See the patterns MANAGING ON-SITE WATER & CARING FOR TREES).

Growing conditions – soils

The designated area for street trees is restricted requiring site and technical planning for adequate soil volume.  Soil depth is important, but more essential is the soil area for roots to spread and access oxygen. Because of competing users in streets – pedestrians, services, vehicles – the maximum area for lateral root growth can be achieved through porous paving and grass from property boundaries to kerbs.

In paved plazas, the use of Structural Soil Systems (SSS) composed of a two-part system of cellular modules for strength and structural support (load bearing) and filler soil can enable tree growth. In this way, structural stability is provided, plus interconnected voids for root penetration, air and water movement.

Growing conditions – water

Water-sensitive urban design can ensure that water reaches the roots of the trees; this includes redirecting kerb water through spaced inlets into the tree pits.

Tree Selection

• Ten criteria are relevant to street trees in urban environments:
• Drought & Heat tolerance are increasingly important in UHI. Isolated trees can experience high heat and radiation loads in urban areas, increasing tree water stress; plant trees in groups to mitigate this.
• Wind tolerance is relevant as urban trees have to withstand turbulence.
• Tree longevity is reduced in urban environments.
• Pollution tolerance: the effects of pollutants on trees can cause the tree to weaken and die. Most pollutants to trees are airborne, including oxidants, sulphur dioxide and carbon monoxide; sunlight reacts with oxidants to form tree pollutants such as ozone.
• Pathogen and pest susceptibility and manageability
• Potential as allergens to humans and animals: of the 50,000 different kinds of trees, less than 100 have been shown to cause allergies; most allergies are specific to one type of tree or to the male cultivar of certain trees.
• Shade cast (see the pattern SELECTING SHADE TREES)
• Maintenance required; this is generally higher in a younger tree in order to attain the form needed to suit site constraints.
• Tree litter: all trees will shed litter, leaves, bark, flowers or fruit at some time during a given growing season. Plane trees drop hard fruit which can be a pedestrian hazard.

Size Matters

One strategically located large-stature tree more impact on conserving energy and mitigating the urban heat island effect than a corresponding quantity of smaller trees. Larger trees do more to:

• Reduce stormwater run-off. 
• Improve local air, soil and water quality.
• Reduce atmospheric carbon dioxide.
• Provide wildlife habitat.
• Enhance the attractiveness of an area.
• Promote human health and well-being.

The bigger the tree, the larger the benefits and, ultimately, the better the community’s quality of life.

Siting

Shade trees in public open space can be situated beside paths, around gathering spaces and can provide shading for play equipment. The canopy shape determines most effective shading (see the pattern SELECTING SHADE TREES).

Street Tree Considerations **

• A shade rating greater than moderate to heavy:
• Requires high maintenance; requires formative pruning; has low litter drop;
• The minimum height clearance required under canopy is 4.6 metres on roads and 2.5 metres on footpaths.

Pattern Conditions

Enablers:

• Landcom (2008) has Street Tree Design Guidelines for Landcom Projects.
• Establishing an “Urban Forest Stewardship” group (www.treefolk.org/ufs) where residents care for trees through local civic groups and social networks.
• City of Sydney & City of Melbourne both have guidelines and technical guides for street trees. City of Sydney Urban Forest Strategy sets targets to increase average total canopy cover by 50% by 2030 and 75% by 2050, meaning that canopy cover percentages should increase from the current 15.5% coverage to 23% in 2030 and 27% in 2050.
• Local Government is responsible for trees in parks; Roads, Maritime Services are responsible for street trees; AUSGRID is responsible for pruning street trees.

Constraints:

• Financial costs of urban tree management
• Environmental hazards and  accident hazards due to branch drops and fallen trees

Commoning Concerns

Access: Unrestricted. 

Use: Shade, beautification, gateways.

Benefit: Biodiversity protection provides important habitat and corridors for birds and animals; heat mitigation through transpiration, shade and passive cooling; energy efficiency by reducing the need for air conditioning in offices and homes; improved air quality; water absorption captures stormwater, reduces peak flows and improves water quality; noise pollution reduction.

Care: First 5 years Landcom and community, after 5 years Local Government.

Responsibility: First 5 years Landcom and community, after 5 years Local Government.

Ownership: First 5 years Landcom and community, after 5 years Local Government.