Thought Piece Series

March 2009

Urban Forest

By Debbie Tikao, Landscape Architect, Natural Habitats

In New Zealand, we tend to use the term urban forestry to describe tree populations within the urban environment. In other countries however, especially in the USA, the term also refers to a type of green infrastructure that aims to improve air quality and reduce the heat island effect within our cities through increased canopy cover.

There has been a significant amount of study undertaken which has measured, researched and analysed the beneficial environmental effects of trees within the urban environment. Additionally, there have been numerous studies on the economic and social benefits of an Urban Forest.

Even developers are beginning to embrace these benefits. Not so long ago getting developers to plant the bare minimum of trees, whatever that may have been, resulted in a heated discussion (to put it mildly!).

Interestingly enough, those bare minimum standards, set by some Councils', remain relatively unchanged despite this new knowledge (one street tree per 40m of road frontage does not conjure up images of an urban forest!). These minimum standards are particularly obvious in industrial areas, which is where I would have thought that we need far greater intensity of planting to reduce carbons.

This is not to say that Council won't support the idea of a more densely planted street environment or park, but there are still far too many streets across the city that are sparsely planted. Minimum standards need to change to reflect Council's vision for sustainable futures.

Our road reserves within the Auckland Isthmus (excluding roads) are 10 times greater in square area than our publicly owned open space (parks, reserves etc). Admittedly, these figures are my approximations only, but it serves the purpose of illustrating that this vast area of under utilised space could be our future urban gardens or urban forest.

Berms are a functional component of our street environment; they accommodate underground services, provide safe pedestrian footpaths, flat surfaces for rubbish collection and allow for visibility for motorists and pedestrians at intersections and driveways. Nowhere does it say that through careful species selection and design, they could not also provide a habitat for deep rooting ground covers, shrubs and native grasses. This would improve ground water recharge and more intensely planted street trees would increase canopy coverage. The pros of utilizing our berms to this effect would be enormous:

• Improve the amenity value and identity of residential and commercial areas
• Improve the quality of natural hydrologies
• Reduce stormwater run off
• Provide wildlife habitat
• Strengthen ecological corridors
• Improve air quality
• Contribute to urban cooling
• Reduce the use of fossil fuel in maintenance
• Bring nature and natural cycles back into our daily lives

The only con that comes to mind is the added maintenance and installation costs. The maintenance that would be required would be the ongoing care as well as the replacement or replanting of small shrubs and groundcovers if access was required to underground services. However, the long-term economic benefits (reduced infrastructure costs, reduced air pollution, increased land value, and reduced urban heating) would outweigh these costs.

Various scenarios can create an urban forest. Firstly, developer led, where the developer is encouraged to plant out entire berms with ground covers, native grasses and low growing shrubs. There are a number of small examples of this already, especially in Christchurch, and Auckland's North Shore. Additionally developers could be encouraged to plant appropriately sized trees either in clusters or at more intense centres.

Secondly, Council led, where the Council converts existing grassed berms into landscaped strips and intensifies street tree planting. The final scenario is Community led, where the Council provides guidelines and assistance for individuals to landscape their own berm. This latter scenario is presently operational within many Councils throughout Australia, and are referred to as the beautification of naturestrips.

There are a few simple principles I personally adopt in every project I am involved in (naturally to varying degrees):

• Increase diversity, the greater the diversity, the greater the ability for vegetation to sequester carbons
• Increase tree canopy cover and wildlife habitat by closer spacing and cluster plantings of smaller growing native tree species
• Select tree species which not only suit the situation but are ideally evergreen and have large leaf surface areas at maturity and/or sticky or hairy leaf characteristics, all of which improve a trees ability to absorb carbon
• Reduce lawn areas, replace with deeper rooting shrubs and grasses

Although not an arborist, I do know that trees are more likely to flourish when in the company of other trees and plants leading to cooler and cleaner the air. The better the soil structure and natural hydrology cycles, the healthier the tree. The tree is then less likely to fail which will reduce replacement and maintenance costs.

Naturally, this would improve amenity value, land value (trees appreciate in value) and social and environmental well-being.

It all makes perfect sense to me.

Natural Habitats is a leading New Zealand landscape design and build business

References / Links:

www.fs.fed.us

www.epa.gov

www.moreland.vic.gov.au

www.greenvalues.cnt.org