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Why should successful urban tree planting matter to engineers? 

31 January 2021

To mark the upcoming Trees, People and the Built Environment 4 conference hosted by the Institute of Chartered Foresters, Sue James, a founding member of the Trees and Design Action Group, explains why engineers should take note.

Why should successful urban tree planting matter to engineers? 
Holistic approach to street trees in Lyon. Image credit: Sue James

To answer the headline question, we could ask: 'What single thing could we invent for cities that would help to:

  • calm traffic
  • increase the speed of patient recovery in hospitals
  • reduce crime
  • reduce pollution and improve air quality
  • reduce external air temperatures in our increasingly hot summers
  • increase the environmental performance of low-rise buildings
  • increase the health and wellbeing of people and place
  • increase biodiversity
  • raise property values
  • reduce the rate of storm water run off
  • encourage active travel and walking
  • make great streets for living
  • make great places for children to enjoy active play

…and more?'

What would this great invention be? Well, it already exists and it's a tree.

Why do we need more urban trees?

The list of tree benefits as outlined above is extensive and backed by evidence-based research.

Sadly, there's a gap between recognising the value that trees can bring and support for urban trees. Not everyone likes trees (and there is a current Forest Research project underway assessing this).

Many people find trees a nuisance – all that muck on the car; the tree in the footway that stops them creating a cross-over to their now paved front garden for parking their car; the dreaded leaves in the gutter, and so on. But, before we welcome a treeless Valhalla, let us consider the implications of removing existing - or not planting new trees.

River bank linked with trees comparison
Embankment with trees and Embankment without trees. We can cut the trees down or fail to replace any that may die. Which scene do you prefer? Image credit: Trees and Design Action Group

Beyond all the well-documented benefits listed in our invention design brief above, trees also satisfy an intangible psychological need – we respond to places with trees, they offer a sense of longevity, stability, continuity which is important to those living in and visiting our towns and cities where the rate of change in our built infrastructure can be disorienting.

Why should we regard trees as an integral part of our urban infrastructure?

Trees should be regarded as assets and not as liabilities. Using valuations such as CAVAT [Capital Asset Valuation of Amenity Trees] show that trees increase in asset value throughout their long lives, unlike other elements of urban infrastructure, such as lamp standards, bollards, benches et al which need to be replaced.

Furthermore, trees deliver increased benefits after the first 30 years and these benefits continue to be delivered until they may slow down as the tree approaches ‘end of life’, which could and should be hundreds of years into the future. This demonstrates that the investment made in trees on a whole life basis is remarkably cost-effective.

Forest Research are undertaking an ‘i-Tree’ canopy study of many of the towns in England and have found some concerning results on tree canopy cover.

Key points on urban canopy cover were set out in a leaflet by the Urban Wood and Forestry Advisory Network. What these studies have shown so far is that an assessment of 283 towns showed an average of 16% canopy cover, but there's a great variability, with 3% in Fleetwood, Lancashire and 45% in Farnham, Surrey and, even within towns, the canopy cover is not spread equitably. The tree canopy cover of a place can be compared with the Index of Multiple Deprivation and can help to identify areas for increased tree planting.

According to Forest Research, a first step would be to achieve an equitable urban tree canopy cover across our towns and cities of about 20%. Of course, levels of canopy cover beyond this and especially when it comes to some of the challenges of climate change such as the need to increase summer cooling, will vary geographically.

Street trees combined with sustainable urban drainage systems (SuDS) have been shown to reduce the rate of storm-water run-off. Given the changing rain event patterns, this is now an important consideration.

In many towns and cities, not only are SuDS required for new developments, but also need to be retrofitted to overcome existing problems with storm-water capacity and this provides a further opportunity to retrofit street tree planting at the same time.

There's now political leadership for more urban tree planting and especially street tree planting with the ambition to include street trees in all streets in new developments. This provides the opportunity to review street design, to reapportion what might have been road space for vehicular traffic to satisfy more uses and, of course, to integrate trees and their multiple benefits at the same time.

How do we provide the delivery and long-term performance of urban trees?

Collaboration is the key here. Collaboration and greater understanding of which skills are needed and when. Towns and cities which have been most successful at integrating trees as part of their urban infrastructure are those where engineers, arboriculturists and all engaged in the built environment, including politicians, have worked together from the outset of any project.

When it comes to street trees, which is one of the areas where so many benefits can be delivered, it is the collaboration between highway engineers, urban designers and tree specialists which have had the greatest success. Lyon in France is a leading example of this approach.

However, the willingness to collaborate needs to be matched by an understanding of what needs to be done to create the right places in which to plant the right trees.

There is, of course, no one way to plant a tree. How the tree should be planted and which species of tree should be planted depends on the purpose for planting the tree and the location in which it will be planted. The method and cost can be very different if it's an inner city hard landscape environment compared to a suburban street with wide grass verges, or an area of park or urban green space.

There is a great deal of guidance available. The Trees and Design Action Group has focused on producing good practice guidance as well as disseminating the latest research into practice. It offers guidance on all aspects of urban trees, including how and where to plant them and which species to select, especially in regard to climate change. Some examples include Trees in Hard Landscapes – A Guide for Delivery and Green Infrastructure- A Guide for Specifiers.

So, back to the opening question: what is this single thing that could provide so many benefits?

We have the answer. Are we ready to seize the moment and deliver at the scale needed? Urban trees are not a ‘nice to have/add on/if we can’ element; they are, and need to be, a serious component of our urban infrastructure and this means that all of us, and especially engineers who can influence so much, really do need to work together to make it happen.

Get involved

  • Trees, People and the Built Environment is an international urban tree research conference hosted every three years by the Institute of Chartered Foresters for a wide range of partners including the ICE.
  • Sue James, Convenor, Trees & Design Action Group