The benefits of large species trees in urban landscapes: a costing, design and management guide

I have written the following review of the CIRIA guide, C712, for the Arboricultural Association, but thought I might share it with you here.

The benefits of large species trees in urban landscapes: a costing, design and management guide (“the Guide”) has been written by three experienced landscape architects working for a research team from Ove Arup & Partners Limited under contract to CIRIA.   The Project Steering Group included some eminent arboriculturists, and others contributed to the development of the Guide.

The 128 page Guide is confusingly presented: 21 pages of Executive summary, Contents, Glossary, Abbreviations and acronyms and Introduction preface parts A and B.   Part A refers to the financial, social and environmental benefits that large species trees in an urban environment may bestow, and B refers to technical guidance for the incorporation of large species trees in an urban environment, from planning through to management.   The narrative is complemented by in excess of 160 references listed on 14 pages.   The text is punctuated with a number of figures, case studies (of the 18 included five refer to UK experience, the remainder are from the United States or Canada) and text boxes.

The positive ambition of the Guide is clearly stated in the opening paragraph the Executive summary on page v:

The UK’s urban trees are special and have played an important role in combating the effects of urbanisation for many years.   It is impossible to image towns and cities without them, yet they are in decline and this guide is intended to inspire a call to action to reverse this trend.

In the Introduction on page xx under the Aims and objectives we are told that:

The information has been collated, analysed and summarised to present it in a practical way so that the benefits and values of large species trees can be clearly understood.

However, it saddens and disappoints me to say that the Guide fails to live up to its promise.

As the title of the Guide is so specific I would have expected to see a very early statement of the range of benefits that large species trees offer, over and above vegetation of any other scale, drawn from experience, intuition or research.   Instead, we are told on page xvii, that:

while all urban trees are special, it is large species trees that are particularly significant as the most important single elements of the green infrastructure (GI) or the “urban forest” – the trees and woodlands within and around towns and cities.

However, the Forest Research paper from 2010 referred to makes no special claims for large species trees over any other, the authors miss the opportunity to discuss how they have reached their conclusion, or refer to the considerations that will have informed their opinion, and are only able to refer to very limited research from America to support their assertion.

In Section 1.1 the text makes the claim that:

This guide has been structured to clearly highlight the financial, social and environmental benefits of large species trees.

Section 1.2 on page 5 refers to two American case studies that have derived cost benefit ratios (CBRs) for large species trees (the first was the Tree guidelines for San Joaquin Valley communities by McPherson et al published in 1999 and the second was A cost-benefit analysis of ten street tree species in Modesto, California, USA by McPherson from 2003), but queries whether:

The principles of value transfer may be an appropriate system to convert the CBRs from international case studies to another similar urban environment.

Putting aside the issue of which UK urban environment may compare with either the San Joaquin valley or Modesto the brief synthesis of the evidence presented for each case study did not permit any analysis, the authors themselves state that:

It would be useful if a similar level of research could be undertaken in the UK.   This would give valuable evidence to demonstrate the positive financial benefits of large species trees to developers, local authorities and designers.

This thought is shaped into the Guide’s sixth and final recommendation on page 86 (albeit research should set out to test a hypothesis rather than prove one).   The irony is of course that this recommendation undermines the Guide by demonstrating the paucity of research data available to the authors when they were drafting this Guide and so the difficulty that the Guide will have in trying to inform its target audience of the benefits of large species trees.

I sought in vain in Section 2 The financial benefits of large species trees for data and evidence that would convince the sceptics within the target audience to commit to the inclusion of large species trees in new schemes, or to retain them in their existing settings, but found none that could be relied upon.   For example on page 10 in 2.2 Increasing property prices and land values the Guide states:

Trees have been shown to have potential to raise property prices by between five and 18% by a series of international studies (CABE Space, 2005 and Morales et al, 1983). . . .

A US study (Wachter and Gillen, 2006) in New Kensington, Philadelphia, demonstrated that properties close to new tree plantings increased in price by about 10%. . . . The potential increase is dependent on several factors including the local socio-economic profile, characteristics of the area, type and density of development and climatic conditions.

The authors do not state that either reference categorically demonstrates that those increases are due to large species trees.

In 2.3 Decreasing sales time on page 11 the authors assert:

These findings [a study on Portland’s housing market by Donovan and Butry, 2010] are supported in the UK, with local authorities such as Warwick District Council (2011) highlighting that trees contribute significantly to the saleability and desirability of residential properties.

I looked up the Warwick reference, which actually states something rather more subtle:

. . . an informal telephone survey of estate agents in the Warwick area suggests that tree cover has a positive effect on saleability, if not directly on price.   Properties on tree lined street were said to be in more demand and to sell faster.

In paragraph 2.4 Encouraging investment and growth on page 12 the authors sate that:

Stockley Park (near Heathrow Airport, London) and Canary Wharf (East London) are good examples where significant private investment was made in planting large species trees to provide immediate effect in urban and business park setting.   Businesses are now prepared to pay a premium to be located within this type of green environment.   This illustrates that initial investment in tree planting and GI can lead to higher rental yields.

In the absence of any data to support their conclusion over yield then other interpretations are equally valid, for example the location of the business park in relation to essential infrastructure or commercial partners is more significant than its setting.

Section 3 The social benefits of large species trees sets out to explain rather than quantify social benefits.   The narrative relies rather too much on the authors’ intuition that, I paraphrase, “large species trees, because they are big, make a greater contribution than other forms of vegetation”; but no evidence is presented to support this assertion.

There are some valuable findings reported, for example on page 25 the Guide states:

Trees absorb gases such as carbon monoxide, nitrogen dioxide and sulphur dioxide, which are known to cause respiratory problems and can also increase sensitivity to allergens (Baines, 2005).   High ambient levels of carbon monoxide, generated primarily by vehicular traffic, are also linked to hospital admission for cardiovascular problems (Bell et al, 2009).   Trees close to the sources of pollutants can help to minimise these health problems.

But the authors do not provide data to support their conclusion and I believe they fail their readers by not providing a recommendation as to what they believe ought to be done.

Where data had been included it was, of course, impressive, but already widely known and perhaps rather loosely applied?   For example in 3.2.6 Improving workplace productivity the authors refer on page 27 to a 1998 study by Wolf that demonstrated that for desk workers a view of a green environment (but not specifically large species trees?) reduced the time they took off due to ill-health compared to their colleagues with no such view: a more recent reference to work by CIPD showed that sickness absence cost an average of £692 per employee in 2008.   The suggestion was that these data could be compared, but there was no evidence presented of a link.   Unfortunately the very thorough 2010 Forest Research literature review from O’Brian et al Urban health and health inequalities and the role of urban forestry in Britain: A review is not included amongst the references.

Section 4 The environmental benefits of large species trees opens with the bold assertion that:

This chapter illustrates how large species trees can immeasurably improve the quality of people’s lives in urban areas by reducing the effect of climate change and improving the day to day environmental conditions.

Perhaps because the benefits are immeasurable no data is presented?

To address the urban heat island effect the work of Gill et al from 2007 is referred to on page 39:

by increasing green cover by 10 per cent, surface temperatures could be maintained at or below the 1961-1990 baseline temperature

But green cover means what?   An increase of 10% of what?   The typical reader from the target audience is not going to find the reference, locate the paper and read what Gill et al meant, the research findings need to be clearly presented here.

On page 41 the authors state:

Carefully planned tree planting schemes can provide shade and insulation to buildings, not only reducing energy consumption for cooling and heating, but also reducing emissions of pollutants from power plants.

This is an extremely worthy, but why are no concrete recommendations given for the designers to consider when drafting those “carefully planned schemes”?

In Section 4.4 Reducing flood risk there is no discussion at the top of page 42 of the difference between the benefits conferred by a mass of planting rather than one planting mass, but perhaps that should not be a surprise as that issue has not been conclusively addressed previously.

I had greater hopes of Part B, Technical guidance remembering that the three authors are all experienced landscape architects.

In 6.2.1 Protecting large species trees from inappropriate development on page 52 a number of controls are mentioned, but with the startling omission of the Tree Preservation Order and Conservation Area.

In 6.2.2 the authors state, at the bottom of page 52:

To ensure that large species trees retain their positive image and public support it is important that good practice procedures are followed to ensure their long-term inclusion within towns and cities.

But, there is no indication of what they might mean by good practice procedures.

In 6.3 Guidance for planting large species trees the authors fall back lazily upon the technical departments of the product manufacturers for their technical drawings: I would have welcomed some imaginative proposals from the authors’ portfolios for new paradigms, for example for tree planting in a car park setting (be it for a housing association or a supermarket), or for street tree planting.   Where technical drawings would have been useful, for example to help illuminate the narrative describing the Construction of hard surfaces in 6.5.1 Techniques for avoiding structural damage, unfortunately none were provided.

On page 68 under 6.6 Successful planting and retention of large species trees close to utilities and underground structures the Guide perpetuates the myth:

Poplars and willows are known to have particularly invasive root systems

On page 70 under Planting techniques in 6.6.2 the authors throw in a delicious morsel to tempt the reader:

The exact below ground shape of a tree pit or planting corridor is not critical whereas achieving the adequate soil volume for large tree establishment is.

Regrettably the authors elect not to include guidance as to what that volume might be, or how it might be calculated.

The lack of arboricultural understanding and experience amongst the authors is most noticeable in this section: at the very least some anonymous data from real planting schemes would have added some credence to the text.

The reader might imagine that Section 7 Planning and designing with large species trees would naturally lend itself to lots of illustrations and sketches, but sadly not.   The opening paragraph of the Section states:

A summary of the benefits and cost implications of particular relevance to clients, developers and local authorities is also included.

but I have yet to locate it.   I hope the authors are not referring to an information box on page 73 that states, without citing any supporting evidence, that:

The cost of this professional advice will be a fraction of the financial benefits that can be realised by incorporating large species trees into a development.

A second information box on page 74 concludes, again without quoting any supporting evidence, that:

Although the costs of tree protection will vary considerably from site to site it will consistently be a fraction of the benefits that may be attributed to retaining large species trees.

The third information box on page 74 quotes Spon as its source for cost information for diverting utilities into shared ducts in an existing urban area.   If it is intended to inspire the hard nosed engineer to increase his costs so that a private householder may benefit from an increase in his property value then perhaps the best can be said is that the authors are disarmingly naïve and optimistic.

Section 8 Management and maintenance of large species trees uncritically rehearses a number of the recommendations from Trees in Towns II: in 8.4 Minimising the risk of damage from structural failure the authors include a very lazy reference:

Specialist arboricultural advice should be sought with regards to the suitability of different species for urban planting schemes, particularly along highways and within car parks, to avoid creating an ongoing maintenance liability.   For more information refer to the National Tree Safety Group (2012).

Arboriculturists will know that the NTSG’s A4 leaflet does not include information that will aid the reader of the Guide, nor does the 20 page Landowners Summary, and the complete text Common sense risk management of trees is 104 pages long – which of the other disciplines targeted by the Guide is going to read all of that searching for details concerning the suitability of different species for urban planting schemes?

The final section 9 Conclusions and recommendations was an opportunity for the Guide to redeem itself, but even here it fails to do so.   The opening paragraph on page 85 is beautifully phrased but must refer to a different draft of the Guide than the one that has been published:

The findings in this guide provide significant evidence in support of environmental, social and financial benefits of large species trees in the urban environment.   Trees should be regarded as a vital component of an urban landscape rather than a poorly planned ornamental addition to a street scene.   A new more systematic, technical and informed approach to tree planting is require to ensure the towns and cities of the UK remain attractive and liveable in the context of a changing climate.

The text continues on page 85 with six recommendations; however their wording is generally vague and anodyne offering few model outputs or mechanisms, plans or structures to use to seek to persuade those opposed to the central theme, that large species trees are good.   The strongest worded recommendation would appear to be the one that chimes most with the authors’ combined experience, yet rehearses the local planning authority’s existing statutory duties under the UK planning system to consider the protection and planting of trees, all the while failing to be assertive, bold or clear:

4 Embed the requirement for trees in planning policy

Strong policy direction (and implementation of) remains one of the most effective mechanisms to ensure compliance with principles of sustainability and urban greening.   Stating the need for inclusion, protection and management of large species trees will provide a mechanism to help determine planning applications and highlight the importance of this issue.

and, tellingly, as far as this Guide is concerned:

6 Undertake UK based cost-benefit analysis of urban trees

The work that is missing in the UK is the analysis of both costs and benefits together.

It would appear to me from some of the text that the authors set out on their journey full of hope, eager to fulfil the brief given by their title, but part way through their journey, for whatever reason, they abandoned their belief and brief.   Rather than embrace the opportunity to provide a new reference work for those managing trees in the public realm they have offered only vague platitudes and encouraging noises: the Guide offers nothing new.   It is regrettable that there was no arboriculturist amongst the authors; perhaps the commissioning editor wanted the enthusiasms and positivity of the landscape architect rather than the problem-solving experience of the arboriculturist to permeate the narrative?   Perhaps this was a limitation imposed by the tender action that lead to the commissioning of the title: maybe the process was too cautious and lead to the selection of a safe team guaranteed to produce a narrative, rather than take a risk with an unknown team that would address the brief?

Aside from the lack of content little things began to irritate me as I read the Guide: it has been badly poorly designed and laid out, it has been very poorly edited throughout, in general the photographs have been poorly selected, a considerable number of the illustrations are superfluous, the majority of the line drawings are weak often lacking a scale or a key, and why are some web-based references given a tiny.url and others not?

If the commissioners of this Guide hoped it would provide useful tools that could used be the target audience when considering the placement or retention of large species trees in the urban landscape then they will be sadly disappointed.

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