Tree Canopy Cover – a strategic solution?

A recent study* has used i-Tree Canopy^ (a free-to-use tool developed by the USDA Forest Service) to survey the tree canopy cover (TCC) of 282 towns and cities in England.

TCC, also called ‘urban canopy cover’ or ‘urban tree cover’, has been defined as the area of leaves, branches, and stems of trees covering the ground when viewed from above. It is an easily accessible measure that can be used to estimate the percentage of tree cover that any urban area enjoys.

It is now internationally accepted that properly managed forests and trees in urban environments make important contributions to the planning, design and management of sustainable, resilient landscapes – they help make cities safer, more pleasant, more diverse and attractive, wealthier and healthier.

Research also suggests that even moderate increases in canopy cover within cities can aid adaptation to the adverse effects projected under a changing climate. However, a baseline TCC value for many of the UK’s towns and cities is unknown.

Nor is it known whether canopy cover is changing and, if it is, whether it is increasing or diminishing. There is also a knowledge gap when it comes to knowing the numbers of trees in towns and cities, or their species, age composition and health. The level of canopy cover required to deliver meaningful benefits in UK towns and cities is also unknown, though there is some evidence to suggest that it is in decline.

This study* has now gone some considerable way to answering these questions and revealed a wide range of baseline tree canopy cover across the country – from a TCC of 45% in Farnham to just 3.3% in Fleetwood; with a median TCC of 15.8% and only 132 (47%) of sampled areas exceeding this.

Bristol, for example, ranks 176/282 samples if the 14% TCC identified in a study undertaken by the city council in 2011 is used. If the 18.6% cover estimated in this recent study is used (the study only looked at TCC in the urban land classes, rather than at the whole administrative area covered by BCC), then its TCC is above average and it would rank 83/282 samples. This suggests that “…boundary choice can impact TCC results and should be driven by the overriding question: ‘what is the tree canopy cover in the urban land classes of a given local authority?’, compared to ‘what is the tree canopy cover in a given local government jurisdiction?'”‘.

BCC NPA Survey 2011
BCC TCC 2011 Study – Neighbourhood Partnerships cover

 

Bristol also has the added benefit of having already surveyed many of its public trees, albeit some eight to ten years ago. This treasure-trove of data has been collated and augmented with other data we have collected to make a dataset of nearly 67,500 individual trees (though just 2.4% of predicted TCC) and made available to all via the Trees of Bristol web site.

Conclusions drawn by the study

A TCC target that is city-wide and not targeted at specific wards or land-uses poses a number of challenges. It can be delivered in such a way that does not optimise or diversify benefit delivery. For towns and cities that have a green belt (or similar designation), planting schemes can be targeted within this land. However, with comparatively lower populations than the urban centres, planting here offers fewer benefits on a per capita basis.

Canopy increase targets could equally be met by preserving the existing tree stock and allowing natural growth. As the canopies increase so will total canopy cover, although such increases will be constrained by tree loss/removals, natural wastage and damage by pests and disease.

Such an approach, however, also fails to address social equity. Targets based on land-use-based assessments or ward are more likely to align the provision of ecosystem services with indicators of social inequity. It will be important that such approaches are underpinned by a robust baseline and a commitment to repeat canopy cover surveys using a consistent approach.

Species diversity and placing the right tree in the right place are important considerations when planting to achieve a TCC increase as these allow resilience to be built into the urban forest. Knowing the composition of the existing urban forest in terms of species and age structure, condition and appropriateness to location (and therefore life expectancy) can inform such decisions. Given that private ownership of trees can be as little as 24-35% in some cities, but as high as 71-75% in others (Introducing England’s urban forests), TCC baselining studies should be complemented by a field study.

With the wide range of considerations and stakeholders involved in urban forest management, TCC targets should be set both within local planning policy and within a formal urban forest management strategy.

Targets should have a target date, an action plan and a commitment to monitor, review and update. The policies should inform on which tree species to plant. They should also prioritise wards and/or land uses and should protect the existing tree canopy by enforcing best practice, codes of practice and statutory controls in the care, maintenance and protection of trees. Given that the average lifespan of a typical urban tree is estimated to be 32 years, changes in the age profile of the urban forest should also be modeled to at least 50 years distant in order to understand and plan for the likely impact on total TCC of tree planting and loss.

Any strategy will need to focus on partnerships with institutions and on guidance advising residents how they can best protect and look after their tree stock, schemes to assist in management and maintenance, and support future tree planting amongst the different ownership groups.

Finally

City-wide tree canopy cover is a useful indicator of the extent of tree presence across a city. Its assessment can be simple, fast and highly reproducible. Repeat observation could be a cost-effective means of monitoring tree populations, setting targets and tracking effectiveness of planting programs.

The results of this study suggest that:

  • an average TCC of 20% should be set as the minimum standard for most UK towns and cities, with a lower target of 15% for coastal towns;
  • towns and cities with at least 20% cover should set targets to increase cover by at least 5% (i.e. above the ±2% tolerance of i-Tree Canopy) within 10 to 20 years (depending on what is achievable against their baseline); and, targets and strategies for increasing tree cover should be set according to the species, size and age composition of the existing urban forest, based upon a ward/district level and land-use assessment.’

We at BTF commend and recommend this very helpful and timely study.

* Doick, K.J., Davies, H.J., Moss, J., Coventry, R., Handley, P., Vaz Monteiro, M., Rogers, K., Simpkin, P. (In Press). The Canopy Cover of England’s Towns and Cities: baselining and setting targets to improve human health and well-being. Conference Proceedings of TPBEIII. Urban Trees Research Conference. 5-6th April 2017. Institute of Chartered Foresters, Edinburgh.

^i-Tree Canopy

Author: bristoltreeforum

We are a group of volunteers dedicated to increasing the tree canopy cover of Bristol.

4 thoughts on “Tree Canopy Cover – a strategic solution?”

  1. Back in 2011 increasing tree canopy cover (TCC) was built into Bristol’s planning strategy, policy BCS9 has this as a key metric of success. The problem is that changes in councillors, mayors, officers and funding means we all need continual reminding of this but more importantly why TCC is so important.

    TCC became an urban objective almost worldwide following the 2003 heatwaves which caused over 50,000 premature deaths in France, 10,000 in the UK and many tens of thousands elsewhere.

    Academics pulled togather a number of studies and undertook more which showed that TCC can reduce the urban heat island effect. They showed that a 20% TCC would mean a city would be 1 degC cooler than one or an area with 15% for example. And that 1 degree could have saved tens of thousands of lives in 2003. Heatwaves like that are predicted to become more common.

    Additionally other studies showed that trees (as long as the leaves are cleared up) can reduce flooding after storms and reduce the amount of road grease, metal and dirt pollution going into the drains and so saving the water companies the costs of cleaning it from their treatment plants.

    But the benefits continue: trees can reduce air pollution too, particulates especially and NOx, and proximity to trees can help reduce other health problems, reduce stress and promote healing. Its for these reasons that NHS planning policy is to put trees into their new developments. Trees provide shelter, shade (reducing aircon costs), are a home for nature and look good too.

    But some politicians, highways engineers and members of the public don’t know all this and think trees are simply a middle class icon, a cost and bring problems. They are mistaken and need the benefits explained by organisations like the Bristol Tree Forum. A scientific approach to understanding the benefits of trees especially those shading the highway shows how beneficial they are.

    Keep up the good work Tree Forum, education is a key role. That’s where one tree per child scores; not in the quantity of trees planted, many die and they are in green spaces, but because a new generation of children are educated in the benefits of trees…but you BTF have to remind the adults…

    Liked by 1 person

  2. Clive – well said! I’m intrigued by your statement: Academics…showed that a 20% TCC would mean a city would be 1 degC cooler than one or an area with 15% for example. Are you referring to the Susanna Gill work for Manchester? Or another study by any chance? would be keen to know specifically which study you are referring to. Thanks

    Like

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