The results are in!

In a September 2022 blog post titled A Shot in the Dark I wrote about the importance of having baseline data on our urban tree canopy cover (TCC). To briefly recap, baseline TCC data is important because it provides a metric that is easy to comprehend, allows us to monitor change over time, to set targets, to influence policy, and to estimate the extent of ecosystem services delivered by trees.

I have used a free online tool called i-Tree Canopy to estimate TCC within the four Douglas (HoK) constituency boundaries. The software randomly lays points onto Google Earth imagery and the user then decides what each point should be classified as. This classification can be as simple as tree/not tree or more advanced, recording different kinds of impervious surfaces and other types of vegetation. From this classification of points, a statistical estimate of the amount or percentage cover in each cover class can be calculated, along with an estimate of how accurate it is (standard error, SE).

The number of points used in each constituency area was determined by the SE, which needs to be under two to provide a meaningful result, so I kept adding points until a satisfactory SE was achieved. The number of points used ranged from 500 (Douglas South) to 400 (Douglas Central). 1800 points were classified across the whole Douglas area. The results for individual constituencies are as follows:

Constituency% TCC
Douglas North16.9
Douglas East18.4
Douglas Central15.0
Douglas South20.4

TCC for the whole Douglas area came out at 17.5%. To most people this figure will be meaningless; is 17.5% TCC good or bad?? Fortunately, Forest Research (the research arm of the Forestry Commission) have published TCC data for over 250 towns and cities in the UK (Doick K.J et al 2017).

For a fair comparison, it should be acknowledged that TCC is generally lower in coastal towns and cities compared to inland equivalents. The graph below shows the Douglas TCC against nine other coastal towns and cities of a similar size to Douglas (blue bars) and nine larger coastal conurbations (yellow bars). 

From this small sample you can see that Douglas is doing pretty well amongst its peers. The average TCC of coastal towns was 13.7%, but the TCC of Douglas even exceeds the mean TCC of all of England’s towns and cities (coastal and inland) which was 16.4%.

Before we slap ourselves on the back and congratulate a job well done though, we should acknowledge a potential flaw in this result. i-Tree Canopy uses Google Earth imagery, which for some reason is currently a patchwork of imagery from 2006 and 2023. It would be more helpful if all the imagery was the same date, even if it was 17 years old, because it would then give us a valid snapshot in time that we could compare to more recent imagery, when it becomes available.

Tree diseases such as ash dieback and Dutch elm disease, as well new development, are likely to have resulted in the loss of many trees since 2006. On the other hand, new planting is likely to have become visible in this time as well. It would be interesting to repeat this survey over a full set of up-to-date imagery, as we cannot be sure this is an accurate representation of Douglas TCC right now.

Leaving these potential flaws aside, assuming that the result is an accurate representation of TCC for the Douglas area, what conclusions can we draw from this exercise, and what questions remain unanswered?

It seems that, in Douglas at least, the Isle of Man is doing something right. This could be a result of regulation and government policy, such as the Tree Preservation Act 1993 and planning policy, or it could be just a cultural phenomenon. In some specific locations, such as the steep bank of woodland behind Queens Promenade, it could just be the result of the topography: the land is too steep to be built on or used for any other purpose.

Regarding the regulation of tree removal, it is interesting to note that more than 1/3 of the towns and cities included in the abovementioned study had a TCC figure that was higher than Douglas. These locations have achieved this level of canopy cover without the benefit of our very strict legislation. In May 2022 I wrote a blog post titled, Statutory Tree Protection – Time for a Rethink?, which considered whether the significant infringement on individual liberty resulting from the Tree Preservation Act 1993, was worth the benefit.

Whilst a TCC metric is useful, there are many questions which remain unanswered:

  • How does TCC for Douglas compare to other towns and villages on the island?
  • What is the species composition of the TCC? (important for judging biodiversity benefits and population resilience)
  • What is the age structure of the tree population that makes up this TCC? (Will TCC fall off a cliff if all our trees are old and in decline?)
  • What is the physiological condition of the trees? (Is our tree population physiologically stressed as a result of increasing occurrences of extreme weather events?)
  • How clumped/clustered is TCC within each area?

The methodology used by i-Tree Canopy (point sampling) is simple and, with the right software, could be repeated over any set of aerial imagery. i-Tree Canopy is free to access, and you don’t need to be a computer wizard to work it. It is just a shame that Google do not have access to more up-to-date imagery for our little piece of the globe.

If there are people out there who would like to conduct surveys in other areas of the island, or undertake repeat studies in Douglas, using i-Tree or a DIY equivalent, Manx Roots would be happy to help or advise.

Stress, Strength & Stability

Strength plays such a marvellous game –

it moves through the things of the world like a servant,

groping out in roots, tapering in trunks,

and in the treetops like a rising from the dead.

These are lines from a poem by the R.M. Rilke. They were written to provide insight in the human condition, but to me their literal meaning is as interesting as the metaphor. Trees are often perceived, mistakenly, as static and unyielding. Indeed, people can be unnerved when a tree sways in the wind. I imagine Storm Agnes will have been a source of apprehension for many people last week.

The line ‘tapering in trunks’ hints at one of the ways trees have adapted to withstand the forces of nature. Imagine a ballet dancer gracefully gliding across the stage, finding balance with every move. Similarly, trees perform an intricate dance with gravity, wind, and the elements. Biomechanics in trees is the study of how they maintain equilibrium, bending and swaying without breaking.

One key element in this delicate dance is the distribution of stress (the force applied) within the tree. Most trees are naturally tapered, with the thickest part at the base and gradually narrowing towards the top. This ‘design’ is an example of how trees have evolved to efficiently distribute the forces acting upon them.

Trees produce specialized wood tissues known as reaction wood in response to mechanical stresses. There are two types: tension wood and compression wood. Tension wood typically forms on the upper side of leaning or bending stems in hardwood trees. It is called tension wood because it develops in response to tensile or stretching forces. Compression wood is found in conifers. It forms on the lower side of leaning or bending stems and is named compression wood because it develops in response to compressive forces.

Trees ensure they can withstand wind events by reinforcing key points of biomechanical vulnerability. Through the growth of reaction wood, trees exposed to wind develop thicker structural roots and trunks and have increased stem taper. Critically, these growth responses require a stimulus, and so will not occur unless the tree experiences mechanical stress. In much the same way as we can build muscle through exercise, trees need ‘exercise’ if they are to reinforce their structures.

A material that remains rigid under loading is often thought to be strong, but in material science and engineering there are different types of strength. The flexibility of branches in a tree crown means that they can bend in the wind, changing the crown’s shape to reduce aerodynamic drag forces. In this sense flexibility is strength.

So, a healthy tree is a dynamic structure, constantly changing and adapting to its surroundings, and optimising its growth for survival. The fact that strength continues to play ‘such a marvellous game’ throughout the life of a tree is what sets it apart from a non-living structure, which will deteriorate over time without human intervention.

We should keep in mind, however, that mechanical strength is just one way that wood is optimised. The structure of a tree is always a compromise because it must also maintain hydraulic conductivity (the ability to conduct water), be able to store nutrients, resist decay and attacks from pests, and remain competitive to its rivals. This is why even healthy trees can sometimes fail in certain conditions.

Planning to Protect

The Strategic Plan sets out the general policies for the development and use of land across the Island and plays a pivotal role in the determination of planning applications. On the 21st July Government announced a full review of this policy document.

It is of interest to Manx Roots because this is the document that sets out how the impact to the environment (including trees) should be considered in the preparation of proposals for development.

Information about the Government’s current consultation on this review is available here.

A lot of the consultation questions relate to specific issues which are not directly relevant to trees. The last question, however, is open ended. I thought that our response may be of interest to some people, so here it is:

Manx Roots welcomes this review of the Strategic Plan. Trees are fundamental to our wellbeing and quality of life, whether in an urban or rural context. They secure many environmental, economic and social benefits when planned and managed appropriately.

In providing arboricultural consultancy services we aim to:

  1. Prevent trees being cut down or pruned unnecessarily, so that they can provide social, economic and environmental benefits for the Island.
  2. Keep people and property safe.
  3. Help our community mitigate climate change and adapt to a changing climate.
  4. Facilitate sustainable development.

We believe that the Strategic Plan should also contribute to achieving these outcomes.

The current strategic plan only promotes the protection/planting of trees through General Policy 2 and Environment Policy 3. Given the many, well-documented, benefits trees provide, this review represents an important opportunity to provide more weight to the protection/planting of trees in the planning process.

Trees have a positive impact on the quality and sustainability of development by:

  1. Creating welcoming, attractive and distinctive places.
  2. Creating boundary definitions.
  3. Creating wayfinding and landmark links.
  4. Encouraging walking and cycling.
  5. Making density work and moderating land use transitions.
  6. Creating visual and noise barriers.
  7. Enhancing thermal comfort.
  8. Dealing with stormwater sustainably and cost-effectively.
  9. Enhancing biodiversity.

These benefits have been documented in UK planning policy, and various place making and design quality frameworks:

Better, B., & Building Beautiful Commission. (2020). Living with beauty: Promoting health, wellbeing and sustainable growth. Ministry of Housing, Communities and Local Government.

Building With Nature Ltd. (2023). Building with nature standards Framework.

Building Research Establishment Environmental Assessment Method (BREEAM)

Homes England. (2020). Streets for a healthy life: A companion guide to building for a healthy life.

International Well Building Institute. (2016). The WELL Building Standard. Delos Living LLC.

Ministry of Housing, Communities and Local Government. (2020). Planning for the future; White paper August 2020.

Police Crime Prevention Initiatives. (2023. Secured by Design: Homes 2023.

Trees can contribute to meeting some of the Island’s challenges and policy goals over the next 50 years. For example, by contributing to the creation of attractive and distinctive places, trees can help to create a ‘sense of place’ and have a positive impact on community cohesion.

Trees will make active travel more likely by making cycling and walking routes feel like attractive places to be. By slowing down traffic, providing a barrier from vehicular traffic, and helping drivers read key changes, such as approaches to intersections and built-up areas, trees can also make the active traveller feel safer. 

Trees can contribute to the Isle of Man Government’s economic strategy/aspirations by creating environments that attract investment, and that are perceived by skilled professionals as an attractive place to live and work.

There is a lot of attention being paid to how planning policies might help the Government meet its commitment to reach net zero by 2050. There is not much discussion, however, on how the island will adapt to a changing climate, which will happen whether net zero is achieved or not. Trees can help to mitigate extreme weather events by intercepting rainwater, facilitating stormwater infiltration and attenuation, removing stormwater pollutants, and by providing shade and shelter.

It is likely that the new strategic plan, like the old one, will be topic based. One concern that may be raised with this format is that trees deliver such a wide variety of benefits (social, environmental and economic) that the need to plant and protect them cannot be adequately reflected by just considering them as part of the ‘the Environment’. 

Another concern is the lack of data relating to trees. Diagram 1 in the main consultation document states that evidence gathering is the first stage of a development plan. The island has no reliable data on its tree population. Even the most basic of metrics, such as urban tree canopy cover are lacking. It will be very difficult to be confident that the correct weight is being given to tree protection and planting when there is so little evidence of the Island’s baseline.

It is acknowledged that this is only the first stage in the consultation process and so, perhaps, not the time/place to discuss the wording of specific policies relevant to trees. However, it may be helpful to discuss the kinds of policies that should be included in the new strategic plan.

We believe the new policy document should:

  1. Make a clear statement that trees are critical to delivering high-quality sustainable development.
  2. Provide a level of aspiration for larger new developments. For example, the UK NPPF states that ‘Planning policies and decisions should ensure that new streets are tree-lined, that opportunities are taken to incorporate trees elsewhere in developments (such as parks and community orchards), that appropriate measures are in place to secure the long-term maintenance of newly-planted trees, and that existing trees are retained wherever possible’.
  3. Clearly state that adequate tree-related information should be submitted with applications. This needn’t be onerous, and the information required should be proportionate to the design and complexity of the development.
  4. State that trees will be subject to the planning system’s mitigation hierarchy, whereby loss is avoided, if possible; losses or negative impacts are minimised; or, as last resort, compensation is provided for unavoidable loss.
  5. Give a lot of weight to the protection of existing good-quality trees (those categorised as A & B under the BS5837:2012 category rating system).
  6. Where compensation is required, provide a clear framework for this to occur. For example, a tree-loss replacement standard should be included.
  7. Encourage trees to be incorporated into the design in a way that creates a harmonious relationship between trees and structures that can be sustained in the long-term.
  8. Secure adequate tree protection for existing retained trees through the construction process of the proposed development.
  9. Seek adequate levels of new tree planting, including aftercare to ensure that newly planted trees reach independence in the landscape.

We look forward to reviewing the policies of the new strategic plan.

An elm sanctuary?

When the ‘elm tunnel’ at St. Mark’s was threatened by a proposed development there was a huge public outcry. At the time, it struck me as odd there was so much public support for the campaign to save those trees, but so little awareness of the disease which threatens the entire elm population on the Island.

The disease which ravaged the English countryside in the 60’s and 70’s reached our shores in the early 90’s. ‘The Forestry Board’, followed by DAFF, and then DEFA, funded a control programme and introduced new plant health legislation to help reduce the rate of spread and prevent new sources of infected material reaching the Island. For the last ten years, however, funding of the control programme has been significantly eroded. Budget reductions and long-term budget freezes meant that the Department’s ability to find and deal with diseased trees was considerably hampered.  In the summers of 2020 and 2021 we observed huge increases in the numbers of diseased elm trees. The distribution of the disease on the island also increased; for the first time ever, diseased trees were identified as far south as Colby.

My understanding is that DEFA’s control programme has now all but ceased completely. As a result, we are seeing an ever-increasing number of diseased elms around the island. This elm tree on the corner of Saddle Road in Douglas is an example of a large urban street tree which has been infected this year.

If the disease spreads down Castletown Road, the junction to Anagh Coar could look very different in a few years’ time. These huge ‘Wheatley’ elms are iconic in the local area.

The impacts of Dutch elm disease have the potential to be far reaching; for amenity, biodiversity, landscape, and heritage, not to mention our finances.

Unfortunately, for most elms in the countryside, it is now too late to act; the disease is so widespread that it would be very difficult and costly to get it back under control. It may still be possible, however, to save our urban elm trees, particularly in the far south of the island where the disease is less prevalent, but we would need to act quickly. Control and monitoring zones, sanitation felling, and new planting in Douglas and Port Erin could create an ‘elm sanctuary’ to rival that which still exists in Brighton today: