Tree benefits; the missing part of the street tree cost benefit analysis equation

Sheffield was widely hailed as one of Europe’s greenest cities, but it is rapidly gaining an international reputation as the place where they are felling street trees on an industrial scale.  Local democracy seems to be unravelling before an international audience as the wishes of local communities are ignored and healthy trees with decades of life left in them are felled causing significant loss of tree benefits.  It is a political problem and it will be for the politicians to find a solution, with issues way beyond the remit for tree experts to resolve.  However, in the melee for the high ground, tree management principles are being misapplied as Sheffield City Council clamours to justify its actions, and that certainly is a matter where tree professionals can assert authority.

At the heart of the rhetoric is the contention that the trees are mature, with little useful life expectancy, and it is in the best interest of the community and good management, to fell and replace them immediately.  Two technical pillars support that position;  life expectancy, and the optimum felling point, or rotation length.  Taking life expectancy first, I have seen a significant selection of the condemned trees and I assess that most of them have decades, if not centuries, of life left in them, so the justification of “they are at the end of their useful life” falls away under professional scrutiny.  Of course, that is my opinion, and there will always be scope for disagreement, but the trees are there for all to see, so doubters can form their own opinion.

This leaves us with rotation length, and more specifically, what is the optimum point in time to fell street trees.  While there is plenty of research on forestry rotation length, there is very little guidance for street trees, and thus this post.  How can we as professional tree managers assist in deciding the optimum time to remove and replace street trees?  Foresters grapple with this concept on a daily basis, with the principles of current annual increment (“CAI”) and mean annual increment (“MAI”) being the foundation for many decisions where optimising the timber volume is the priority.  From research and practical experience, we know that the optimum time to fell is the age where the curves of CAI and MAI cross, but can this principle be reasonably transferred to urban tree management, where the product is tree benefits rather than timber volume?

Which reveals the heart of the problem;  conventional street tree management decision making has been focused almost entirely on costs, with no balanced consideration of benefits.  There are plenty of models identifying how much it costs to buy, plant and maintain a new tree through to removal, but very few factoring in the multiple benefits that trees provide.  Accountants are exploiting this knowledge gap at the expense of communities, creating an urgent need for the tree profession to stand up and inject some balance into the decision-making process.  Towards this end, I have worked up a draft conceptualisation of what an urban tree benefits model may look like (see attached figure).  It reflects the concepts of CAI and MAI, with the optimum rotation length being where they cross.  Conceptualising tree benefits is tricky for multiple reasons;  primarily because most are difficult to reliably value, but also because some are linked to size, e.g. pollution buffering, water buffering, health and wellbeing, etc, while others are more related to age, e.g. ecology, heritage, etc.  I accept that the precise form of the curves will vary with species, growing conditions, local benefit values, etc, but that aside, the primary question is whether this conceptualisation reasonably captures the principle.

With increasing instances of street trees being prematurely removed across the country based on costs rather than a balance of costs and benefits, this is becoming an increasingly important built environment management issue, which is why I would be grateful for feedback on any fundamental flaws that this CAI/MAI approach may have.

 

Sheffield-Rotation length graphic - Jeremy Barrell FICFor

Disclaimer: The views and opinions expressed in this article are those of the author and do not necessarily reflect the views of the Institute of Chartered Foresters. 

About the Author

Jeremy Barrell FICFor, Director, Barrell Tree Consultancy
Jeremy Barrell FICFor
Managing Director
Barrell Tree Consultancy
  • Kieron

    At Forest Research we have been using the i-Tree Eco tool to model the benefits of urban trees over their life – cycle, focusing on carbon uptake, air pollution removal and stormwater interception. Our results very much reflect yours. We are about to send the work for review and hope to share it soon.
    It’s incredible how many benefits are delivered by a mature tree, so it would seem logical to retain mature trees wherever right and safe to do so.

    • Jeremy Barrell

      Kieron, thanks for that i-Tree observation. I am mindful of the current limitations to the i-Tree approach, namely that it does not yet account for some important benefits, e.g. wildlife & habitat, health & wellbeing, heritage, which I suspect is selling the whole benefits package short by some considerable amount. Of course, it is the best that we have right now, and it is improving all the time, so it is relevant, but I wanted to try for a conceptualisation of all the benefits, although we are not at all sure what the full extent is at the moment. I appreciate that there is no simple or reliable answer, but it is reassuring that your figures reflect some similarities with my best guess. My inclination is that something is better than nothing, as long as the shortcomings are appreciated and the approach is used with caution. I am grateful for your input; it is obviously an area where we need a lot more work, and forest management principles seem to be an important facet of teasing out some meaningful conclusions. Thanks.

      • Steve Cox

        Hi Jeremy and Kieron. This looks to be a very helpful way of organising our thinking about urban tree values. I’m pleased Forest Research is working on this. Are you intending to publish in the Arboricultural Journal as well as Forestry, Kieron? I think that would be a good idea. I agree with you, Jeremy, that at present we can only model a fraction of the factors contributing to total tree values. iTree is a great basis on which to work. Your basic model for tree benefit values does depend on the inputs and many of these can only be estimated. I’d love to see more data that we can use to plug into the model. What is the next step that we need to take?

        • Jeremy Barrell

          I think arboriculture urgently needs a model because of the way that good street trees are being removed on spurious grounds, although it will certainly be some time before we can reliably put figures to it. It seems just the sort of research project that FR would be qualified to undertake. It is all very well me speculating in the absence of an alternative, but it would be really useful to have a credible and academically oriented analysis that we could use on the ground to refute calls to fell trees prematurely. It is good news to hear that FR will soon publish and I am looking forward to seeing that, but I do hope the work continues to ensure that realistic tree values are factored into the built environment decision making process. This is not happening in many places at the moment, which is resulting in huge benefit losses to local communities, often the very places where those benefits are most needed.

      • Ian McDermott

        Hi J&K, The advantage of the iTree approach is that it uses science, or as close to it as we can get, to value the trees and the issue with giving values around esoteric benefits is well known to us as tree managers. It is relatively easy to explain (to an elected official for example) how much it would cost to remove the same amount of pollution as a tree using “machinery” but we all know how difficult it is to explain visual amenity and although I am 100% with Jeremy here the more difficult it is to evaluate benefits of trees the easier it is for highways managers to brush them aside with statements about the cost of tarmac repair to the ratepayer.

        • Jeremy Barrell

          I am sure that i-Tree is going to become more valuable as time passes and more benefits are accounted for, so it is certainly useful. I suspect that one of the most difficult hurdles with it is the lack of recorded data for ages beyond 100, e.g. what happens to carbon sequestration, what are the life expectancies of various species, etc. We know that plane for example, can survive in streets for hundreds of years because we have examples in London and some other cities, but I am not sure if there is recorded data on performance, although we know they survive. I would like to see a responsible and credible body such as FR begin to look at such trees, so that our extrapolations are based on investigation more so than the current observation/speculation approach that I have adopted. Getting the basic curves using i-Tree would be great because it will give us the minimum to which we can speculate on the added benefits that come from those factors not accounted for, so health & wellbeing, heritage, etc. If we are to put more balance into decision making, then we need some help on this, before all those trees with lots left to give are felled because the wrong figures didn’t stack up!!

  • Tommy

    I actually find it somewhat disturbing that we need to go to such lengths to even show the benefits of specimen, mature etc. trees in the urban environment. It certainly is a reflection of the times we’re in… There’s a lot of research that shows, undoubtedly, the benefits of urban trees, so may be it’s an issue with communicating the benefits of urban tree to the right people? Anyhow, the above article and the comments below are moving in the right direction, and we shouldn’t underestimate the urgency for such a model that will help prolong the benefits that urban trees provide.

    An holistic approach in finding values, benefits, services etc. from a tree is needed. Thus, this will produce a more diverse number of ‘good points’ for a tree, which will then relate to more people.

    From some of the insights in the comments below I feel that the ‘Ecosystem Services’ approach has not been full incorporated into the i-Tree approach may be?

    • Jeremy Barrell

      Tommy, thanks for taking the time to contribute. I agree that this is disturbing, but that is the world we are in, where accountants rule and unless we can reliably demonstrate value, then it will not be factored into the decision making process. This is happening throughout the planning system, which is about weighing and balancing competing values, and trees consistently lose out because although the research is there, no one has really put a package together to explain it. So, we have heaps of data demonstrating a vast array of tree benefits and yet no one it taking any notice, in fact, quite the opposite is happening, in that this information is being actively ignored. This has to be our fault for failing to put the case in an easy to understand and compelling way. We need the case to be put by academics, not practitioners, in a way that cannot be ignored. Until we find a way to do this, trees will continue to be lost and communities deprived of benefits that are demonstrably improving peoples’ life prospects.

      • Tommy

        Wow, forgot about this. Too many email addresses and notificaions spread all over…

        We need people who can synthesis and channel the information in a understandable way to the ‘right people’, as you say. Something I’m certainly working on. 🙂

  • Andy Greenwood

    Jeremy,

    The problem faced in Sheffield is that a large proportion of the mature trees are causing damage to the highway, primarily the kerbstones and there is no extra money (at a time of austerity) for sustainable, long term engineering solutions such as build outs. Removing kerbstones, is a cheap short term solution that would allow the felling to be spread over the 25 years, but that is difficult to sell to highway engineers and managers concerned with delivering a quality highway repair. It is also arguably delaying the inevitable for a future generation.

    How do you consolidate your rotation length model, in a typical northern city with narrow footways and little space to allow trees to fully mature before they are threatening to encroach into the highway. Surely the lifespan of a highway tree, ignoring lack of quality soil volume, is effectively governed by the distance it is planted from the highway and its aftercare.

    I think the answer for Sheffield lies not in expensive engineering solutions to extend the life of the current trees but in planting a lot more trees to compensate for the loss in benefits.

    Having said all that, I agree that communities should certainly be given a greater say. I suspect that is the real issue in Sheffield.

  • Catriona Clark

    Trees are one of Sheffield’s saving graces. I’d also like to see new housing developments in Sheffield making provision for century old trees.

Conifer stands 450

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