A letter to our Councillors

Dear Bristol City Councillors,

We recognise the fundamental importance of the natural environment, the value that nature has in an urban setting and the global threat posed by the ongoing climate catastrophe. We also recognise that trees are a crucial component in all these concerns.

We are supportive of Bristol City Council’s declaration of a Climate Emergency and an Ecological Emergency and the goals detailed in the One City Climate Strategy, including the commitment to carbon neutrality by 2030 and doubling the abundance of wildlife by 2050. We are also supportive of their commitment to doubling the tree canopy by 2046.

However, we have a real concern that the commendable words are not being matched by effective actions.

A principle aim of the BTF is to promote the planting and preservation of trees in Bristol for the well-being of its citizens, the sustainability of urban habitation, the enhancement of nature in the cityscape and as our contribution to combating climate change (see A Manifesto for protecting Bristol’s existing Urban Forest).

A recurrent concern we have is the continued loss of trees as a result of environmentally insensitive developments that are not sympathetic to the City’s declared commitments outlined above. On the other hand, the BTF supports developments that favour a sustainable environment over high density occupancy, and those that prioritise retention of existing trees.

Bristol’s policy on replacing trees lost to development – adhering to the Bristol Tree Replacement Standard (BTRS) – is widely well regarded. As such, decision makers believe that tree loss is mitigated by subsequent tree replacement. However, recent studies undertaken by the BTF have shown that this is not the case over the timescales committed to by Bristol City Council and the Green Party.

Typically, tree planting undertaken under the BTRS takes between 30 and 50 years to recover the biomass (and therefore the CO2e) lost by felling, well beyond the 10-year commitment on carbon neutrality, and even beyond dates set for doubling the tree canopy or doubling wildlife abundance.

The BTF study has been developed into a versatile online tool for calculating the extent and timescale of the carbon deficit, with a wide range of inputs. This can be accessed via the link Tree Carbon Calculator, and we encourage you to try this yourself. See also the BTF blog Tree replacement and carbon neutrality.

In the example shown here, a mature tree felled in 2020 is replaced by four trees (as per BTRS) of the same species. The carbon released (2 tonnes CO2e) is not recovered until 2064, a full 34 years beyond the date Bristol aims to be carbon neutral.

This model can also be used to determine how many replacement trees are needed to recover lost carbon within a particular timescale. In the example shown, to be carbon neutral by 2030, a reasonable expectation as this is the declared aim of BCC, the felled tree would need to be replaced by 37 plantings of the same species. Scaled up to, for instance, 500 trees, new plantings would need to number 18,500 to mitigate the lost carbon.

This new information represents a fundamental change in the evidence base for tree replacements, and emphasises the need to retain existing mature trees, and not to consider replacement by new plantings as adequate mitigation.

We request that you consider this new information with urgency and make a commitment to oppose developments where mature trees are removed and tree replacements do not deliver carbon neutrality by 2030.

Tree replacement and carbon neutrality

The UK aims to be carbon neutral by 2050. Bristol is more ambitious and aims to reach that goal in 2030. Both are massive challenges in which trees have been enrolled to play their part in mitigating the carbon dioxide (CO2) created by human activity.

Background

There are plans for extensive tree-planting.  The government pledged to plant 30 million trees a year, nationally. This a huge challenge partly because seedlings and land has to be found for these trees. However even when planted, these trees will take a long time to grow and extract CO2 from the air.  We in Bristol Tree Forum are concerned that not enough attention is given to the role of existing mature trees.  

Trees grow and add to their mass each year. Most of this mass is in the form of cellulose and lignin and about 50% of those organic compounds is carbon, obtained through photosynthesis using the energy of sunlight and CO2 from the atmosphere.  The rate at which mass is accumulated increases with age so whilst a 10 year old tree might put on a few kilograms a year, a 50 year old tree might add 50 kg.  So the older the tree the better for CO2 fixation. However mature trees are constantly under threat – from development for housing and industry, from home owners overshaded by large trees, from councils assessing maintenance costs and risks.

Here in Bristol, the Bristol Tree Replacement Standard (BTRS) is part of local planning regulations and specifies how many replacement trees are needed to be paid for by the developer and planted to mitigate the loss of mature trees. The BTRS is a very welcome and forward-thinking strategy, but is it enough to support the Carbon Neutrality goals? Should BTRS  apply also to council-owned and indeed privately owned trees for which no funded replacements are available?

The Bottom Line

In an attempt to understand how this standard works in practice, we have developed an on-line calculator to explore different scenarios.

Tree CO2 Calculator

The general conclusion from this analysis is stark:  it will take 25 to 40 years before the replacement trees are able to compensate for the loss of the mature tree.

The graph shows the scenario of the replacement of a mature tree such as a Maple with a diameter of 60 cm by the 6 trees as determined by BTRS which are faster growing but shorter lived such as Rowens.

Assuming that the original tree is felled, chipped and used as fuel in a biomass boiler (the practice in Bristol), the carbon stored in the mature tree is returned to the atmosphere within months of felling.  The replacement trees start to grow, but absorb much less carbon than the original mature tree would have done, so they take many years to catch up. In the case shown in the graph, it takes 35 years (ie, to 2055) before the new trees mitigate the loss of the original tree.

Modelling

A model of this scenario needs to take into account:

  • the rate at which different species of tree grow at different ages in different conditions.
  • the estimated mortality of the tree over time.
  • the calculation of a tree’s biomass from its girth for different species.
  • the relationship between the tree’s biomass and the amount of carbon stored.

There is a lot of uncertainly in these relationships, partly because of the paucity of data on urban, as opposed to forest, trees. Urban trees are under threat not only from natural processes and disease, but also from the vagaries of vehicles and humans. Planting sites are often less than optimal and urban trees have no support from the ‘wood wide web’.

The interactive calculator allows the user to vary the parameters of the model using the sliders. This allows the sensitivity of the overall outcome to variation in values to be tested. Different policy choices can also be explored and can be used in a predictive sense to determine the number of replacements needed to achieve a given carbon neutral date.

Summary

Documentation on the website explains the thinking behind the model in more detail, and the sources of data used. The model is still under development, in particular to make it easier to select conditions for different species and situations, and to improve the quality of the model itself. The research literature is extensive but often of limited applicability to urban conditions.

We would be grateful to receive additional or better sources of this information, and indeed any comments on the model itself at co2@bristoltrees.space.

Chris Wallace, Bristol Tree Forum