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.
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.
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.
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 email@example.com.