Are street trees worth their weight in carbon?

Trees in hard paving require extensive below ground infrastructure. © Jai Warya

Trees are always a good thing – or are they? Jai Warya argues that it is necessary to try to quantify and compare the carbon emissions of planting a typical street tree with its anticipated carbon sequestration before claiming it is 'good for the environment'.

In August 2021, in the middle of the COVID pandemic, the New York Times published an article about the harm caused by the production of cotton tote bags; the crop’s water intensiveness, the challenges faced in recycling them due to PVCbased logos, the carbon footprint of textile mills, and the connections with forced labour in major cotton-producing economies. As someone who has at least half a dozen tote bags, I read the article with a growing sense of alarm and eventual dismay. Was nothing sustainable? Is there nothing we can do as consumers that is not harmful to the environment?

Apart from their sturdiness, the popularity of cotton tote bags can also be attributed partly to the sense they created in users of ‘doing the right thing for the planet’ by eschewing fossil fuel based, single use, non-recyclable plastic bags that have contributed significantly to the ‘plastification’ of the oceans and the killing of marine life. Now, however, it has become clear that the picture is much more complicated; cotton totes may not find their way into the stomachs of fish, but they also tend not to find their way to recycling plants.

Around the same time, with the fourth Extinction Rebellion protests in London reaching a crescendo, I began coming across several articles, podcasts and talks by prominent practitioners exhorting fellow architects and landscape architects to plant more (and more diverse) trees. Indeed, a recent study by Forest Research and DEFRA valued existing UK trees planted outside woodland as providing benefits worth £1.39 billion and £3.83 billion annually.¹ What’s not to like?

Well, as it was with the cotton tote bag, the devil lies in the details.

In the period between 2021-2022, the UK Forestry Commission recorded an estimated 2,255 hectares of new woodland planting in England, of which 2,022 hectares (approximately 3.4 million trees) was supported by Government funding.² Defra claims to have increased tree planting and woodland creation in England to c.2,700 hectares in 2021-22, corresponding to approximately 4.5 million trees.³ Big numbers, and potentially big carbon sequestration.

Compare this with the UK Government’s Urban Tree Challenge, launched in May 2019 with the stated goal of planting 130,000 trees across England’s towns and cities by 2021. Smaller numbers, and therefore a smaller amount of carbon sequestration, one would think. Yet this would only be the case if one assumes that planting urban trees actually involves net negative carbon emissions, something that has not actually been proven in any study. In fact, with a significant percentage of urban trees being planted in plastic-based structural soil cells underneath pavements, I suspected that the net result of extensive tree planting in urban areas might be to add to atmospheric carbon rather than sequester it, at least in the short term. In order to test this hypothesis, it became necessary to try and quantify and compare the carbon emissions of planting a typical street tree with its anticipated carbon sequestration.

Avenue of Trees in soft margins, Manzanares Park, Madrid

Planting trees within hard surfacing requires extensive underground and above ground infrastructure, a lot of which is ultimately invisible to the public. This includes plastic soil cells, stone aggregate, geosynthetic membranes, metal guying wires, anchor plates, irrigation and aeration pipes, grilles, pavers, metal or mineral edging and sometimes metal guards. Each of these elements is manufactured separately, transported to suppliers, procured by the contractor, transported to the site and then assembled; a process involving significant amounts of carbon emission. With a number of these items being petroleum-based products, the actual carbon footprint of the process is likely much higher, starting the moment the ground is disturbed to extract oil from it. Consider also that a large percentage of trees planted in the UK are transported from European nurseries along with the soil they are grown in, entailing further carbon emissions. If the street tree being planted is of a larger variety, there are additional emissions from mechanical forklifts. Furthermore, proper relocation of a tree from any nursery to a tough urban environment should involve a very healthy watering regime, especially if occurring in summer time, thereby increasing the tree’s carbon footprint even more.

And that’s leaving out many things a tree actually needs to survive and thrive; soil, water, nutrients in the form of compost, additives such a mycorrhizal fungi and biochar. It is not always easy (or possible) to quantify the amount of carbon involved in using these products. Apart from biochar, perhaps, it is safe to assume that procuring, supplying and applying these items to a tree pit results in net positive carbon emissions.

On the other hand, trees do sequester substantial amounts of carbon as well. Quite how much they do is not very clearly established in research, as it depends greatly on the species, foliage type, age and health of the tree. While mature trees with larger canopies can sequester more carbon due to their greater surface area, younger trees that grow rapidly may photosynthesize faster and therefore sequester more carbon per square metre of canopy. Evergreen trees can photosynthesize (and thus sequester) all year round, while deciduous trees can only do so from spring through autumn, and both drop plant material in the form of either leaves or needles which, when they biodegrade, add carbon back into the atmosphere. In short; it’s complicated, and there is no definitive figure one can attach to how much carbon a typical tree sequesters per year or in a lifetime.

The most common figure that crops up when researching the subject is approximately 20kg CO2 per annum, for a mature broadleaf tree. But this figure is by no means universally accepted. The European Environmental Agency estimates sequestration at 22kg per annum; the United States Department of Agriculture cites the Arbor Day Foundation’s figure of 48 pounds (21.8kg) per annum; Ecotree, a Danish B-Corp Company focused on sustainable forestry, states 25kg per annum; and Viessmann, a UK boiler company, states 21kg per annum. All these figures are contested and should be taken with a massive pinch of salt. For the calculations in this article, I used the figure of 25kg CO2 per annum because the purpose was to compare the best-case sequestration scenario with the best-case emissions scenario.

As a control case, I decided to calculate the carbon emissions of a hypothetical tree planted in a pavement using structural soil cells. I assumed the pavement to be three metres wide and the street trees to be planted in hard paving at 10m centres, allowing each to grow to a canopy size of 10m diameter. For the tree species I selected Platanus x hispanica (London Plane); a fast growing, long-lived, stately species resistant to pollution and tolerant of most soil conditions, making it a very popular and commonly planted urban street tree. A Plane tree growing to and being maintained at a canopy size of 10m diameter would require, as per advice from the two most commonly specified soil cell suppliers, approximately 50 cubic metres of soil to enable its survival and growth, equating to about 333 plastic soil cells measuring 0.6 x 0.5 x 0.5m. Having the space to accommodate such a high quantity of cells is very rare unless one is designing a plaza or squares, so one has to ultimately settle for what will fit in the available area. In this case, a three-metre-wide pavement would only allow for a 3m x 10m wide grid for each tree, consisting of 120 cells providing 18 cu.m of soil.

Trees in soft beds, Tate Modern, London

© Jai Warya

In a nutshell, with a single plastic soil cell emitting 38kg of Carbon per installation according to a Building Research Establishment (BRE Group) calculation⁴ in 2017, this Plane tree would require approximately 180 years to sequester the carbon that was emitted to put it in the ground. While this estimate includes the manufacturing, moulding and transportation emissions of the soil cell, it does not include the carbon emitted in transporting the tree itself from a nursery, the imported topsoil, the paving around it or any of the other products and activities mentioned earlier that are required to successfully install a tree in a pavement, so the actual situation is likely to be even worse. Information about carbon emitted for many of these products and processes is difficult to come by, but it is safe to assume it would add significantly to the ‘break even’ sequestration figure for a tree. Note also that the 25kg per annum carbon sequestering figure used here is for a mature broadleaf tree, while street trees are rarely, if ever, planted larger than semi-mature specimens.

How, then, should landscape architects be thinking about the argument for planting street trees? Trees provide many benefits other than carbon sequestration; they create shade, cool micro-climates, provide habitat and food to wildlife, help reduce flood risk, reduce air pollution, provide visual amenity, connect people to nature, and they do all of this without significant costs to councils and developers. Thus, despite it seeming nearly impossible for a tree planted in hard paving to sequester enough carbon to balance out the carbon emitted during its installation, it is still essential that we redouble efforts to plant more street trees. While plastic cells remain the best way of providing street trees sufficient soil volume because they most closely approximate natural soil mixes below ground, it is clear that trees planted in soft beds will always be environmentally more sustainable than those planted within paving. However, urban street configurations rarely allow adequate space for such a scenario, so it is essential we find alternative ways of minimising carbon emissions. One way would be to explore substitutes like structural soil wherever feasible, though this approach brings its own limitations that are beyond the remit of this article. Another possibility would be for local authorities who are responsible for street tree planting to commit to offsetting the carbon emissions of street trees by planting a commensurate number of trees in soft beds, be it in urban parks and gardens or in the countryside. Either way it is of the utmost importance that the profession is clear with its clients and collaborating professions about the benefits as well as harms of street tree planting. Yes, it provides multiple benefits to the local urban environment and to us as urban residents, but carbon sequestration is not necessarily one of them.

References:

1 https://www. endsreport.com/ article/1807177/treesplanted-outsidewoodlands-valued38-billion-annually

2 https://assets. publishing.service. gov.uk/government/ uploads/system/ uploads/attachment_

3 https://deframedia. blog.gov. uk/2022/09/30/ daily-mail-andindependentcoverage-ongovernment-treeplanting-targets/

4 This figure will vary for each manufacturer, and could be brought down through the adoption of more sustainable supply chains, renewable energy use at factories and the use of recycled or upcycle raw materials

Jai Warya

A Landscape Architect at Allies and Morrison.