Carbon storage: Which trees should you plant?

IN June 2019, parliament passed legislation requiring the government to reduce the UK’s net emissions of greenhouse gases by 100 per cent, relative to 1990 levels, by 2050. Doing so would make the UK a ‘net zero’ emitter.

The fact that trees have a critical role to play in the achievement of this target has been recognised by government both centrally and locally. On 5 December last year, the UK government committed to a £4 million fund to plant more trees up and down the country and made a claim that this fund would “see hundreds of thousands of new trees planted, including in towns and cities near rivers to reduce flood risk, and help meet the government’s commitment to increase planting to 30,000 hectares per year across the UK by 2025”.

A pot of £2.5 m, which must be assumed additional (although this is always confusing when considering government cash injections), has been identified to support schemes that establish new ways of planting trees in our towns and cities. The government also announced a £640 m Nature for Climate Fund to support increased tree planting.

Consultations on the England Tree Strategy have largely been completed with publication eagerly awaited. Consultations continue about the Environmental Land Management scheme which will replace the Common Agricultural Policy Grant System with urban forestry seemingly an integral part of the proposed new grant system.

Local authorities across the country have committed to extended tree-planting programmes and have set ambitious tree canopy cover targets. It is beyond the scope of this article to question whether those targets are realistic and achievable, but the fact that such targets are being set consolidates the fact that trees are recognised, at all levels within the political spectrum, as being of critical importance in delivering a wide range of public benefits, with the removal of CO2 from the atmosphere being a significant and well-recognised societal good.

The work of Treeconomics and partners using i-tree and other techniques has led the way in identifying the public goods which trees deliver for communities across the country, as well as providing valuable and detailed information on local urban forests and data which can be used to manage urban forests into the future in a sustainable way. With regard to carbon capture, the work of Treeconomics has clearly demonstrated the role trees play in removing carbon dioxide from the atmosphere by quantifying the amount of carbon sequestered and stored and putting a monetary value to the service trees provide in this respect. Details of the i-tree reports carried out in the UK can be found on the Treeconomics website (www.treeconomics.co.uk).

Mature yew in a Cornwall churchyard. How much carbon is stored here?

As always with any analysis, there are gaps in knowledge which need to be filled. With regard to trees and carbon storage the gap in the knowledge is which tree species are most efficient in capturing and storing carbon. There has not been an easily accessible guide to illustrate which tree species should be prioritised if carbon capture is the prime public good to be achieved. The question as to which species delivers what and over what time period has never been articulated in an uncomplicated way which is easy to follow by both professionals and the public alike.

It is an undisputed truth that trees and their capacity to capture carbon is scalable. The amount of carbon different species store during their lifetimes can be calculated with some confidence and the rate at which they capture and store carbon assessed using known information about species growth rates.

Over the last 10 years, measurements from thousands of urban trees have been collected in towns and cities across the UK by Treeconomics and project partners. This detailed information has been run through a sophisticated computer model called i-tree Eco.

The entire Barcham catalogue of trees has been run through the same model to produce an environmental carbon storage model for each species. The result is that all the trees in the Barcham catalogue have been graded A to E with A being the most effective and E being the least. It has also been possible to numerically grade all the trees in the catalogue in order with regards to carbon storage.

The protocol used during the work assessed each species up to 300 years of age, if the anticipated life expectancy reached that age. This means that the carbon stored by long-lived species such as oak and sequoia is an underestimation because of the 300-year ceiling.

READ MORE: Millions of trees to be planted across Glasgow by 2030

A tree tag has been produced for each individual species and can been seen in the illustrations accompanying this article. The tag clearly indicates the grade of each species in a clear and easily interpreted way.

An assessment of the growth rates of different species allowed further interpretation of the data to take place. About 50 per cent of wood by dry weight is composed of carbon and the DBH of any given species can be used to estimate carbon storage. Combining growth rate with DBH at certain periods of the life of any species provides an indication of the amount of carbon at defined periods during the species’ life. To produce the Barcham tree tags, estimations of DBH beginning at 0 and increasing by 10 up to 50 years and thereafter at 75, 100, 150, 200, 250 and 300 were calculated, and the amount of carbon potentially stored at each stage recorded. This is represented graphically on the tree tag illustrating not only the total amount of carbon stored, but a visual indication as to the rate of accumulated carbon stored at defined stages of the species’ life.

The carbon cost of production and dispatch was also built into the model allowing a ‘break-even’ point to be calculated and displayed on the tree tag. This is the point at which the carbon cost of production and dispatch is matched by the carbon stored and the point at which the species is showing net carbon gain.

It is intended that the tree tags will appear on the Barcham website next to each species as can be seen from one of the images accompanying this article.

It is also intended to include a carbon usage calculator on the website so members of the public and others can easily assess their own carbon use and estimate how many trees, of what species and over what time period they can expect to mitigate this use. While some of us have problems with the concept of carbon trade-off as opposed to a real reduction in carbon use, this form of environmental carbon credit rating does provide a means whereby individuals, both professional and non-professional, can make a realistic and informed decision as opposed to just planting trees by guesswork.

It is obvious that the Barcham catalogue contains many cultivars of species where the necessary data and knowledge of performance is unavailable or unknown. In these instances, there has been a deferment to the species type, or an educated and informed estimation made using the data available.

For further information on the Barcham Environmental Credit Rating, please contact Keith Sacre at keith@barchamtrees.co.uk.

For further information about the services offered by Treeconomics and details of i-tree projects completed in the UK, please visit www.treeconomics.co.uk or contact Kenton Rogers at kenton@treeconomics.co.uk.

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