​​Carbon sequestration of trees​

Where are we now?

Climate change is on the rise due to an increase in greenhouse gas (GHG) emissions which is leading to extreme weather events becoming more frequent. To help combat climate change, reducing our GHG emissions is one of the many things we should be implementing into our design as engineers and architects. Building operations, building materials and construction are responsible for 39% of all carbon emissions globally which is why we at Cundall are committed to lowering GHG’s associated with our projects.

To reduce our emissions, we must determine which areas we are doing well in and which areas improvements can be made. Our civil engineering Embodied Carbon Tool is almost ready for beta roll out and will play a vital role in calculating and recording the embodied carbon associated with Cundall’s projects. Once the embodied carbon values are figured out with the most carbon-intensive areas identified, we can work towards improving on these areas.

We all must do more to reduce our carbon emissions wherever possible, work towards net zero carbon design and construction if we want to limit the effects of climate change. It is difficult for a construction project to have zero carbon emissions; therefore, we must find sustainable ways to reduce and offset carbon emissions where possible.

One way to offset carbon emissions is by planting trees to absorb and store carbon dioxide from the atmosphere. This case study has been conducted to investigate the carbon sequestration rate of different tree species with the aim of offsetting the embodied carbon emissions associated with construction projects and calculating the time it would take to do so.

What did we find out?

Putting a value on the amount of carbon sequestration by tree species is challenging due to the influence of various factors such as site conditions, climate, age of the tree etc. Accurate measurements could only be calculated following site-specific studies considering the different factors such as the age of the tree, environmental conditions, site management practices and so on.

Existing information

A tree is generally 50% water and 50% dry mass, approximately 47.5% of which is carbon. Therefore, it can be said that the amount of carbon that a tree absorbs depends on its mass. The EcoTree organisation has calculated that on average a tree absorbs 25kg of CO₂ annually. As trees grow quicker when they are younger, they will absorb CO₂ faster, however during their lifespan the density is far greater leading to more CO₂ being absorbed. EcoTree have stated that their figures give an average range of 10 to 40kg of CO₂ sequestration over the lifetime of different tree species. For example, the Swamp White Oak can live for over 300 years compared to the Pin Oak (~150 years) or Paper Birch (typically 60-90 years), and therefore will have a higher CO₂ sequestration over its lifetime.

Studies are limited when it comes to carbon sequestration by species. However, one investigation called ‘Trees for Carbon Sequestration and Wildlife Support’ by Anderson (2018), has found that in a 50-year period the White Swamp Oak tree has the highest value of ~6804kg (or 15,000lbs) CO₂ and the lowest being the Ginkgo tree with under ~453kg (or 1000lbs) CO₂. It is determined that the disparity in CO₂ absorption between these two trees is a result of the growth rate with the White Swamp Oak tree growing relatively quickly (2-3ft per year) compared to the Ginkgo tree (1-2ft per year).

Factors to Consider

The rate at which a tree absorbs CO₂ depends on other factors such as the weather conditions and the type of soil in which the trees grow from. When planting trees with the goal of offsetting carbon emissions, careful planning is required considering which species (or combination of such) can store the most carbon and which areas are best for planting. In some cases, the planting of trees could end up having a negative impact on the carbon sequestration of the proposed location. An example of this would be the Scottish peatlands which store a huge amount of carbon and if trees were to be planted here it would prompt draining of the peatlands and therefore result in more carbon released than that of which the forest would be able to absorb.

Younger trees growing absorb carbon and can grow faster by using the additional carbon in the atmosphere, but mature trees have limited growth capacity due to environmental factors. When these mature trees have reached their growth limit, they transition into low absorption state and the extra carbon is cycled through the soil and released back into the atmosphere via the trees themselves or fungi and bacteria in the soil.

It was previously thought that having older trees removed and replaced with younger trees would lead to more carbon absorbed as the younger trees grow quicker and absorb more carbon in the earlier stages of their lifecycle. However, a study by Imperial College London researchers showed that the carbon released by soil and rotting wood outpaces the carbon absorbed by new growth. The results showed that felled areas continue to be a source of carbon even 10 years after felling has occurred which prompts the need for careful consideration when planning to maintain forests.

Current Emissions

According to Statista, the average global carbon emissions in 2022 were estimated at 37.15 billion tonnes. This is expected to rise in subsequent years but let’s assume that it remains steady. With the above figures from Anderson's investigation taken into account; we would need to plant over 5.4 billion White Swamp Oak trees to offset one year’s emissions. However, the offset wouldn’t be realised for 50 years. Considering the carbon sequestration value for an average tree being 25kg a year, 1.486 trillion trees would be required to offset our annual GHG emissions , which to put into context is equivalent to four times the coverage of the Amazon rainforest.

CO-benefits

As detailed above, the climate crisis cannot be planted away, although it is just one of many ways in which we can reduce CO₂ in the atmosphere ultimately reducing the effects of climate change.

Trees can also be used in other ways to tackle climate change and one of these is using trees to mitigate ‘heat island’ effects. Heat islands are locations which have higher temperatures due to the high density of pavements, buildings and other materials that absorb and retain heat. These effects can be mitigated by planting trees to reduce the density of these heat absorbing materials, and in some cases providing shade which can also reduce the temperature significantly. Reduced temperature results in reduced energy demand (from air-conditioning etc.), reduced air pollution levels and a reduction in other issues that arise from high temperatures.

Further study

One study by The Carbon Community commenced in May 2021 which aimed to ‘uncover a new reforestation approach, rooted in science, to accelerate and enhance the sequestration of carbon dioxide in trees and soil to tackle the climate crisis. This study looked at 9 different randomised test cells (400 trees in each cell) replicated 8 times to allow for ‘changes in microclimate and statistical validity’. The first two years of the study measured the carbon stored in the soil and trees, with the results being used to identify which combination of treatments that allow for the most sequestration of carbon. The results of such are yet to be published but yearly measuring has been taking place and you can keep up to date on the progress here.

Further study is required to provide an accurate estimation of the carbon sequestration for each tree species. Even then, an average of values may only be possible due to the factors that influence the results such as ground conditions, environmental conditions, site/forest management practices and so on. The suggestion that carbon released by soil and rotting wood outpaces the carbon absorbed by new growth prompts the need for consideration of what happens with a maintained forest where carbon dioxide is removed from the atmosphere and into the earth which will eventually release carbon from end-of-life emissions from the timber product produced.

References

https://ecotree.green/en/how-m...
https://www.yourgardensanctuar...
https://www.science.org/doi/10...