We often asked by Constructco2 users, can we offset by planting trees on site and if so how much carbon is ‘offset’? The response can be complicated – but that’s not a reason not to balance construction carbon emissions through increasing living carbon in trees and other living eco systems.
Before diving in to the role of trees within managing construction CO2 – we need to understand current carbon footprint thinking – so take a look at this fairsnape blog post – Carbon Is Not The Enemy – on why we need to balance the Fugitive Carbon with Durable and Living Carbons
Trees and Carbon
The answer is not at all straight forward and publications / papers / articles found on the internet do not agree. However, the amount of carbon stored by a tree depends on its size, and age: young trees will absorb carbon dioxide quickly while they are growing, but as a tree ages a steady state would eventually be reached. At this point the amount of carbon absorbed through photosynthesis is equal to that lost through respiration and decay.
Ecometrica study found a one tonne carbon tree locks up around 3.67 tonnes of carbon dioxide from the atmosphere
A tree can absorb as much as 24kg of carbon dioxide per year and could potential sequester 1 ton of carbon dioxide by the time it reaches 40 years old.
On average, each National Forest tree will sequester 79kg of carbon, equivalent to 290kg of carbon dioxide, over an 80 year period of growth.
A recent study carried out at Kielder Forest has calculated that the Forest’s 150 million trees lock up 82,000 tonnes of carbon* annually. This means that as a rough estimate each tree at Kielder is locking up 0.546 kg of carbon per year.
It is better to offset in forests rather than individual trees asm within the UK, forest soils contain around four times as much carbon as the trees.
CO2 is absorbed from the atmosphere by trees during their growth through photosynthesis. The carbon element of the CO2 absorbed remains locked into the timber until its End of Life. The sequestered carbon should though only be considered a benefit in the scope of (any) carbon assessment when the timber is sustainably sourced – certified by FSC, PEFC or equivalent. This is to ensure that any trees felled are being substituted with a minimum of the same number of trees planted and therefore not contributing to deforestation and not compromising the overall carbon- absorbing capacity of woodlands.
Understanding definitions. The language used when talking about carbon in trees and other eco systems is important.
Biogenic carbon. The carbon sequestered in timber or other bio-based materials.If we are concerned with using trees to offset our construction carbon emissions then we need to address the tree’s sequestration and storage of CO2.
Sequestration The natural process removing (ie seizing) CO2 from the atmosphere and storing it within biological material.
Sink A carbon ‘sink’ is where there is a net transfer of carbon from the atmosphere to the (tree/forest) A forest only remains a sink while its carbon stock continues to increase.
Store Wood products are a store of carbon, as they themselves do not capture carbon dioxide from the atmosphere, but keep it locked up throughout their lifetime
Perhaps value engineering to increase materials that have a high carbon store (eg timber) in lieu of materials that have a high embedded carbon footprint through processing (eg concrete) may prove a more viable carbon option.
Importance of locking carbon into long lived, circular economy based, timber products …
When a tree dies the carbon that is stored in its biomass is either released to the atmosphere or added to the carbon in the soil through decomposition. The rate that carbon is released back to the atmosphere can be controlled by reducing the rate of decomposition, for example by using timber to create long-lived wood products. However, eventually most of the carbon sequestered by the tree will be returned to the atmosphere where each tonne of carbon will be converted to about 3.67 tonnes of carbon dioxide.
More than just Carbon
UK woodland, especially native species, in addition to providing the habitat for our incredible natural biodiversity, provide a wide range of “ecosystem services” such as the control and condition of water supplies, mitigation of surface water flooding, provision of shade, shelter, control of pollution. Woodland plays a far greater role in the move to a low carbon economy than simple carbon sequestration by trees.
- If planting (additional) trees on site obtain a carbon figure from the projects ecologist or landscape architect. (ConstructCO2 can arrange one for you). You cannot count the landscape design as offset for your construction emissions.
- A very rough figure for guidance, for each additional young tree planted on the project 1kg CO2 per month that the tree will be growing.
- Consider and promote the regenerative benefits of trees, which will be far greater than simply carbon offsets.
- If looking to offset your construction CO2 through tree planting offsets – use a certified organisation and ensure that the offset is an additional measure, and not counted elsewhere.
- Consider offsetting to schemes that protect, enhance soils and bring peat bogs and moss lands back into healthy, carbon sequestration eco systems. This can be a higher co2 sequestration than trees.
- Consider increasing project materials with a high carbon store – locking greater levels of carbon into the building through sustainability focused value engineering.
- A single tree can absorb CO2 at a rate of 12kg per year.
- Trees act as natural pollution filters by absorbing pollutants through the stomates in leaf surfaces.
- Trees lower temperature by transpiring water and shading surfaces.
- Trees reduce heat sinks.
- Trees reduce erosion.
- An acre of trees absorbs enough CO2 over one year to equal the amount produced by driving a car 26,000 miles.
- Trees provide food and wildlife habitats.
- Planting trees remains one of the cheapest, most effective means of drawing excess CO2 from the atmosphere.
- Trees recharge ground water and sustain stream flow.
- One large tree strategically placed can replace 10 room-size air conditioners operating 20 hours per day.