Monday, 18 November 2019

Trees and carbon storage

Tree planting seems to be a hot topic in the election at the moment, with the Tories and the Lib Dems both offering lavish promises. The Committee on Climate Change also advises tree planting. How much difference will this make to climate change? How much carbon is stored in a tree, or in an area of forest and how much is added to this each year? Read on for the answers.


How much carbon is stored in a tree?
Trees store carbon, not carbon dioxide but in this article I use carbon dioxide units throughout, or CO2e (carbon dioxide equivalent), since that is what we normally use. In the wood itself, you can assume about 0.9 kg CO2e for each kg of green wood (50% moisture).

Whatkg CO2eNotesSource
One oak tree, 222 years old, 23.9m tall
18,000
This includes the root system as well as the branchesSylva foundation ‘One Oak’ project [1]
Average tree in London
281
Inner London trees are on average a little larger and store moreLondon’s urban woodland [2]
Average tree carbon storage per year
9.2
dittoditto


As for how much a tree weighs, well there is a nice project measuring a single oak tree ‘One Oak’ that grew in Blenheim Palace and was studied by the Sylva Foundation. The tree in question was 222 years old, 23.9m high, 18m wide and weighed 7.9 tonnes (above ground wood, dry mass) plus another 2.1 tonnes for roots. This is equivalent to 18 tonnes CO2e.

From the point of view of offsetting our emissions, the rate of carbon storage is much more important than the stored carbon. The ‘One Oak’ tree has stored 81 kg CO2e/year on average. However, the rate of storage depends on tree size and that was a big tree!

Modelling carbon in trees is not a trivial exercise as it is hard to weigh a tree. The ‘OneOak’ tree was cut down and weighed in pieces (and 50 seedlings from acorns from the same woods were planted to replace it). i-Tree is a tool for estimating carbon uptake and other benefits from trees that are living; one of the case studies on its website is for a study of London. Volunteers surveyed 724 sample plots, each of 400m2, recording the number and species of trees, the height including trunk diameter and the size of the canopy, also the health of the canopy, the type of land and the ground cover. Using this data and the i-Tree tool they estimated the stock of carbon in the whole area of the city. It seems London has about 8.4 million trees containing 2.4 million tonnes of CO2e. On average, each tree stores 9.2 kg CO2e/year. However the biggest benefit of these trees is in removing pollution. This was valued at £126 million/year compared to £5 million for the carbon storage [2].

How much carbon in a hectare of woodland?
One hectare is 10,000 m2  or approximately 2.5 acres.

Whattonnes CO2eNotesSource
Carbon stock in 1 ha woodland
1,200
CO2e equivalents, for all biomass including soilForest statistics [4]
UK GHG emissions per person per year
7.0
As of 2018. Population 66 million, GHG emissions from all sectors except land use change and forestryUK GHG emissions Inventory [3]
Carbon stored annually, per ha of woodland
6.7
This is based on the estimated increase in carbon storedForest statistics [4]
Carbon stored/ha including wood products
8.1
My estimate based on the mass of wood sent to saw mills.My estimate


The per tree carbon statistics above only count the carbon in the tree itself. In a mature temperature woodland there may be three times as much carbon stored in the soil as in the trees.

In the UK, as of 2015, there was about 3 million ha woodland and stored carbon (including soil carbon) was estimated to increase by 21 million tonnes CO2e/year [4]. That means about 6.7 tonnes/ha/year stored.

In addition, another 11 million tonnes green wood was harvested and some of this was used for making furniture or for building which could be considered carbon storage. Around 6.5 million tonnes was sent to saw mills but there must be some wastage, say, 5.5 million tonnes of this was stored – that implies an additional 1.7 tonnes CO2e/ha/year.

Based on this estimate, each UK resident would need about 0.9 ha of woodland to offset their GHG emissions each year.

How much carbon is stored in UK woodland?

There are 66 million people in the UK so, we would need 59 million ha of woodland to offset all our emissions – but the land area of the whole of the UK is only 25 million ha. In fact the annual carbon storage in the whole of the UK woodland amounts to less than 6% of our annual emissions.



Whatmillion tonnes CO2eNotesSource
Carbon stock in UK woodlands
3,200
As of 2015. About 70% is stored in the soilForest statistics [4]
Annual UK GHG emissions
460
All sectors excluding land use, land use change and forestryUK GHG emissions Inventory [3]
Annual carbon storage in UK woodland
21
Change in woodland inventoryForest statistics [4]
Annual Carbon storage in UK woodlands including wood products
26
Estimate based on the mass of wood sent to saw mills.My estimate

There is no way that the UK can offset all of its emissions by planting trees. However, as we get close to zero emissions, trees could play a significant part in offsetting the remainder. Also, carbon storage is by no means the greatest benefit of tree planting. Trees can clean the air in cities, support biodiversity, support agriculture, improve human health – and well they are just nice.

[1] One Oak (Sylva Foundation)

[2] Valuing London’s Urban Woodland (i-Tree case study) 2015

[3] 2018 UK GHG Emissions (www.gov.uk)

[4] Forestry statistics 2018 (Woodland Research)


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