Carbon storage of trees

Carbon dioxide: how much help is it to plant a tree? (Author: Simon Maddrell)

What you get when you plant a tree is:

  • Something that will when mature, absorb and store significant amounts of carbon dioxide from the air – about 10 kg per year in the Isle of Man. In its lifetime, a tree will absorb on average about one tonne of carbon dioxide. An acre of moderately mature trees absorbs 2.5 tonnes of carbon dioxide per year, about the same as a family car might emit per year (perhaps 3.5 tonnes per year)..

The IOM Government plans to plant – or have planted for them! – 85,000 trees in response to demonstrations against fossil fuels.

When moderately mature, say in 25 years time, they might absorb 850 tonnes of carbon dioxide per year or the emissions from about 250 family cars. There are approximately 70,000 vehicles registered on the Isle of Man (40% more per person than in the UK!), so this planting might in 25 years time only absorb about 0.35% of the vehicular emissions of carbon dioxide.

Or, to put it another way, it would need about 25 million mature trees to absorb all the carbon dioxide emitted by our vehicles. At 400 trees per acre, that would require 70,000 acres of woodland, or 50% of the total area of the Isle of Man.

But vehicular emissions are only a small fraction of the total amount of carbon dioxide emissions. The total net emission for the UK is about 370 million tonnes per year, or about 6 tonnes per person per year. The comparable figure for the Isle of Man is considerably worse: 10 tonnes per head.

So it is nowhere near feasible for tree planting alone to solve the emissions problem.

  • However, by planting trees we do more than increase carbon capture; we increase the number of ecological niches for wildlife. For example, there are nearly 300 species of insect that live on an oak tree; holly, hornbeams or lime trees are much less good– see table below showing how many insect species live on different species of tree. Conifers* do not support much biodiversity on the island, though they have more insect species on them in colder climes.
  • Apart from absorbing carbon dioxide, a tree will give out lots of oxygen for us to breathe – one mature tree produces enough oxygen for two people to breathe.
  • And, curiously, a tree doesn’t slow down; rather, as it ages, it absorbs carbon dioxide faster and faster. A single large tree can add the same amount of carbon to a wood in a year as has been stored by a mid-sized tree during its entire life to date. So fully mature trees need protecting!
  • A stand of trees will act to slow run-off of water after a rainstorm and so help avoid flooding – and much reduce soil being washed away.
  • And let’s not forget trees are beautiful and can provide unforgettable sights, especially in Autumn – see the mountain ash, a native Manx tree, showing itself off.
Mountain ash (Rowan)