Trees and CO2

CO2 in our atmosphere has been a hot topic for some time but the evidence of its effects on our trees, and the way in which our trees can mitigate its effects, is slowly coming to the fore. The following information is a collection of facts outlining how increased levels of CO2 may be both a bad thing and a good thing for the future of our forests and woodlands.

Trees as carbon sinks:

  • Trees require CO2 to live. In the complex process of photosynthesis the tree will take CO2 from the surrounding air and use the sunlight to break the molecule into it’s carbon and oxygen parts.
  • The oxygen is released back into the atmosphere and the carbon is used in a variety of essential living processes including the creation of sugars (carbohydrates) and the creation of substances, like cellulose, that are used in the formation of new cells and ultimately the formation of woody tissue, this is called ‘carbon sequestration’.
  • A tree will also produce CO2 in the process of respiration where by oxygen is used to release energy stored in the carbohydrates (sugars), much the same way as humans.
  • A tree will take more CO2 from the atmosphere in the process of photosynthesis than is released in respiration and therefore a tree is locking the carbon in it’s cells and is referred to as a ‘carbon sink’. It’s a kind of savings account.
  • The CO2 taken in compared to the C locked in the tree’s tissues is measured at around 3.7 parts CO2 to 1 part C, meaning for every 100 tons of CO2 taken in by a forest it will lock approx 27 tons of carbon in its mass.
  • The living carbon storing mass i.e. wood, leaves and roots is referred to as ‘biomass’ but in temperate regions this may only account for 25% of carbon stored by forests.
  • As much as 65% of carbon stored by temperate forests may be in the surrounding soil and organic litter. The carbon in the substances like cellulose and lignin, especially, are slow to break down in the soil therefore accumulate greatly.
  • The remains of dead tree parts such as dead branches, fallen or standing deadwood and dead roots account for the remaining 10% or ‘sequestered’ carbon in the forest.
  • In 2009 it was estimated that UK woodlands contained a carbon stock (soil and biomass) of 790 MtC (million tons of carbon) with a further 80 MtC contained in harvested timber (products).
  • It’s worth noting that carbon is also stored in harvested timber such as that used to replace high energy requiring building materials such as concrete and steel. Providing the timber doesn’t travel to far from plantation to site and the trees are restocked this is highly sustainable.
  • The future – Although the rising CO2 levels may be posing a significant risk to our climate, the fact that climate experts along with international governments are recognising the importance of creating and maintaining forests and woodland to at least mitigate the increase in atmospheric CO2. This is good news for the fact that we should see a increase in forest cover proportionate to land mass should the goals be met. Ultimately this could mean more trees!

Acid rain:

  • Acid rain is a rain that has a low pH and is therefore deemed acidic.
  • The acidity in this rain is caused by a reaction between the water molecule (H2O) and a the  substances sulphur dioxide (SO2) and nitrogen dioxide (NOx).
  • The reaction between these substances and the hydrogen in the water molecule causes the hydrogen to become ionised. High levels of hydrogen ionisation is what we recognise as acidity. The resulting substances are sulphuric and nitric acid.
  • Substances such as SO2 and NOx are generally in the emissions from human activity, such as factory production, the burning of fossil fuel for energy and soil disturbance from building or excavating, although some of these substances are naturally occurring in such instances as volcanic eruptions and electrical activity in the atmosphere (lighting).
  • Acid rain has negative effects on forests worldwide by increasing the acidity of the soil to damaging levels.
  • Increase in soil acidity causes aluminium (Al) to become soluble in water. High levels of Al makes it very hard for tree roots to take up essential water along with nutrients vital for growth and healthy function.
  • The acid solution has the ability to break down essential nutrients in the tissue of leaves and needles which can hinder photosynthesis and therefore food production.
  • This combination of foliage damage and water/ nutrient uptake impediment can lead to stress in the individual tree leaving them susceptible to drought and infection. These secondary problems are usually the cause of decline and death of the trees and forests.
  • Wind can carry these substances for many hundreds of mile before they condense to fall as rain.
  • The future – it’s reported by the UK based ‘National Atmospheric Emissions Inventory’ that SO2 emissions have been steadily decreasing since the 1970’s and were 93% lower in 2009 than they were in 1970. The ‘European Environment Agency’ have reported that between 1990 and 2011 NOx emissions have decreased by 44%. These figure are promising in that humans are aware of the damage caused by acid rain, but what the long term effects of high soil acids will have on the future of our forests is not so easy to calculate. I hope that the natural buffering ability of the forest soils will be enough to keep forests in good enough health until a time when acid rain is a thing of the past.

Increased atmospheric CO2:

  • CO2 levels as of April 2014 were recorded as 401.3ppm which equates to around .04% of the atmosphere. This may sound low but it’s believed that CO2 levels are between 25% and 39% higher now than in 1800, and probably higher now than at any point in the last 20 million years.
  • The sudden rise in CO2 levels coincides with the dawn of the industrial revolution (turn of the 19th century) as humans began to intensively burn fossil fuels, such as coal, to produce energy.
  • Fossil fuels basically refers to organic compounds high in carbon arising from dead animals and plants that have, over millions of years, been gradually buried deep in the earth by the natural layering process that occurs at the surface.
  • The high carbon fuels, such as coal and oil, once brought to the surface and burned (burning refers to the oxygenation of a substance) the resulting product is CO2.
  • CO2 is a potent greenhouse gas that act to trap ever increasing amounts of the heat radiation from the sun in the earths atmosphere rather than letting it escape to space. This trapping of the suns energy acts to increase the mean temperature of the atmosphere.
  • Research has shown that the cheapest and easiest way to remove this CO2 from the atmosphere, or at least mitigate the excess, is by the creation and maintenance of forests.
  • The future –  Through guidance from United Nations Framework Convention on Climate Change (UNFCCC) and agreements set out by Kyoto Protocol, as well as local bodies such as the Forestry Commission there are goals set for the reduction in CO2 emissions and the mitigation of inevitable emissions by the creation and maintenance of forests and woodlands. Research is showing that, besides a drastic reduction in CO2 emissions by human activity, trees may be the only effective way in which we will off-set and reduce atmospheric CO2. It therefore seems common sense that we will have more trees and better maintained forests and woodlands in the future (wether common sense prevails is another thing).
  • Not to pretend that increased CO2 is a good thing but research recently published in ‘Nature’ has suggested that increased levels of CO2, which is the substance used in the trees’ production of food, are increasing the growth rates of trees by as much as 70% in a stand of trees in Germany that has been monitored closely since the 1870’s. Providing adequate water is available, fast growth rates may be good for locking away carbon more quickly but it may not necessarily be good for the quality of timber products.

Carbon offsetting programmes

  • The UK Government has committed to reduce greenhouse gas emissions by 80% by 2050 using the 1990 data taken on atmospheric CO2 and equivalents (referred to as CO2e).
  • One way in which they will try to do this is by setting budgets for specific periods e.i. 2013 to 2017 they intend to produce no more than 2,782 MtC.
  • To meet the quoters they are placing greenhouse gas emission (GHG) allowances on large public and private sector organisations meaning that these organisations will either have to stay within the allowance, enter into offsetting programmes or face being charged per ton of CO2 (currently £12).
  • Carbon offsetting programmes are a way in which a business can voluntarily invest money into Woodland Carbon Code approved (GHG) projects, such as reforestation projects, as to meet their allowance or offset any overstep of their allowance.
  • It’s possible that by 2016 all large companies will have to have their GHG emissions monitored.
  • Offsetting schemes are also available to individuals who voluntarily wish to neutralise their annual carbon footprint. ‘The Guardian’ suggests that the typical British family could offset their annual footprint for around £45.
  • The future – It’s very difficult to say for sure if this method of reducing the net carbon emissions from big business will work and wether the government will meet their targets. I would say that any scheme that promotes the planting of trees is good thing, but if it turns out that “offsetting” just acts to allow big business to invest money into schemes so they can continue polluting, only for the projects into which they invested to never come to fruition then our atmosphere and our forests will be no better off.

by Ben Brooker (


  1. “Forests and Climate Change” – Forestry Commission
  2. “Trees: the carbon storage experts” – Dept. of Environmental Conservation –
  3. “Essential soil science” – M. R. Ashman and G. Puri
  4. “Acid rain” – Wikipedia –
  5. “Acid rain” – National Geographic –
  6. “Nitrogen oxides (NOx) emissions (APE 002) – Assessment published Jan 2014” – European Environment Agency –
  7. “Carbon dioxide in Earth’s atmosphere”- Wikipedia –’s_atmosphere
  8. “Another benefit of climate change and increased CO2 – trees continue to grow at a faster rate” – Anthony Watts –
  9. “Reducing demand for energy from industry, businesses and the public sector” – UK Government ––2/supporting-pages/crc-energy-efficiency-scheme
  10. ”Reducing the UK’s greenhouse gas emissions by 80% by 2050” – UK Government –
  11. “The Woodland Carbon Code and sequestration in context” – Forestry Commission –
  12. ”A complete guide to carbon offsetting” – The Guardian –