Frequently Asked Questions on Measuring Carbon

Measuring carbon

How do you calculate/measure how much carbon is stored in a woodland and how much is sequestered over time?

Although it is difficult to measure the carbon in a woodland directly, we do know that about half the dry weight of timber is composed of carbon.  The Forestry Commission has therefore developed two tools help estimate the amount of carbon in a woodland.

• Predicting future carbon:  Carbon Lookup tables.  These predict the rate at which carbon will accumulate in various forest types (from a new native woodland to a fast growing energy crop) given various site conditions.  Estimates are presented in a series of ‘lookup tables’, so for any given woodland type with given site conditions, the amount of carbon that is likely to be sequestered per hectare over a given time period can be identified.  This method provides a robust and consistent approach to predicting the amount of carbon that will be stored in a woodland.

• Measuring the carbon that’s there now:  Direct tree measurements.  It is not possible to directly measure the amount of carbon in a tree, but it is possible to measure some aspects of the size of the tree (e.g. height and diameter of the stem) to give a volume of timber.  This can then be converted to a weight, and we know that 50% of the weight is carbon.  Measurements of a selection of trees in a woodland can be taken at regular intervals to show the change in volume over that time, and therefore weight and carbon.  This method of measurement is more accurate estimate of the carbon stored as the trees grow.


How do the scheme’s calculations compare to others’?

There are no existing examples of widely agreed international standards for evaluation or assessment of forest carbon projects. However, a Guide to Monitoring Carbon Storage in Forestry and Agroforestry Projects (pdf)produced by the Winrock Organisation in the United States is widely respected and referred to as an approach to monitoring woodland carbon.  The Forestry Commission protocol has been developed independently but has broadly similar structure and methods. Many of the methods also build directly on tree and woodland measurement methods which have been agreed as best practice within the UK for over 40 years (Matthews and Mackie, 2006; Mackie and Matthews, 2008). The IPCC has produced a good practice guide on estimation of land and vegetation carbon at large (e.g. national) scales, and related assessment methods have been reviewed by the EU (see the results of the MASCAREF project which looked at Harmonising Methods for Assessing Carbon Sequestration in European Forests (pdf)).  These approaches are also broadly consistent with those described in the Forestry Commission protocol (although the larger-scale methods are still under development).

The carbon lookup tables on this website have been produced from the outputs of a forest carbon accounting model called CSORT. Models like CSORT are often used to appraise the potential of woodland carbon projects, for example, the CO2FIX model has been widely applied. CSORT has been developed by the Forestry Commission as a successor to the previous CARBINE model, which was the world’s first forest carbon accounting model to be developed (Thompson and Matthews, 1989a, 1989b; Matthews, 1994, 1996). CSORT has common features of structure and functionality with other forest carbon models developed around the world such as C-Flow (Dewar, 1990, 1991; Dewar and Cannell, 1992), CO2FIX (Mohren and Klein Goldewijk, 1990; Nabuurs, 1996; Mohren et al., 1999) and CBM-CFS3 (Kurz et al., 2009). The modelling methodology behind the current version of CSORT is based on conventional forest yield models (Edwards and Christie, 1981), coupled to models of tree carbon content and wood decomposition.

References without online link:

Matthews, R.W. (1994) Towards a methodology for the evaluation of the carbon budget of forests. In: Carbon balance of the world’s forested ecosystems: towards a global assessment (ed. M. Kanninen). Proceedings of a workshop held by the Intergovernmental Panel on Climate Change AFOS, Joensuu, Finland, 11-15 May 1992. Painatuskeskus: Helsinki.

Matthews, R.W. (1996) The influence of carbon budget methodology on assessments of the impacts of forest management on the carbon balance. In: Forest Ecosystems, Forest Management and the Global Carbon Cycle (ed. M.J. Apps, D.T. Price). NATO ASI Series Vol. I40. Springer-Verlag, Berlin.

Mohren, G.M.J. and Klein Goldewijk, C.G.M. (1990). CO2FIX: a dynamic model of the CO2-fixation in forest stands. “De Dorschkamp”, Research Institute for Forestry and Urban Ecology Report 624: Wageningen

Thompson, D.A. and Matthews, R.W. (1989b). CO2 in trees and timber lowers greenhouse effect. Forestry and British Timber 19: 21-24.


How many trees does it take to capture one tonne of CO2?

Over the long-term average, taking into account the whole cycle of a commercial rotation, the sequestration rate can be as high as 3 tC (tonnes of carbon) or 11 tCO2 (tonnes of carbon dioxide) per hectare per year. 

Over a number of forest rotations, this commercially managed stand can be assumed to maintain around 100 tC (or 370 tCO2) per hectare on the site on average.  A typical conifer plantation would start out life with around 2,500 trees per hectare but some of these would be ‘thinned’ out whilst the remainder of the trees continue to grow.  They would then typically be felled, in accordance with agreed felling plans, from about the age of 40 years, and the area would be replanted. 

If the same commercially managed forest were left to grow old naturally the long term average stock, taking into account natural disturbance, is estimated to be around 170 to 220 tC (or 620 to 800 tCO2) per hectare.  See FC Information Note 48.


My emissions are happening now: don’t trees take ages to grow?

In commercial conifer crops, the trees take around 40 years to reach maturity.  As the forest goes through different phases of growth the trees take up carbon at varying rates.  During the establishment phase (approx first 10 years) carbon uptake is relatively low (and can even be negative due to carbon loss from the soil associated with ground preparation).  This is followed by the full-vigour phase, when the trees sequester carbon at a relatively rapid rate.  This levels off as the stand reaches the mature phase.  Ultimately the forest stand reaches the old growth stage where the carbon balance is in equilibrium and the carbon sequestered during growth is equal to the carbon loss through mortality.  So to summarise, trees are sequestering carbon at a maximum rate between the ages of around 10 and 40 for conifers, and over a longer period for broadleaves.

Only verified Woodland Carbon Units (ie units which represent carbon which has already been sequestered) can be used to help compensate for emissions.


How does management affect the amount of carbon a woodland sequesters?

The Woodland Carbon Code currently deals with carbon stored within a woodland, in the living biomass, dead trees, litter and debris on the forest floor and in the soil on which the woodland stands (balanced by the emissions required to establish and manage that woodland).  It does not account for carbon stored in any timber which is removed from the site.  The amount of carbon that can be sequestered and retained on a particular site will vary depending upon the management of the site:

  • Woodland planted and all trees retained:  If a woodland is created and no timber removed, then the carbon sequestered on that site increases quickly for a number of years, and once the woodland becomes mature (50-100’s of years depending on species) the carbon retained on the site remains roughly static, with increases from sequestration balanced by decreases from emissions – from decaying dead trees, litter and from the soil.  This long-term average (over 100's of years) amount of carbon is the maximum amount that can be claimed from such a woodland (see Figure 4 in FCIN48). 
  • Woodland planted and some trees extracted for timber:  If a woodland is managed and timber is extracted from the woodland, then some of the carbon is effectively removed from the site and sold, within the timber, on to another owner.  The clearest example of this is if the site is managed on a clear-fell regime.  In this case, the woodland would grow and sequester carbon for say 50 years, but then the entire above-ground timber (and carbon) stock would be removed and sold to a different owner.  The woodland would then be replanted and grow again.  In this case, it is harder to see how much carbon is stored on the site in the long-run as it varies from zero to a maximum each 50 years, each time returning to zero.  The long-term average (over 100's of years or many rotations) is the maximum about of carbon that can be claimed from such a woodland.  The long-term average tends to be around 30% to 50% of the maximum amount of carbon on the site in any one rotation, depending upon species and rotation length (see Figure 5 in FCIN48). 

Figures 4 and 5 in FCIN48 clarify this difference.


Can planting trees really help achieve the scale of change needed?

UK Government targets presented in the UK’s carbon budgets are to reduce GHG emissions 34% by 2020, based on 1990 levels.  There is a longer term commitment to reduce GHG (Greenhouse Gas) emissions 80% by 2050, based on 1990 levels.  Ideally these changes will be made by reducing our direct emissions of CO2 and other greenhouse gases.  It is estimated that if we plant an additional 15,000 hectares per year on top of the current planting rate of 8,000 hectares per year, then by 2050 these woodlands, together with those already planted since 1990, could be sequestering an amount of CO2 equivalent to 10% of GHG emissions, assuming the target above is met.  So if we plant trees now, they will be compensating for a small but significant proportion of our emissions by 2050.


Doesn’t large scale forestry create greenhouse gas emissions?

Managing woodlands does cause some emissions, in terms of fuel used by vehicles, harvesting machines, chainsaws etc.  Recent calculations suggest that in 2008 the amount of CO2 emitted during forest operations to manage the UK’s woodlands was around 2% of the total carbon sequestered.  Overall the UK’s forests currently (in 2010) soak up around 10 Million tonnes of CO2 per year.

Woodland Carbon Code projects have to account for these onsite emissions due to establishing, thinning and felling a woodland.


Last updated: 03/02/2014