Carbon sequestration calculations
The rate of carbon sequestration in a forest is approximately proportional to the growth rate of the trees. The term used by foresters is Yield Class (YC), meaning the annual increment of stem timber in cubic metres per hectare per year over the growing life of the forest.
Foresters use a system devised by Forestry Commission Research called Ecological Site Classification (ESC). This is a computer programme which allows the forester to enter site data including grid reference, altitude and soil type. The programme will advise which tree and shrub species are best suited to that site and the expected yield class for each tree species.
Thus a composite yield class may be estimated for a given species mix over a specific area. Foresters normally express YC as whole even numbers , eg 6,8 etc. However for this purpose we recommend use of YC to one decimal place, as calculated. Conversion of yield class to mass of carbon dioxide sequestered is done arithmetically, eg:
1) YC, stem timber volume, is converted to dry weight by reference to tables of wood specific density, generally in the range of 0.33-0.45 t/cu m for softwoods and 0.49-0.56 t/cu m for hardwoods.
2) The carbon content, typically 50%, is used to convert dry weight to carbon content.
3) Carbon content is converted to CO2 equivalent by a factor of 3.67, the ratio of molecular to atomic weight.
Thus a stand of Oak growing at YC6 is laying down 3 tonnes of stem dry matter, comprising 1.5 tonnes of carbon, equivalent to 5.5 tonnes of CO2 /ha/yr.
Similarly a stand of Spruce growing at YC18 is laying down 6 tonnes of stem dry matter, comprising 3 tonnes of carbon, equivalent to 11 tonnes of CO2.
In order to account for carbon in non-stem components as well as stemwood the estimates are increased by a factor known as “expansion factor” which depends on the tree species, age, management and environmental conditions. The range of values published for mature trees ranges from 1.3-1.8.
Taking a mid-range value of 1.5, the Oakwood described above is sequestering 8.3 tonnes of CO2 /ha/yr over its growing lifetime. Over time a similar or greater quantity of carbon will be sequestered in the associated soil.
The Yield Class method of carbon accounting will thus typically give a value in the order of approximately one third of total carbon sequestered. We believe this conservative approach provides a sound basis for the trading of forest carbon credits, to provide an insurance “margin” against the premature demise of all or part of the forest and the simple possibility that it may not grow as fast as originally anticipated.
Foresters use a system devised by Forestry Commission Research called Ecological Site Classification (ESC). This is a computer programme which allows the forester to enter site data including grid reference, altitude and soil type. The programme will advise which tree and shrub species are best suited to that site and the expected yield class for each tree species.
Thus a composite yield class may be estimated for a given species mix over a specific area. Foresters normally express YC as whole even numbers , eg 6,8 etc. However for this purpose we recommend use of YC to one decimal place, as calculated. Conversion of yield class to mass of carbon dioxide sequestered is done arithmetically, eg:
1) YC, stem timber volume, is converted to dry weight by reference to tables of wood specific density, generally in the range of 0.33-0.45 t/cu m for softwoods and 0.49-0.56 t/cu m for hardwoods.
2) The carbon content, typically 50%, is used to convert dry weight to carbon content.
3) Carbon content is converted to CO2 equivalent by a factor of 3.67, the ratio of molecular to atomic weight.
Thus a stand of Oak growing at YC6 is laying down 3 tonnes of stem dry matter, comprising 1.5 tonnes of carbon, equivalent to 5.5 tonnes of CO2 /ha/yr.
Similarly a stand of Spruce growing at YC18 is laying down 6 tonnes of stem dry matter, comprising 3 tonnes of carbon, equivalent to 11 tonnes of CO2.
In order to account for carbon in non-stem components as well as stemwood the estimates are increased by a factor known as “expansion factor” which depends on the tree species, age, management and environmental conditions. The range of values published for mature trees ranges from 1.3-1.8.
Taking a mid-range value of 1.5, the Oakwood described above is sequestering 8.3 tonnes of CO2 /ha/yr over its growing lifetime. Over time a similar or greater quantity of carbon will be sequestered in the associated soil.
The Yield Class method of carbon accounting will thus typically give a value in the order of approximately one third of total carbon sequestered. We believe this conservative approach provides a sound basis for the trading of forest carbon credits, to provide an insurance “margin” against the premature demise of all or part of the forest and the simple possibility that it may not grow as fast as originally anticipated.
