## 3 Calculation Methods
### Carbon Accumulation in the Litter, Dead and Soil Pools  
While tree biomass represents the main carbon sink within forestry it does not represent the total carbon sink. Net ecosystem productivity (NEP) for a forest area can be crudely simplified as the change in carbon stocks in biomass, above ground litter, deadwood and soil. The annual carbon gains through litter fall were calculated for each tree species at each individual year. Equation 2 forms the basis for this calculation. \[G_{n}=Vn\times D\times ABEF\times CF\times 0.2\times\big{(}9.6/100\big{)}\times 1.67 \tag{2}\]  
In Equation 2, G\({}_{n}\) refers to the gross carbon gained through litterfall accumulation and is measured in tonnes of carbon per hectare per year. The value '0.2' refers to the annual conifer leave turnover (1 for broadleaves). The value '9.6' refers to the percentage of above ground biomass allocated to leaves. As age increases, the leaf biomass remains relatively constant while stem wood increases; therefore, this value is reduced to 4% beyond 17 years. The value '1.67' represents an attempt to account for the wood fraction of litter fall, which was found to comprise up to 40% of coniferous forest floors: this value was changed to 21% for broadleaf floors as the wood fraction was considerably less [25]. The loss of carbon due to decomposition is more complicated due to the fact that while litter accumulation happens on an annual basis (and is therefore easily identifiable), decomposition of the litter fall from previous years occurs continually. Carbon losses due to decomposition (L\({}_{n}\)) were estimated as a function of leaf carbon litter gains from previous years. As with Equations 1 and 2 above, it is measured in tonnes of carbon per hectare per year. \[L_{n}=\sum_{1}^{15}G_{n-(n-1)}\times\big{(}1-k\big{)}^{(n-1)}\times k \tag{3}\]  
In Equation 3, an attempt was made to account for the compound nature of litter decomposition within a given year. G\({}_{n}\) refers to carbon gains within a given year, estimated using Equation 2 and 'k' refers to litter turnover or decay rate, assumed to be 0.14 yr\({}^{-1}\)[26]. Effectively, the carbon loss for any given year, n, is estimated as the gain of the current year multiplied by the turnover rate plus the net carbon gain from the fall of previous years multiplied by litter turnover. Because this is a compound method, the actual loss from a given year's litterfall will decrease with each subsequent year. If the amount available for decomposition decreases by 14% each year, then 90% of a given aboveground leaf litter fall is decomposed within 15 years. A similar method was adopted for deadwood, using whole tree biomass as opposed to above ground biomass, whereby an average annual tree mortality rate was estimated by dividing the volume of recently dead trees by the total growing stock volume; both estimates taken from [16]. This resulted in an annual mortality estimate of 0.35%. Losses due to the decomposition of deadwood were calculated in the same manner as in Equation 3. A decay rate of 0.1 yr\({}^{-1}\), as used within the Australian greenhouse gas (GHG) inventory report, taken from Mackensen and Bauhus [27], was applied. The compound nature of turnover means that 23 years are required before 90% of dead tree matter is decomposed. Estimates of soil stock change due to afforestation were taken directly from relevant literature. The 2008 National GHG inventory report [28] suggests that newly afforested organic soils emit 16 tonnes of carbon per hectare per year (t C/ha/yr) over a transition period of four years [28]. Because newly affected areas are represented within the national forest inventory as the area planted less than 10 year ago, this emission factor was assumed to apply to 40% of the planted area and therefore was reduced to 1.6 t C/ha/yr. This new emission factor was applied to only 30% of forested area under 10 years of age, as organic soils are though to represent 30% of affected areas planted from 2003 onwards.