## 5 Discussion  
### Factors Affecting Forest Sequestration Estimates  
The method for calculating carbon sequestration due to annual tree biomass increment is relatively straightforward. A significant point of debate is the inclusion of trees younger than 10 years: many studies [34, 35] exclude the sequestration of tree stands below a certain age as it is assumed that they do not have biomass. Because trees below 10 years represent the single largest age category in Ireland, it was considered preferable to include this biomass and potentially overestimate carbon sequestration rather than exclude it and risk a significant underestimation of current annual sequestration. The inclusion of litter fall, deadwood and soil carbon fluxes represents a new addition to ecological footprinting. This is relevant to all footprint studies, as the energy footprint as currently calculated is based on the carbon stored in forest areas through annual tree growth. Because both the carbon accumulated in the litter and dead pools is estimated as a function of annual growth, and includes the compound carbon losses due to decomposition, the results are extremely sensitive to both the values used within the tree biomass calculation as well as the decay rates. One of the difficulties in modeling the decomposition of litter is that the decay of an individual piece of leaf litter or coarse woody debris will change over time. (While most of the following discussion relates to leaf litter, it is equally relevant to the dead pool). The decomposition rate constant refers to the forest floor as a whole but, it will differ from the decay rate of both recently fallen and older litter. This is due to the fact that the most biochemically labile elements will decompose first. Ideally, an annual decay rate (\(k\)) of 0.10 yr\({}^{-1}\) suggests a litter residency time of 10 years. [PERSON] and [PERSON] [36] report on an early model [37], which suggests that due to the incremental reduction in litter decay rates an individual litter fall may take a period of 5 /\(k\) to lose 99% of its original mass. The method of calculating decomposition losses within this paper was compound based but maintained the same decay rate throughout. However, it is difficult to deduce the result of such a change as it has been suggested that while broadleaf and coniferous decay rates converge over time, the changes to \(k\) leading up to convergence may vary significantly [38]. There appears to be considerable debate within the literature as to what are the main determinants of leaf litter decomposition. Indeed, the variable--and often contradictory--estimates of forest litter dynamics may negate the value of using decay constants extrapolated from experimental data. In many cases correlation between litter decay rates and external factors may not adequately reflect cause and effect and may mask the impact of potentially more influential factors, such as temperature of precipitation [39].  
The estimate for the carbon soil flux represents a conservative estimate: this value was adopted due to the large variance in soil respiration rates [40]. As NEE is defined as the difference between GPP and combined autotrophic and heterotrophic respiration, an increase in soil respiration will decrease the carbon stored in soil. [PERSON] _et al._[26] publish soil respiration estimates ranging between 991 and 564 g C/m\({}^{2}\)/yr for three different mineral gley sites (with a stand age of 10 to 31 years) and a 47 year old gleyic brown earth. [PERSON] _et al._[8] in examining European soils state that respiration, because of its variability in comparison to NEE, is the main determinant of sequestration. Because of these uncertainties, the lower value for the sequestration rate of Irish forest soil, taken from [28], was applied to all stands older than 10 years.