Model
Overyield’s carbon model is based on an allometric equation, which estimates biomass based on anticipated tree measurements across a 30 year timeline.
Wood Density
The amount of wood material in a unit volume of wood, calculated by finding the ratio between the mass of dried wood vs green wood.
Although slight variation in wood density across time and species morphology may occur, Overyield assumes this value is consistent across 30 years.
Trunk Height
Trunk height is species-dependent, but may be management dependent as well, as certain practices such as pollarding may cease upward growth.
Branch Mass as a % of Trunk
This factor will depend on management practices, such as the intensity of pruning.
Diameter at breast height (DBH)
Many factors influence DBH, not limited to species growth rate and growth habit, tree age, environment, and management factors.
DBH is typically measured at 4.5ft.
Root Mass as % of Above
Below-ground biomass varies across species, though on average root biomass constitutes 27% of a tree’s above-ground biomass.
Soil Carbon % of Above
Carbon from leaf litter, stems, branches and root exudates may contribute to soil carbon.
This value is again species and management dependent, but may average around 7%.
Calculation
Per-tree carbon sequestered per year is determined by the numbers entered in the Carbon tab of your crop template. Years 1-30 in the life cycle of a tree are represented by the fields labeled 1 - 30, which can be adjusted for each value (DBH, Trunk Height, etc.).
This carbon data works in tandem with your design decisions (e.g. sections drawn, start year selected) and crop template data (e.g. loss events, mortality) to influence your farm’s total tree count and tree age, which in turn impacts per-tree carbon sequestered.
Calculation Steps:
1. First, trunk carbon is calculated.
BasalArea = pi * (DBH/2)^2TrunkVolume = TrunkHeight * BasalAreaTrunkCarbon = TrunkVolume*(WoodDensity/2}
2. Next, Above Ground Carbon (AGC) is calculated by adding trunk carbon and the branch mass, measured as a percentage of AGC.
BranchCarbon = TrunkCarbon*(BranchMassPercentOfTrunk/100)AboveGroundCarbon = BranchCarbon + TrunkCarbon
3. Soil carbon and below ground carbon are likewise calculated as a percentage of AGC.
BelowGroundCarbon = AboveGroundCarbon*{RootMassPercentOfAbove/100}SoilCarbon = AboveGroundCarbon*{SoilCarbonPercentOfAbove/100}
4. Finally, the values from steps 2-3 are added and convert into CO2 equivalents:
TonsCO2=(AboveGroundCarbon+BelowGroundCarbon+SoilCarbon) * CO2PerCwhere
Co2PerC= 11/3
Assumptions
When a tree dies, Overyield assumes that that carbon remains stored
Wood density is assumed to remain constant over the life of the tree
If there are 0s/empty values in the list, Overyield fills in these values by linearly interpolating between non-zero values.
For the first two years following the start year, Overyield assumes 0 carbon sequestered to allow for prep and planting.
3.667 tons CO2 per ton C is the assumed conversion equivalent (ratioale - the atomic weight of carbon is 12 atomic mass units, while the weight of carbon dioxide is 44, resulting in a ratio of 44/12, or 3.667 tons CO2: 1 ton carbon)
If you’ve defined “mortality” within your Crop Settings, dead trees that are replanted will be one year younger than their survived counterparts.
If you’ve defined any Loss Events, that percent of trees will die in that year of their life based on the cadence, start, and end year of the event entered.