Appendix IV. Notes on Methodology
i-Tree Eco is designed to use standardized field data from randomly located plots and local hourly air pollution and meteorological data to quantify forest structure and its numerous effects, including:
- Forest structure (e.g., species composition, tree health, leaf area, etc.).
- Amount of pollution removed hourly by trees and its associated percent air quality improvement throughout a year. Pollution removal is calculated for ozone, sulphur dioxide, nitrogen dioxide, carbon monoxide and particulate matter (<2.5 microns).
- Total carbon stored and net carbon annually sequestered by trees.
- Effects of trees on building energy use and consequent effects on carbon dioxide emissions from power plants.
- Structural value of the forest, as well as the value for air pollution removal and carbon storage and sequestration.
- Potential impact of infestations by pests, such as Ash Dieback, Asian Long-horned beetle and a variety of threats to oak populations.
The 0.04-hectare plots were randomly distributed. All field data was collected during the leaf-on season to properly assess tree canopies. Within each plot, data collection includes land use, ground cover, stem diameter, height, crown width, percent of crown missing, percent dieback and condition.
Once the data has been uploaded to i-Tree, the software is able to determine current carbon storage, biomass for each tree which was calculated using equations from the literature and measured tree data. Open-grown, maintained trees tend to have less biomass than predicted by forest-derived biomass equations. To adjust for this difference, biomass results for open-grown urban trees were multiplied by 0.8. No adjustment was made for trees found in natural stand conditions. Tree dry-weight biomass was converted to stored carbon by multiplying by 0.5.
To estimate the gross amount of carbon sequestered annually, average diameter growth from the appropriate genera and diameter class and tree condition were added to the existing tree diameter (year x) to estimate tree diameter and carbon storage in year x+1.
The amount of oxygen produced is estimated from carbon sequestration based on atomic weights: net O2 release (kg/yr) = net C sequestration (kg/yr) × 32/12. To estimate the net carbon sequestration rate, the amount of carbon sequestered as a result of tree growth is reduced by the amount lost resulting from tree mortality. Thus, net carbon sequestration and net annual oxygen production of trees account for decomposition.
Recent updates (2011) to air quality modeling are based on improved leaf area index simulations, weather and pollution processing and interpolation, and updated pollutant monetary values.
The Unit Value is determined by the CAVAT steering group and published online. The Unit Value for 2023 is £24.59. DBH is taken directly from the field measurements. The CTI rating is determined from the London Tree Officers Association approved list and is calculated on an area by area basis. Functionality was calculated directly from the amount of canopy remaining from field observations. For the purposes of this report, trees with data entered only at genus level were not represented in the figures so as to more accurately represent species level results.