Urban forestry has increasingly been a focus of interest as a means for reducing atmospheric carbon dioxide (CO₂) for climate protection while at the same time increasing quality of life in communities. This attention parallels a growing recognition that well-planned urban forestry projects can provide multiple benefits. Given these developments, a stakeholder-driven process was launched in 2007 to develop a credible approach for accounting and reporting greenhouse gas (GHG) benefits from tree planting projects to the California Climate Action Registry. Reporting carbon reductions to the California Registry guarantees their credibility and accuracy in the eyes of regulators and investors. An important milestone in that process was reached in August 2008, when the Urban Forest Project Reporting Protocol was adopted by the Registry's Board of Directors. Although originally designed for California users, the Protocol can be readily transferable to other states and countries.

The Urban Protocol consists of two products. The first is a written set of guidelines that provide accounting, reporting, and verifying guidance for registering urban forest projects. This presentation deals with the second product, the Center for Urban Forest Research Tree Carbon Calculator (CTCC). It is an Excel spreadsheet tool that provides numerical data on carbon storage, energy savings and corresponding GHG emission reductions, and biomass volumes for common tree species in California cities. The CTCC is still in the development phase, usable for those with moderate experience with spreadsheet software, and is currently undergoing testing and evaluation.

To determine carbon storage and annual sequestration, the user enters data on the project's climate zone and the species and size or age of the tree. Tree growth curves were developed from data collected from population samples of about 900 street trees representing approximately 20 predominant species in each of six regional reference cities (a total of about 5,400 trees). Biomass equations incorporating these data are used to derive total CO₂ stored (fresh weight), total aboveground stored (dry weight), and annual CO₂ sequestration.

Additional inputs related to building heating and cooling loads and tree shade are required to determine emission reductions from energy conservation. Simulations were conducted for each reference city using different combinations of tree sizes (9), locations (24), and building vintages (3) for each studied tree species to determine effects of trees on building energy performance. Basic information on heating and cooling systems is also required. Simulation results are tabulated for later lookup by the CTCC. Changes in energy use are converted to changes in emissions based on default regional or user-supplied emissions coefficients. Emissions of carbon dioxide, methane and nitrous oxide are addressed. CTCC outputs can be used to estimate GHG benefits on a per tree basis for a single point in time for reporting purposes. A time series can be developed to forecast future benefits.

Several new or enhanced features are planned for inclusion in future CTCC versions. These include addition of co-benefits (dollar value of energy savings, hydrologic impacts, air quality improvement, increases in property values), costs of tree planting and maintenance, treatment of tree populations vs single trees, web-based delivery, real-time calculations vs. table lookups, and annual reporting and forecast modes.