Energy Reduction

UFORE estimates the effects of trees on building energy use and consequent emissions of carbon from power plants. Methods for these estimates are based on a report by McPherson and Simpson (1999). For each tree within 18 m of two-story or less occupied building (e.g., garages were excluded unless there was a residence above the garage), information on distance and direction from the building was recorded. Tree heights and distances from building were classified into three groups

Tree Size:
Large: tree height > 15 m
Medium: 10 m < tree height <= 15 m
Small: 6 m <= tree height <= 10 m

Tree Distance:
Adjacent: distance <= 6 m
Near: 6 m < distance <= 12 m
Far: 12 m < distance <= 18 m

Any energy tree that was smaller than 6 meters in height or farther than 18 meters from a building was considered to have no effect on building energy use.

Using the tree size, distance, direction from building, climate region, leaf type (deciduous or evergreen) and percent cover of buildings and trees on the plot, the amount of carbon avoided from power plants due to the presence of trees was calculated based on methods in McPherson and Simpson (1999). The amount of carbon avoided was categorized into the amount of MWh (cooling), and MBtus and MWh (heating) avoided due to tree energy effects. Default energy effects per tree were set for each climate region, vintage building types (period of construction), tree size class, distance from building, energy use (heating or cooling) and/or leaf type (deciduous or evergreen) depending upon the energy effect of the tree (tree shade, windbreak effects, and local climate effect) (McPherson and Simpson, 1999). Default shading and climate effect values were applied to all trees; heating windbreak energy effects were assigned to each evergreen tree; and climate effects. As shading effect default values were on given for one vintage building type (post-1980), vintage adjustment factors (McPherson and Simpson, 1999) were applied to obtain shading effect values for all other vintage types.

Tree Condition Adjustment
To adjust for varying energy effects of trees due to tree condition, the default energy effect values (McPherson and Simpson, 1999) were adjusted for the tree condition as follows:
Energy adjustment = 0.5 + (0.5 * tree condition) ( 28 )
where tree condition = 1 - % dieback.

This adjustment factor was applied to all tree energy effects for cooling, but only evergreen trees for the heating energy use effects.

Carbon Values

To estimate monetary value associated with urban tree carbon storage and sequestration, total tree dry-weight biomass was converted to total stored C by multiplying by 0.5 (USDA For. Serv. 1952; Chow and Rolfe 1989). Carbon values were multiplied by $20.30/tC based on the estimated marginal social costs of carbon dioxide emissions (Fankhauser, 1994). Standard errors given for C report sampling error rather than error of estimation. Estimation error is unknown and likely larger than the reported sampling error. Estimation error also includes the uncertainty of using biomass equations and conversion factors, which may be large, as well as measurement error, which is typically very small.