Guidelines for Developing and Evaluating Tree Ordinances

Evaluating parking lot shading

Shade provided by trees in parking lots reduces excessive heat buildup which can adversely affect the local microclimate and air quality (Center for Urban Forest Research 2001a). Recognizing this fact, many cities have adopted ordinances that require set amounts of tree planting or shading in parking lots (see provision 25). Parking lot shade ordinances lend themselves readily to retrospective analysis to determine whether the goals of the ordinance are indeed being met. Greg McPherson and coworkers at the USDA Forest Service Pacific Southwest Research Station Center for Urban Forest Research evaluated shade in parking lots in Davis and Sacramento, CA. Like many other California communities, these cities have ordinances that require parking lots to be landscaped so that 50% of paved area is shaded 15 years after development of the lot.

In Davis, five parking lots were selected for evaluation. Ground surveys were undertaken to identify tree species, size, condition, and management needs. Tree canopy cover in these lots was evaluated via remote sensing techniques. Aerial color infrared photographs were taken of the parking lots. The researchers then used image analysis and GIS software to determine the percentage of paved area shaded by existing tree canopy. An overlay was created based on the original landscape plans to show the planned mature size of trees (below).

Image courtesy of USDA Forest Service, Pacific Southwest Research Station, Center for Urban Forest Research.

The researchers found that current shade coverage ranged from 8 to 45% of the paved area of the lots. Furthermore, they found that the original designs showed projected pavement shading of 18% to 47% by 15 years after development, all less than the ordinance standard of 50% shading. The Davis ordinance, which was adopted in 1979, was updated in 1997 based on information from the parking lot shade survey.

A similar analysis of 15 parking lots in Sacramento, CA (McPherson submitted) was made with the help of volunteers from the Sacramento Tree Foundation. Using ground surveys only, volunteers recorded the following data:

In addition, the researchers used original site plans on file with the Planning Department to calculate the total paved area in each lot. Shading provided by each tree was estimated assuming that crowns were circular in outline. For the 10 lots in which the trees were less than 15 years old (range 1-14 years), the researchers used empirical data on tree spread at different ages collected from the nearby city of Modesto to estimate crown diameter after 15 years. Since much of this data was based on street trees growing in residential yards, it probably overestimates parking lot tree growth.

This survey found that the ordinance, enacted in 1983, was only partially effective in meeting its goals. One of the lots over 15 years old actually exceeded the shading standard (55%), and another was close to the standard (47%). However, projected tree shade 15 years after development averaged only 21% overall. The analysis also showed that tree shading was generally lower in retail business lots than in office or apartment parking lots.

Researchers found that the following factors contributed to the failure of parking lots to meet shade standards. Several of these factors are also likely to contribute to failure of other types of parking lot planting ordinances.

Trees growing in parking lots are often stunted because soil compaction and impermeable pavement limits the amount of rootable soil volume available and because temperature and soil moisture regimes in parking lot islands are often unfavorable for tree growth. In addition, sometimes soil is treated with chemicals during the construction process (e.g., high amounts of lime) that may render it unfavorable for plant growth. By measuring crown spread of trees in Sacramento parking lots, researchers were able to determine the likely crown spread that various tree species could attain when grown in parking lots. Tree crown projection areas (i.e., area of shade provided by trees) after 15 years, as measured by McPherson's group, were considerably less than those listed in the supporting regulations.

McPherson also conducted an economic analysis to calculate both the loss in benefit value associated with the lack of compliance with the ordinance (estimated at about $2.2 million per year citywide) as well as the likely costs of various remedial actions. In addition to suggestions for improving the parking lot shading ordinance, this analysis provided insight into elements of site planning and parking lot design that could be modified to reduce total amounts of area devoted to parking and increase shading of paved areas (McPherson submitted).