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Tree Ordinance Guidelines
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:
- tree species
- DBH
- average crown diameter (determined by measuring canopy two radii at 90
degrees to each other to the nearest 0.5 meter by tape)
- the percentage of crown that shades parking lot pavement to the nearest
25% excluding overlapping shade
- management needs
- vacant planting sites.
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 shown on plans were not planted or else were removed shortly after
planting. This was a particular problem near store fronts where business owners
feared trees would block their signs.
- Tree species planted in lots were not those shown on plans.
- Crown diameters listed for tree species in the supporting regulations were
larger than trees would actually attain under parking lot conditions.
- There was no crown spread data in the supporting planning regulations for
some of the species commonly planted in parking lots.
- Submitted parking lot plans used the wrong crown spreads for the trees in
the plans and the errors were not caught by the Planning Department.
- Tree shade was overestimated in cases where overlapping shade from adjacent
trees was counted twice.
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).
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