Urban Forest
Research
January 2002
Center for Urban Forest Research • Pacific Southwest Research Station • USDA Forest Service
EPA Adopts
Center’s Parking Where Are All the “Cool” Parking Lots?
Can you think of a parking lot
where you can always find a parking
Lot Research
space in the shade? It’s not easy, so
When the U.S. Environmental
Protection Agency (EPA) needed
data on how well shade reduces pollution from parked cars, they went
to parking lot shading studies of the
Center for Urban Forest Research.
The EPA was looking for ways to
cut pollution from vehicles in the
Chicago, IL ozone-nonattainment
area where parked vehicles contribute approximately 5.2 tons of
volatile organic compounds (VOC)
per million vehicles per day.
Because no studies were found
for locations in the midwest, calculations from studies in California
had to be adapted for conditions in
Chicago where the average ozoneseason temperature is approximately
82°F rather than the study temperature of 104°F. In addition, Chicago
receives less direct sunlight, so the
shading effect is less.
By extrapolating from the pilot
studies in Davis and Sacramento CA,
the potential reduction in VOC was
estimated to be 0.645 to 1.132 lbs
per 1000 vehicles parked per day in
parking lots with a 25% and 50%
canopy of shade, respectively.
“Effects of tree cover on parking
lot microclimate and vehicle
emissions” can be found at http://
cufr.ucdavis.edu. EPA contact:
Steve Marquardt, 312-353-3214.
last year we investigated this issue.
Parking Lot Shading Ordinances
Parking lots occupy about 10% of
the land in many of our cities. Since
the energy crisis in the 1970s, there
has been increasing interest in parking-lot-shading ordinances. In some
cities (e.g. Sacramento, Davis, and
Los Angeles, CA) ordinances require
50% of the total paved area to be
shaded within 15 years of issuing a
development permit. Planners use
tree lists containing the 15-year
crown diameter and crown projection area of recommended species to
calculate shaded area.
Other parking lot ordinances specify one tree for a certain number of
parking spaces or a certain amount
of landscaped area per space. Under
Cars parked in shade evaporate less
hydrocarbons from gas tanks, hoses, and
fabrics than cars parked in full sun.
these ordinances, however, trees can
be clustered in islands or along the
lot perimeter, often resulting in large
areas of unshaded pavement.
Benefits of Shaded Parking Lots
Trees provide important benefits
in parking lots. They moderate the
heat absorbed by asphalt. Cooler air
(continued next page)
Center for Urban Forest Research
Pacific Southwest Research Station, USDA Forest Service
c/o Department of Environmental Horticulture
University of California
1 Shields Avenue, Suite 1103
Davis, CA 95616-8587
ADDRESS SERVICE REQUESTED
PRSRT STD
US Postage
PAID
Hayward, CA
Permit #3335
2
Cool Parking Lots (continued from page 1)
Another Benefit
temperatures reduce ozone concentrations by lowering hydrocarbon
emissions. The cooler the car, the
lower the rate of evaporation from
gas tanks, hoses, and vehicle fabrics.
Trees in Davis, CA parking lots
reduce surface asphalt temperatures
by as much as 36oF, vehicle cabin
temperatures by over 47oF, and fueltank temperatures by nearly 7oF.
number of existing spaces was 6%
more than required. When surveyed
at peak occupancy periods, 36% of
the spaces were empty.
We assumed that just 25% of all
empty spaces are excess parking
that could be converted to nonimpervious surfaces. Calculations
showed that these excess spaces
occupy more than 10% of the land
covered by parking lots.
Reducing the amount of impervious
surface in parking lots can reduce
polluted runoff and the size and costs
of stormwater facilities needed to
store and treat that runoff. The quantity of pollutants in parking lot runoff is
related to vehicular traffic, vehicle
condition, and atmospheric
deposition. Parking lot runoff has
relatively high concentrations of trace
metals, oil and grease.
Parking Lot Shade
about $2 million per year. This is half
of the $4 million per year in benefits
that will be realized from these
“cool” and aesthetically pleasing
parking lots—a wise investment
yielding $2 for every dollar invested.
Sacramento Study
Sacramento, CA was chosen for
an investigation of how one “pretty
good” ordinance was working. The
study was designed to answer the
following policy/planning questions:
Are current parking demand ratios
adequate?
Are requirements for parking lot
shade being met?
What are the environmental and
economic costs of compliance and
non-compliance?
How can the ordinance and its
implementation be modified to
increase effectiveness?
A random sample of 15 parking
lots was examined to evaluate
parking capacity and compliance
with the 1983 ordinance that
requires 50% shade of paved areas
15 years after development.
Parking Demand Ratios
Over 38% of Sacramento is
covered with impervious surfaces—
roofs, streets, sidewalks, and parking
lots. Parking lots account for 13% of
the impervious surface and occupy
5.6% of the total land area. The total
Urban Forest Research
is a publication of the Center
for Urban Forest Research,
Pacific Southwest Research
Station, USDA Forest Service. For more
information, contact the Center at the
Department of Environmental
Horticulture, University of California,
1 Shields Ave, Suite 1103, Davis, CA
95616-8587. (530) 752-7636
Editor: Laurie Litman, InfoWright
Urban Forest Research
The Sacramento ordinance
requires shading of 50% of the total
parking lot surface, including
covered parking. Not one of the lots
surveyed even came close to this
target using just trees. In fact, the
average shade provided by existing
trees was only 8%. After “computer
growing” younger trees to their
projected 15-year size, tree shade
increased to only 21%. Many of the
lots planted with large-statured trees
will probably exceed this figure, but
lots with crab apple, crepe myrtle,
and pear will never come close.
Costs of [Non]-Compliance
Annual benefits provided by the
current parking lot trees was valued
at approximately $700,000. Benefits
measured included air quality, CO2
reduction, energy savings for cooling, storm water runoff, and aesthetics. The benefits will increase to
about $1.8 million when all trees are
at least 15 years old. By increasing
shade to 50% in all lots in the city,
annual benefits will increase to $4
million. By not achieving the ordinance’s 50% shade target, Sacramento
is annually foregoing nearly $3.3
million in benefits.
If we factor in the annual costs of
maintaining parking lot trees, we
still see good reason to achieve the
50% target. The cost of maintaining
existing trees is about $1 million per
year. If all of the lots had the right
trees in the right locations this
maintenance figure would double to
Increasing Effectiveness
Is this a pipe dream? Retrofitting
existing parking lots to get 50% shade
will be a relatively expensive, longterm process, but it can be done.
First, to get more extensive shade it
is necessary to add more trees,
increase soil volume for roots, and
provide tree care information to
property managers and arborists.
Second, and perhaps more
importantly, it is necessary to make
key planning decisions prior to
starting the retrofitting process.
We found that updating the ordinance’s Tree List to include more
accurate estimates of 15-year crown
diameters for a wider range of
species should be a high priority. Of
equal importance is providing
planning staff with adequate time
and training to review parking lot
shade plans. Sacramento’s existing
ordinance requires a site check after
construction, but inspections may
not be as systematic and thorough as
needed. Teaching inspectors to identify common problems is one way to
remedy this. Requiring landscape
architects to certify that parking
spaces and trees are located as per
the plan is another key to success.
See Fact Sheet #3: Making parking
lots more tree friendly.
January 2002
Fact Sheet #3: Making Parking Lots More Tree Friendly
After Installation
Site Planning and Design
Reduce Paved Surfaces
Reduce parking ratios to decrease the number of unused parking spaces.
Identify peripheral and overflow parking areas, especially in retail lots,
and determine the appropriate landscape treatment (e.g., pervious
paving, stormwater infiltration areas)(Girling, et al. 2000).
Narrow the width of aisles between rows of spaces. In many cases aisle
widths exceed the standard.
Increase the ratio of compact to full-sized spaces.
Increase use of one-way aisles, angled parking spaces, and shared parking
to reduce overall imperviousness (ULI, 1983; Center for Watershed
Protection, 1998).
Promote Tree Growth
Reduce soil compaction in tree planting areas.
Increase tree well and planting island minimum dimensions to 8 feet.
Require soil in tree wells to be excavated to a depth of 3 feet and
amended as necessary.
Use structural soil mix under paving to retain parking spaces while
increasing soil volume (Grabosky and Bassuk 1996).
Increase Environmental Benefits
Use vegetated swales instead of tree wells or convex-shaped islands to
treat stormwater, promote infiltration, and increase soil volume for trees
(Richman 1997).
Convert double-loaded full-size spaces to compact spaces with a tree in
between to increase shade without reducing the number of spaces.
Reduce conflicts between trees, lighting, and signage by coordinating
location of trees, light poles, and signs. 1. Reduce the maximum height of
parking lot light poles to the height trees are typically pruned for
clearance. 2. Amend sign ordinances to allow monument signs (eye-level
signs located near the street) and promote site designs that locate
businesses closer to the street and move parking behind the buildings.
Insure adequate species diversity.
Develop a master tree list, omit species that are not suitable for parking
lots (e.g., pines, poplars, birch, etc.) and consider specifying recommended tree spacing and minimum planting island widths for each species.
January 2002
Promote adequate tree care after
installation to increase tree vigor,
crown growth, and shade density.
Require that proper tree care
practices are used by qualified
professionals.
Remove stakes as soon as young
trees can support themselves.
Prune young trees early to train
their growth.
Allow tree crowns to reach their
full potential.
Make property owners, managers
and arborists aware of shade
benefits as well as the benefits of
a commitment to professional
care on a regular and long-term
basis.
Enforce the ordinance to ensure
that trees are growing at
acceptable rates, properly pruned
and watered, and promptly
replaced after removal.
Replace removed trees with trees
of equivalent size or value.
Strengthen Ordinances
Link inspection fees to the
issuance of a building permit.
Develop a monitoring and
enforcement program that
records information on the
management needs of every
tree and results in a letter sent
to the property manager
requesting corrective action in
a specified time.
Establish a mechanism to
collect fines or place a lien on
the property if the owner fails
to make the requested
improvements.
Require interest-bearing bonds
to pay for landscape improvements throughout the life of
the project.
Urban Forest Research
References: Making Parking Lots More Tree Friendly
For more information on parking lots, refer to the following publications written by
Center researchers and associates:
Center for Watershed Protection.
1998. Better site design: a handbook for changing development
rules in your community. Center
for Watershed Protection. Ellicott
City, MD. 174 p.
McPherson, E.G. 2001.
Sacramento's parking lot shading
ordinance: environmental and
economic costs of compliance.
Landscape and Urban Planning
57:105–123.
Girling, C.; Kellett, R.; Rochefort,
J.; Roe, C. 2000. Green
neighborhoods: planning and
design guidelines for air, water,
and urban forest quality. Center
for Housing Innovation. University
of Oregon, Eugene. 132 p.
McPherson, E.G.; Simpson, J.R.;
Scott, K.I. 2000. Actualizing
microclimate and air quality
benefits with parking lot tree
shade ordinances. Wetter und
Leben. 50: 353–369.
Grabosky, J.; Bassuk, N. 1996.
Testing of structural urban tree
soil materials for use under
pavement to increase street tree
rooting volumes. J. Arbor. 22: 255–
262.
Richman, T. 1997. Start at the
source: residential site planning
and design guidance manual for
stormwater quality protection. Bay
Area Stormwater Management
Agencies Association. Oakland, CA.
75 p.
Increasing Effectiveness in Parking Lots
It is necessary to make key planning decisions prior to starting the
retrofitting process. Some actions need to be taken and others avoided.
Consider the following during the planning phase:
Avoid double-counting tree shade where tree shade overlaps.
Do not allow planting of trees not on the ordinance’s Recommended
Tree List. Improve the Tree List if necessary.
Be sure crown diameters on parking lot plans correctly reflect crown
diameters specified in the Tree List.
Scott, K.I.; Simpson, J.R.;
McPherson, E.G. 1999. Effects of
tree cover on parking lot
microclimate and vehicle
emissions. J. Arbor. 25: 129–141.
Scott, K.I.; Simpson, J.R.;
McPherson, E.G. 1999. Green
parking lots: can trees improve air
quality? In McPherson, E.G.;
Mathis, S., editors. Proceedings of
the best of the west summit. CAES.
University of California, Davis,
Davis, CA; 86–87.
Urban Land Institute. 1983. Shared
parking. Urban Land Institute,
Washington, D.C. 86 p.
Be sure crown diameters for mature trees are not overstated in the
Tree List, thus allowing parking lot plans to reflect more shade than
they can actually achieve. Correct diameters in the List if necessary.
Follow-up to ensure trees are actually planted, as well as not
removed shortly after planting, especially at sites near store fronts
where trees could obstruct signs.
Do not allow smaller-sized substitutions after the plans have been
approved.
Do not allow parking lot ratios to exceed those stipulated in the
ordinance.
Visit our
website at
http://cufr.ucdavis.edu
This fact sheet is provided for you to copy and distribute. Please credit the Center for Urban Forest Research, Pacific Southwest Research
Station,Forest
USDA Forest
Service, Davis, California. January 2002.
Urban
Research
January 2002
3
Getting to the “Root” of Infrastructure Damage
The millions of trees along our
streets and in parks and backyards
provide many environmental, social,
and economic benefits. Retaining
these trees and their benefits should
be a community’s number-one priority. Yet when damage occurs to sidewalks, curbs and driveways, many of
these trees are removed because
they appear too costly to retain.
There are alternatives to removal.
These were demonstrated at a twoday symposium at the University of
California at Davis, held March 31–
April 1, 2000. A Compendium of
Strategies to Reduce Infrastructure
Damage by Tree Roots is expected to
be released this spring. Highlights of
the symposium follow.
Research
Costs—Dr. Greg McPherson of the
Center for Urban Forest Research
related some surprising survey statistics on costs due to root damage.
Of the $70 million spent annually in
California, 61% goes for hardscape
repair, 13% for liability and legal fees,
10% for tree removal and replacement, 8% for prevention and mitigation, and 8% for administration and
inspection. Annual costs for tripand-fall claims are $9 million, with
the average payment being $6,245.
Down-sizing—All over the west, tree
removal and replacement with
smaller-statured species are leading
to a “down-sizing” of the urban
forest and a loss of the benefits that
large-canopied trees offer. In addition, tree managers report that the
most important factors associated
with hardscape damage are restricted planting space, incorrect species,
shallow soil, fine-textured or compacted soils, and inadequate site
design or engineering.
Soil Characteristics—Dr. Larry
Costello of University of California
January 2002
Cooperative Extension
shared the results of a
study conducted in
Modesto, California.
The researchers
looked at
characteristics of soils
at tree sites with and
without sidewalk
damage to see if
factors such as fine
texture, poor
structure, shallow
hard pan, or a high
water table contributed to hardscape
damage. They found no consistent
relationship between soil conditions
and damage to sidewalks.
Root Architecture—Cultivar selection based on root architecture is
being examined by Dr. Dave Burger
at the Environmental Horticulture
Department at the University of
California, Davis. He has evaluated
seedlings of evergreen or Shamel ash
(Fraxinus uhdei) and Chinese
pistache (Pistacia chinensis) in lab
and field experiments for their ability to produce downward growing
roots as opposed to those growing
more horizontally. These selections
are currently being tested to see if
the deep-rooting characteristics
persist under conditions of vegetative propagation. If successful, these
cultivars may provide alternatives
for better rootstocks for plants near
hardscape.
Structural Soils—Dr. Nina Bassuk of
the Urban Horticulture Institute at
Cornell University shared her latest
findings on the use of “structural
soils.” Over the past several years
she has developed and refined a soil
mix that provides increased underground pore space for tree roots
without compromising the loadbearing needs of streets and sidewalks. The mix combines angular
crushed stone and clay loam soil
with hydrogel as a binding agent.
Many field trials are being conducted
worldwide. The advent of supersonic
air evacuation techniques that do
not damage root systems may make
retrofitting existing sites with structural soils possible in the future.
Practitioner Experiences
Water Jet Tools—In Modesto, CA
the city loans out several water jet
tools to sidewalk replacement crews
to help them increase moisture
deeper in the soil to discourage
surface rooting.
Retaining Mature Trees—In
Sunnyvale, CA the Public Works
Department’s right-of-way concrete
maintenance staff uses root barriers
and root pruning techniques in their
efforts to retain mature street trees.
They are also experimenting with
interlocking sidewalk pavers.
Rubberized Sidewalks—Santa
Monica, CA is using rubberized
sidewalks made from recycled tires
to combat surface roots. In addition,
the city forester reviews all new
development plans and includes
“tree protection zones” in almost
every plan.
Allies in the City Council—In
Redwood City, CA the city forester
(continued on next page)
Urban Forest Research
4
Infrastructure Damage (continued from previous page)
works closely with the city council
and the public to retain as many
large street trees as possible by setting limits on removals within each
block and using larger-scale species
for new plantings when space
permits.
Design and Engineering
Perspective
Conflict Avoidance, Root Guidance
or Hardscape Resistance—Jim
Urban, a landscape architect in
Annapolis, Maryland says that conflict avoidance can be easily
designed into a new landscape by
providing larger tree planting spaces,
using a monolithic street and
sidewalk design that eliminates the
parkway strip, or using a meandering curvilinear sidewalk that provides more room for trees. Achieve
root guidance by including root
barriers, gravel layers to direct root
growth, or structural soil in the
design specifications. Design hardscape resistant to root expansion by
using heavier concrete or a compression subgrade like rubber or
Styrofoam.
Appropriate Habitats—Gary Mason,
an Oakland, CA landscape architect,
suggests designing appropriate habitats for urban trees. Instead of planting in straight, evenly spaced rows,
group trees in groves or clusters
along streets in urban wilderness
areas. Use alternative materials for
walkways, and tailor design criteria
to accommodate trees in their
mature stage.
Mixing and Pouring Concrete—
Concrete engineers George
Seegebrecht of Skokie, IL and Dave
Holman of Danville, CA emphasized
the importance of basic design criteria in mixing and pouring concrete.
The ratio of water to cement is
critical, and the amount of trapped
air can affect freezing and thawing.
Joints should be placed in concrete
so panels are approximately square.
If the length-to-width ratio exceeds
1.5 to 1, cracking may occur. The
shrinking and expanding of clay soils
often causes concrete to crack, but
roots are viewed as the culprit.
You can order a copy of the Proceedings of the Symposium, Strategies to
Reduce Infrastructure Damage by
Tree Roots, from the Western
Chapter ISA for $12.50 members/
$13.50 non-members plus $7.00
shipping. Email: mayeve@msn.com
Welcome aboard
Add me to the mailing list / Change my address:
Name ___________________________________________________________________
Organization _____________________________________________________________
A Compendium of Strategies to
Reduce Infrastructure Damage
by Tree Roots is in progress and
expected to be available sometime in
the spring. The Compendium identifies and describes key strategies
used to prevent or ameliorate damage to sidewalks, curbs, and gutters
by tree roots. Strategies include the
use of root barriers, structural soil,
species selection, alternative design,
and soil management techniques.
Each strategy is described in terms of
objective(s), methods, materials, and
limitations. Literature citations are
included and field photos are used to
illustrate techniques whenever possible. This publication will serve as a
quick and complete reference for all
professionals interested in reducing
infrastructure damage: municipal,
consulting, and commercial arborists;
landscape architects; landscape
contractors; and public works
managers.
Upcoming Presentations
MARCH, 8, 2002
Benefits and costs of community
trees in the Pacific Northwest by Jim
Geiger. Oregon Community Trees
Summit. Wilsonville, Oregon.
MARCH 21, 2002
Benefits and costs of urban forests
by Greg McPherson. Trees, People,
and Our Urban Environment.
Anaheim, CA.
MARCH 26, 2002
Economic impacts of street trees by
Paula Peper. Roots and Infrastructure
Workshop. San Jose, CA.
Address _________________________________________________________________
City _______________________________ State ____________ Zip ________________
Phone ____________________________ e-mail ________________________________
Comments or suggestions? __________________________________________________
_______________________________________________________________________
Send to Center for Urban Forest Research, Pacific Southwest Research Station, USDA
Forest Service, c/o Department of Environmental Horticulture, University of California,
1 Shields Avenue, Suite 1103, Davis, CA 95616-8587 or contact jgeiger@fs.fed.us.
Urban Forest Research
Urban forestry
resources and
information can be
found at our website
http://cufr.ucdavis.edu/
January 2002