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Parking lot trees

Parking lots occupy large patches of the urban landscape. Trees in parking plots can help mitigate some of their undesirable characteristics:

• Tree shade helps cool pavement. This helps reduce the urban heat island effect that is associated with paved areas.
• Tree shade cools parked cars. Hydrocarbon vapors emitted by hot cars contribute to photochemical smog formation.
• Trees intercept and channel rainfall, reducing runoff and water pollution associated with runoff from paved surfaces.
• Trees screen and soften the visual blight that parking lots pose.

Many communities have policies and regulations designed to increase the amount of tree cover in parking lots. These regulations generally apply to new or redeveloped lots. Relatively few communities have audited parking lots to determine if they achieved the canopy cover specified in the design approved by the Planning Department. An assessment of parking lot tree canopy can indicate whether design policies are effective or need to be changed.

You can use aerial photos to assess total canopy cover in parking lots. If you have a series of historical aerial images, you can track changes in canopy cover over time. Many parking lot shading regulations specify the amount of shade required after a certain number of years. Changes in parking lot canopy cover can be correlated with the age of the parking lot, tree species, pruning and maintenance practices, planter sizes, or other factors. Assessing the percentage of the parking spaces that are shaded provides another measure of parking lot shade. You can use ground surveys or high-resolution aerial photos estimate shading of individual spaces.

More trees and greater canopy cover do not always result in more shaded parking spaces. Total canopy cover is directly related to rainfall interception. Shaded parking spaces and pavement are related to air quality and heat island effects.

Parking lots are typically poor areas for growing trees. Trees are often grown in small cutouts with compacted soils, poor irrigation, and inadequate drainage. Trees may be subject to heat damage from hot pavement and vehicle engines. Trees are also damaged by vehicles and shopping carts. Trees are pruned to provide vehicle clearance and avoid blocking parking lot lighting. Retailers sometimes have trees pruned inappropriately to enhance visibility of signs or buildings from the street.

Assessments related to management issues may include:

• empty planting spaces
• species
• condition, including damage
• planting site dimensions.

Example

Parking lot shading was an item that the City of Rocklin, CA, considered in its urban forest management plan (section 3.6). An analysis of commercial parking lots based on aerial photo analysis and ground surveys showed that most lots never attained even moderate levels of shade.  Most parking lots had maximum levels of tree shade after about 10 years and then began to lose canopy as the result of both poor growth and tree loss.

Findings

• The City of Rocklin has several regulations designed to increase tree canopy in parking lots.
• Surveyed parking lots had low levels of shading provided by trees.  Less than 3% of surveyed parking spaces were at least 50% shaded.
• Less than half of the surveyed parking lot trees actually provided canopy cover over parking spaces.
• Ratios of parking spaces per tree were variable, ranging from 2.25 to almost 8 parking spaces per tree.
• Small parking lots generally had higher levels of shading and fewer parking spaces per tree than larger lots.
• Most parking lot trees were well below mature size.
• Levels of parking lot shade were not correlated with parking lot age.
• Empty tree planting sites were common, averaging 9% of all planting sites overall.  The number of empty sites increased with increasing parking lot age.
• About 7% of existing parking lot trees were in poor condition.
• Most parking lot trees are of only moderate size at maturity, and 18% are small-statured trees that will not provide substantial shade at maturity.
• Most native oaks retained in parking lots were in poor condition, although some have survived for over 20 years since construction.

Planning for the future of Rocklin's urban forest, 2006

Work plan

For each category listed below:

• indicate with a check in column 1 those that apply to your situation and will be addressed in the UF management plan
• indicate the methods that will be used to collect the data, who will be responsible for collecting data, and their timeline for supplying the data
• indicate who will the be responsible for summarizing or analyzing the data and their timeline for providing the analysis for the plan

Parking lot trees
		Data collection			Data analysis
Include	Characteristic	Methods	From whom?	Target date	Analysis / summary by whom?	Target date
	Species
	Tree condition
	Planting site size
	Age/size
	Number/percent of empty planting spaces
	Parking space to tree ratio
	Percent of parking spaces shaded - can use shade category (e.g., <10%, 10-50%, >50%) to show degree of shading in shaded spaces.
	Other-specify

Save to work plan

Outline

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