The information collected in Inventories and Assessments will be summarized in the plan section: Status of the Urban Forest.

Canopy Cover

Tree canopy cover refers to the proportion of land area covered by tree crowns as viewed from the air. Assess the amount of canopy cover using aerial imagery to get an overall picture of the urban forest. By determining the current canopy cover, you establish a baseline for the range of the urban forest and define canopy cover targets for the future.

Defining canopy cover will help you find opportunities to broaden the urban forest and to determine where management is needed to sustain current canopy levels. Knowing canopy cover helps quantify the ecosystem services provided by the urban forest.

Tools: itreetools.org

Find the outermost canopy boundaries and measure. What percent of that area is covered by trees?

Compute the benefit of the existing canopy cover. 

Can you define canopy cover targets for the planning area as a whole and/or for segments within the planning area (e.g., residential areas)?

How does current canopy cover compare to possible canopy cover targets? 

Are canopy cover levels in the parts of the planning area at or near maximum sustainable levels? If so, should these levels be used to set targets?

What factors (environment/trees/management) are associated with the urban forest in areas that have maximum or optimal canopy cover levels?

Tree-Vegetation Inventory

Do you have an inventory of the trees, or do you need to make one? Assess the current composition and distribution of the community’s trees.

Having a tree inventory is essential if you manage urban tree populations on a tree-by-tree basis. The data record for each tree typically includes information about tree characteristics, maintenance history, and management needs. The data that is maintained for each inventoried tree will depend on the tree program’s needs, who will be collecting the data, and the role that the inventory plays in the tree management program.

Work toward developing a recent tree inventory that contains the following minimum data for each tree:

• Location
• Species
• Size/age
• Condition
• Management responsibility

What do you want/need to know about each tree? In addition to the list above, consider recording:

• hazard risk
• raised roots
• pruning needs
• irrigation needs
• native species
• heritage trees
• site conflicts, such as powerlines above or constrained roots in planters
• sidewalk damage
• short life span
• failure prone tree
• high maintenance needs
• value

What inventory method will be used?

Tree Inventory Resources

List of Inventory Management Software
This list, compiled by the U.S. Forest Service, contains lists of commercial software and public domain program

Urban Tree Considerations

Street Trees

Due to their location, street trees provide specific benefits not provided by other trees. Benefits include traffic calming and extending the life of roadway pavement. Streets shaded by trees contribute to “sense of place”, which can also contribute to increased community pride and property values.

Street trees are often located in very constrained locations. Pavement and utility lines may limit growing space. Other management issues that may be important for street trees:

• Trees are commonly subject to damage by vehicles and street construction activities.
• Conflicts with utilities, hardscape (especially sidewalks, curbs, and gutters) and other built infrastructure are common.
• Branch, trunk, and root failures commonly have a high potential to cause property damage and/or injury.
• Tree canopies typically need to be maintained for street and sidewalk clearance and to minimize risk of branch failures.
• Falling leaves, seeds, and fruits may create hazards on sidewalks and contribute to storm drain clogging.
• Street trees may generate high numbers of service requests and complaints.

Because of these issues, species selection is often a primary consideration. The species used may be specified in a master planting plan or on an approved species list. The palette of potential street tree species may be limited, which can sometimes lead to low species diversity. Low species diversity can pose a risk to the urban forest if one or more common species develop serious problems. The street tree assessment should look at both species diversity and the overall performance of various species in use.

What types of tree and site data should be assessed/analyzed?

Are necessary data available in an inventory or are surveys needed?

What are the most meaningful ways to stratify tree data: by species, size class/age, condition class, maintenance needs, planting situation? 

It is practical or useful to divide street trees into geographic management zones? If management zones have been in use, how well do they work?

How do we identify/quantify empty planting spaces?

Is the long term sustainability of street tree canopy at risk due to a lack of age and/or species diversity?

What is the frequency and severity of conflicts between street trees and other elements of infrastructure (e.g. curbs, sidewalks, overhead lines)?

Facility Trees

Many urban trees fall into the “facility tree” category. These are trees around buildings and other built facilities that are not adjacent to streets. Most trees in sites such as office parks or campuses are facility trees. In cities or counties, facility trees are found around public buildings. Shade provided by trees near buildings can greatly reduce summer cooling costs. Facility trees also modify the visual impact of structures.

Facility trees often grow where soil volume is restricted by hardscape. They commonly occur in landscape beds near structures. These landscape beds can vary widely in size. Facility trees may also occur in small planters or cutouts in sidewalks or plazas.

Some potential management issues:

• Soil near buildings may be unfavorable due to severe compaction and alkaline residues from to concrete, stucco, etc.
• Planting beds may have inadequate drainage or irrigation.
• Competition from other landscape plants may be excessive.
• Reflected heat or excessive shading from structures may affect tree growth and health.
• Pruning may be needed to maintain clearance from buildings and over walkways.
• Potential for root damage to foundations and walkways needs to be considered.
• Underground utility maintenance may damage tree roots.

Assessments should note special management issues. Also consider how tree condition and management vary with distance from structures, planter size, or other factors.

Which departments currently manage facility trees?
Are there maintenance / management issues that are unique to some or all facility trees in the plan area?
Should facility trees be handled separately from other groups of trees in the management plan?
Can we define management zones for facility trees based on planter size, location relative to buildings, or other factors?

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.

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 to estimate shading of individual spaces.

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.

What is the planting site size?

What is the number or percent of empty planting spaces?

What is the parking space to tree ratio?

What percent of parking spaces are shaded? Use shade categories (e.g., <10%, 10-50%, >50%) to show degree of shading in shaded spaces.

Park Trees

Park trees include trees in public parks maintained by cities, counties, parks districts, and the like. Park-like sites may also exist in open areas of campuses, botanical gardens, and other private or public locations.

Parks of similar age may be similar in makeup and management and can be considered as a group. For initial data collection, you may only need to do sample surveys of representative parks in each group. You can use sample surveys to gather data if you lack an inventory.

Compared with street or facility trees, park trees have fewer space constraints for both canopies and roots. This can allow the use of a wider range of species and larger trees overall. However, tree care may not receive high priority where turf or sports fields are primary uses. Crews that maintain lawns may not have adequate training to recognize and address tree maintenance issues.

Other considerations:

• Trees in or near lawns can compete strongly with young trees.
• Soil compaction due to foot and equipment traffic on wet soils may impair root growth and drainage.
• Surface roots in turf may conflict with mowing equipment and may pose tripping hazards.
• Trees can be subject to damage from mowing equipment and park users. This can make it difficult to establish new trees.
• Hazard management may be a primary concern, especially in areas that are heavily used.
• Newly–developed parks often start with even–aged stands of trees. Phased tree replacement and inter-planting may be needed to avoid a future replacement of the entire stand.
• Parks may include heritage trees or other old or unique trees with special maintenance needs.

As an example, the inventory for Fort Greening Park in New York City was conducted focusing on the following elements:

• Tree spatial distribution
• Species diversity and composition along with disease and pest problems of some species
• Tree size distribution
• Tree condition
• Formal landscape features
• Growing conditions
• Human damage
• Historic trees

What maintenance and management issues are common in park sites?

Which parks need to be treated individually for planning purposes?

Can we create useful groupings of park sites based on location, year of construction, or other factors? 

What roles do trees play in various park sites?

Heritage Trees

Do you have trees that have cultural and historical significance, e.g. trees that were planted when the area was first settled or developed? Because these are special trees by definition, they can have special needs relative to tree care activities and inspections. The evaluation of heritage trees should include whether inspection and maintenance is adequately matched to the trees’ needs.

Some communities have enacted heritage tree  ordinances or other regulations to protect significant local trees that are on private and public property. Heritage tree ordinances vary widely. A discussion of various factors used to define heritage trees can be found at the ISA Guidelines for Developing and Evaluating Tree Ordinances, pages 166-168. In jurisdictions that have heritage tree ordinances, it is important to assess the effectiveness of the ordinance to see whether it is accomplishing its intended goals. Assessments may include examining permits related to retention of heritage trees and selecting a sample to see how trees have fared in the years after the permitted activity.

Trees planted as mitigation could also be inspected after a number of years to see if they are still in place and how much of the lost canopy is actually replaced over time. Aerial imagery can be used to track individual trees over time. Aerial photos can provide information on canopy size and can be used to detect evidence of significant canopy decline in large trees.

If heritage trees form an important component of a community’s urban forest but do not have any regulatory protection, protection of these trees via an ordinance or other regulations may be considered in the plan. Information on the numbers of heritage trees, their condition, and trends affecting them can help to determine whether and how this segment of the urban forest resource should be addressed.

Are there “heritage” trees or trees of historical or cultural significance that are important to protect?

Which trees are affected by the permit process or design review? 
What are the reasons for permit requests?
What heritage trees have been removed? Reason for removal? 
What is the amount of mitigation for removed trees? e.g. money and/or trees planted.
Did mitigation trees survive?

Open Space Trees

Tree management in open spaces is usually less intensive than in other parts of the urban forest. In some areas, open space trees may be largely unmanaged. However, these stands can and will change over time.

Active management may be needed to:

• help maintain native stands that have low levels of natural regeneration
• suppress exotic species that may crowd out native trees in riparian areas
• replace flammable exotic species with lower risk, native trees such as native oaks.

Unless the stand is very small, a complete tree inventory of open space trees may not be practical or necessary. You will normally collect data on open space trees at the stand or forest level. Canopy cover can be assessed using aerial images. Use sample surveys to collect information about species, size classes, condition, regeneration and other factors of interest.

Is the composition of existing native trees a factor to consider? 

What is the overall health of forests/woodlands in our open space areas? 

What changes have occurred in these stands?

What changes are likely to occur over time under current management practices?

How do current and projected uses of the open space and surrounding properties affect the management of the forests/woodlands?

Are there information gaps related to management of open space stands? 

Are there other vegetative components that are important to assess?

Is conservation of existing native trees a factor to consider, such as along a waterway? What is or would have been the native vegetation/trees?  

Are there interface issues between urban, agriculture and/or habitat lands, such as escaped exotic species or landscaping with invasive species?

Tree Selection Resources