There is no universally agreed definition of tree, urban forest, or urban forestry among practitioners. Definitions of urban forest range from the very broad that include all forms of vegetation growing anywhere within an urban environment to more focused definitions aligned with the usual understanding of forests as tree-dominated ecosystems. Some of the terminology used as part of this Urban Forest Management project may be unfamiliar to many people therefore, clear definitions should be articulated. For the purposes of Tacoma’s Urban Forest Management Plan, the following definitions have been adopted (derived from a variety of U.S. and international sources):
Arboriculture is the branch of horticulture concerned with the cultivation, management and study of individual trees.
City-maintained land means freehold land that is owned by the City, State land vested in or managed by the City under a statutory order, and land that is leased by the City from an external party.
DBH, or Diameter at Breast Height, is a standard measurement of a tree’s size. It is measured at 4.5 feet above ground.
Ecosystem services provided by trees and the overall urban forest are generated as a result of healthy urban and rural forest ecosystems that serve as ecological life-support systems. Urban and rural forests provide a full suite of goods and services that are vital to human health and livelihood natural assets. Many of these goods and services are traditionally viewed as free benefits to society, or "public goods" - wildlife habitat and diversity, watershed services, carbon storage, and scenic landscapes, for example(3).
Forest means an area where the dominant vegetation comprises trees and large shrubs with a mature height of more than 10 feet.
Green infrastructure, for the purposes of this Plan, is an approach to water management that protects, restores, or mimics the natural water cycle. Green infrastructure is effective, economical, and enhances community safety and quality of life. It means planting trees and restoring wetlands, rather than building a costly new water treatment plant. It means choosing water efficiency instead of building a new water supply dam. It means restoring floodplains instead of building taller levees. Green infrastructure incorporates both the natural environment and engineered systems to provide clean water, conserve ecosystem values and functions, and provide a wide array of benefits to people and wildlife. Green infrastructure solutions can be applied on different scales, from the house or building level, to the broader landscape level. On the local level, green infrastructure practices include rain gardens, permeable pavements, green roofs, infiltration planters, trees and tree boxes, and rainwater harvesting systems. At the largest scale, the preservation and restoration of natural landscapes (such as forests, floodplains and wetlands) are critical components of green infrastructure(2).
Green space means vegetated outdoor space within the urban environment, whether on public or private land, and includes but is not limited to areas of urban forest.
Open space, in urban planning terms, means a non-enclosed area, usually unroofed and/or open on at least two sides. It includes both natural (vegetated) and artificial ground surfaces. Most green space is open space, but not all open space is green space. Public open space is defined in planning legislation.
Right-of-Way is the public easement (typically) over the land of the abutting property owner. According to our TMC 8.30.020, public right-of-way includes the area of land, the right to possession of which is secured by the City for right-of-way purposes and includes the traveled portion of the public streets and alleys, as well as the border area, which includes, but is not limited to, any sidewalks, planting strips, traffic circles, or medians. The City of Tacoma requires abutting property owners to maintain adjoining rights-of-way. This includes streets and alleys extending from the owner's property lines out to the curbs or edges of pavement (includes sidewalks and planting strips) if improved, or if unimproved (unpaved), out to the centerlines. There are several places in the Tacoma Municipal Code where these obligations are stated: Chapters 9.17, 9.18, 8.30, 8.31, and 12.093.
Tree is defined for the purposes of this Plan as any perennial woody plant, including single-stemmed trees and multi-stemmed shrubs, with a potential mature height of more than 10 feet and a canopy of branches and leaves extending from the upper parts of the stem(s).
Tree canopy is defined as the layer of tree leaves, branches and stems that cover the ground when viewed from above.
Tree canopy cover is the percentage of a given area of land that lies directly below the canopy of trees taller than 10 feet. It is approximately equal to the area of midday shade provided by the canopy. Climbing plants (vines) and giant grasses are not counted as part of the tree canopy cover regardless of height.
Urban forest means the trees and associated understory plants on both public and privately owned land, but for the purposes of this Plan does not include significant areas of turf or unirrigated grass.
Urban forestry means the planned, integrated and systematic management of the urban forest for its collective contribution to the physical, social, environmental, and economic wellbeing of the community. For the purposes of this Plan, the terms urban forestry and urban forest management refer to the management of the component of the urban forest growing on City-controlled land.
Urban forest sustainability includes everything needed to assure that the entire forest system achieves and maintains a healthy overall extent and structure sufficient to provide the desired benefits, or ecosystem services, over time. While this definition is narrowly focused on the urban forest resource, it’s important never to lose sight of the broader view that places the urban forest in the context of overall sustainability and a sustainable community. This can include such intersecting areas as waste reduction and recycling, stormwater management, energy use, air and water quality, wildlife habitat, public health, economic viability, social equity, overall livability, and so on. Clearly, the sustainable urban forest fits well within that conceptual framework(4).
Urban Tree Canopy Assessment Definitions
Land Acres: Total land area, in acres, of the assessment boundary (excludes water).
Non-Canopy Vegetation: Areas of grass and open space where tree canopy does not exist.
Possible Planting Area - Vegetation: Areas of grass and open space where tree canopy does not exist, and it is biophysically possible to plant trees.
Possible Planting Area - Impervious: Paved areas void of tree canopy, excluding buildings and roads, where it is biophysically possible to establish tree canopy. Examples include parking lots and sidewalks.
Possible Planting Area - Total: The combination of PPA Vegetation area and PPA Impervious area.
Shrub: Low-lying vegetation that was classified based on interpretation of shadows and texture in vegetation. Shrubs produce little to no shadow and appeared smooth in texture compared to tree canopy.
Soil/Dry Vegetation: Areas of bare soil and/or dried, dead vegetation.
Total Acres: Total area, in acres, of the assessment boundary.
Unsuitable Impervious: Areas of impervious surfaces that are not suitable for tree planting. These include buildings and roads.
Unsuitable Planting Area: Areas where it is not feasible to plant trees. Airports, ball fields, golf courses, etc. were manually defined as unsuitable planting areas.
Unsuitable Soil: Areas of soil/dry vegetation considered unsuitable for tree planting. Irrigation and other modifiers may be required to keep a tree alive in these areas.
Unsuitable Vegetation: Areas of non-canopy vegetation that are not suitable for tree planting due to their land use.
Urban Tree Canopy (UTC): The “layer of leaves, branches and stems that cover the ground” (Raciti et al., 2006) when viewed from above; the metric used to quantify the extent, function, and value of Tacoma’s urban forest. Tree canopy was generally taller than 10-15 feet tall.
Water: Areas of open, surface water not including swimming pools.
Ecosystem Benefits Terms
Ecosystem benefits are values of ecosystem services generated by trees and derived from research. These include(5):
Stormwater Monetary Benefit: Monetary savings due to reductions in annual stormwater runoff due to rainfall interception by tree canopy.
Runoff Prevention (Gallons): Reductions in annual stormwater runoff due to rainfall interception by tree canopy.
Property Value Total: Monetary increases in tangible and intangible benefits of trees reflected in increases in property values.
Energy Savings: Monetary increases due to the contribution of the urban forest toward conserving energy in terms of reduced natural gas use in winter (measured in therms) and reduced electricity use for air conditioning in the summer.
Energy Saved (kWh): Contribution of the urban forest toward conserving energy in terms of reduced natural gas use in winter (measured in therms) and reduced electricity use for air conditioning in the summer (measured in kwh).
Natural Gas Savings: Monetary increase due to the contribution of the urban forest toward conserving energy in terms of reduced natural gas use in winter.
Heat Prevention (Therms): Contribution of the urban forest toward conserving energy in terms of reduced natural gas use in winter (measured in therms).
Air Quality Monetary Benefit: Trees improve air quality when air pollutants (O3, NO2, SO2, Particulate Matter) are deposited on tree surfaces and absorbed, and from reduced emissions from power plants (NO2, Particulate Matter, VOC’s (Volatile Organic Compounds), SO2) due to reduced electricity use (see Energy Conservation definition). This is the monetary amount of this benefit.
Pollutants removed (lb): Trees improve air quality when air pollutants (O3, NO2, SO2, Particulate Matter) are deposited on tree surfaces and absorbed, and from reduced emissions from power plants (NO2, Particulate Matter, VOC’s (Volatile Organic Compounds), SO2) due to reduced electricity use (see Energy Conservation definition). This is the measured amount of this benefit in lbs.
Carbon Monetary Benefit: The dollar value associated with the amount of carbon stored or sequestered by trees based on calculations of the social cost of carbon.
Carbon Stored (lb): All carbon dioxide stored in the urban forest over the life of the trees as a result of sequestration (in pounds). This measurement is not the same as annual carbon sequestered.
Carbon Sequestered (lb): The amount of carbon annually removed from the atmosphere and stored in the canopy's biomass (in pounds).
Carbon Avoided (lb): Annual reductions in atmospheric CO2 due to sequestration by trees and reduced emissions from power plants due to reduced energy use (in pounds).
1 USDA Forest Service, “Ecosystem Services”. www.fs.fed.us
2 American Rivers, “What is Green Infrastructure?”. www.americanrivers.org
3 City of Tacoma, “What is Right-of-Way”. www.cityoftacoma.org
4 Leff, Michael, “The Sustainable Urban Forest A Step-by-Step Approach”. Davey Institute / USDA Forest Service USFS Philadelphia Field Station, 2016
5 USDA Forest Service et al., “i-Tree”. www.itreetools.org