URBAN TREE CANOPY

What is Urban Tree Canopy?

Urban tree canopy (UTC) is the layer of leaves, branches, and stems of trees that cover the ground when viewed from above. In urban areas, the UTC provides an important stormwater management function by intercepting rainfall that would otherwise run off of paved surfaces and be transported into local waters though the storm drainage system, picking up various pollutants along the way. UTC also reduces the urban heat island effect, reduces heating/cooling costs, lowers air temperatures, reduces air pollution, increases property values, provides wildlife habitat, and provides aesthetic and community benefits such as improved quality of life.

Why Set UTC Goals?

Researchers estimate that tree canopy cover in urban and metropolitan areas across the U.S. averages only 27% and 33% respectively (Dwyer and Nowak, 2000). Additionally, the trees that are present are subject to a wide variety of stressors, which significantly shortens their lifespan. As such, it is important for urban communities to take steps to protect and enhance their urban forests through UTC goal setting processes. Few communities have developed land cover strategies such as UTC that mitigate urbanization effects regardless of land use type. Several recent efforts have explicitly included UTC in planning efforts to address community, environmental and human health concerns:

The Chesapeake Bay Program has included UTC in its strategies to improve water quality in the Bay by reducing sedimentation and nutrient loads. The 2003 Riparian Forest Buffer Directive states the following goal of: “by 2010, work with at least 5 local jurisdictions and communities in each state to complete an assessment of urban forests, adopt a local goal to increase urban tree canopy, and encourage measures to attain the established goals in order to enhance and extend forest buffer functions in urban areas.” The 2007 Forestry Conservation Initiative has a goal of: “By 2020, accelerate reforestation and conservation in urban and suburban areas, by increasing the number of communities with commitments to tree canopy expansion goals to 120.”

Using the Urban Ecosystem Analyses, American Forests has pioneered the idea of measuring and calculating the value of UTC in our metropolitan areas. These analyses have been conducted for more than 30 cities and metropolitan areas across the country. As an example, results from the Montgomery, Alabama analysis include:

  • As of 2002, 34% of the city was covered by tree canopy
  • The stormwater retention capacity of the city’s urban forest is 227 million ft3
  • The cost to manage this volume of runoff is estimated at $454 million
  • The city’s urban forest removes 3.2 million lbs of pollutants from the air annually and this benefit is valued at $7.9 million.
  • The city’s urban forest sequesters 11,263 tons of carbon each year and stored a total of 1.45 million tons of carbon

Some states are including UTC in State Implementation Plans to improve air quality by mitigating ground level ozone formation.

Other UTC efforts have focused on individual development sites as opposed to entire cities or metropolitan areas. Because unshaded parking lots can become extremely hot and contribute to both the urban heat island effect and increased air pollution, many communities in hot climates require that newly constructed or reconstructed parking lots be shaded by incorporating tree plantings into the parking lot design. Parking lot shading provisions are sometimes enacted though a specific parking lot shading ordinance, but the code may be incorporated into sections of the city code related to trees, landscaping, parking lots, or elsewhere. For example, Sacramento and Davis, California have parking lot tree shading ordinances that require 50% shading of paved areas in parking lots 15 years after development.

 

Conducting UTC Assessments and Goal Setting

In order to set UTC goals, communities must first have an idea of how much current canopy is present. The process for conducting UTC assessments and goal setting generally includes the following steps:

1. Measure current UTC

  • Usea top-down remote sensing or bottom-up on-the-ground tree surveys to measure existing urban tree canopy.
  • Identify the different types of forest in the community, including public (street trees, riparian corridors, parks, etc.) and private (residential, commercial, industrial areas, etc.)

2. Estimate potential UTC

  • Use remote sensing imagery and Geographic Information Systems analyses to identify locations with potential for UTC.
  • Identify priority locations where UTC increases will support identified community priorities (e.g., water quality, wildlife).

3. Adopt a UTC Goal

  • Determine a goal based on the results of the assessments and specify a timeframe.
  • Formal adoption of the goal is preferable to ensure that the goal comes to fruition (e.g., institutionalize UTC goals in local ordinance, regulations and comprehensive planning efforts).

This aerial photo of the city shows a top-down view of the urban tree canopy

Various methods and tools are available for conducting UTC assessments:

Top-down:

  • The University of Vermont Spatial Analysis Laboratory has developed a remote sensing approach to UTC measurement that has been applied in a number of Chesapeake Bay communities. The approach evaluates both current and potential UTC.
  • The National Land Cover Database can be used to estimate UTC for your community.  This data is pretty coarse (30 m) so it is not typically recommended for small areas, but the Forest Service has developed some correction factors for the data that accounts for the fact that NLCD underestimates UTC and IC.  The NLCD files by state, county, and county subdivision can be downloaded here: http://nrs.fs.fed.us/data/urban, along with the reports that include the correction factors.  This data is free and relatively easy to use.
  • I-Tree VUE pulls in the NLCD data for an area and allows the user to view maps of UTC and IC, get estimates of their coverage and the benefits they provide, and also see how changes in tree planting rates will affect the benefits.
  • Free maps are available for Google Earth; these can be photo interpreted to estimate UTC for the project area.  This method doesn’t result in a map, just a numeric estimate.  It requires some knowledge of photo interpretation but is still much simpler and lower cost than other options.
Bottom-up:
  • The Forest Service’ s i-Treetools and utilities can be used to collect plot data for on-the-ground tree surveys.  i-Tree Eco – is an adaptation of the Urban Forest Effects (UFORE) model which provides a broad picture of the entire urban forest. It is designed to use field data from complete inventories or randomly located plots throughout a community along with local hourly air pollution and meteorological data to quantify urban forest structure, environmental effects, and value to communities.  i-Tree Streets focuses on the benefits provided by a municipality’s street trees. It makes use of a sample or complete inventory to quantify and put a dollar value on the street trees’ annual environmental and aesthetic benefits. Streets also describes urban forest structure and management needs to help managers plan for the future.  i-Tree Hydro will be available in February 2011 and it allows the user to model changes in streamflow and water quality based on changes in IC and UTC.  The model shows that for every 1% increase in IC, a manager needs to plant 10-12x as much area in forest to offset the changes in hydrology. 

The USDA Forest Service's national UTC program website provides a map and summary of communities across the country that have completed UTC assessments.  The National Conference of Mayors also have a report on national UTC goals.  A smaller number of communities in the Chesapeake Bay Watershed and surrounding area have gone through the assessment process and formally adopted UTC goals. A summary is provided in the chart below. To assist with this process, the USDA Forest Service provides a Guide for Chesapeake Bay Communities on UTC goal setting.


The U.S. Conference of Mayors Community Trees Task Force, created in 2006, shares information on urban forests among municipalities and disseminates information on federal and private resources of value to cities in their community greening efforts.  The Task Force completed a survey of current efforts in cities to expand and protect their UTC.  Responses were tallied for 135 cities in 36 states nationwide and provides information on how community tree activities link to other sustainability and climate change efforts,  how they partnered with other organizations to conduct activities relating to trees, how they conducted residential and community organization outreach efforts on preserving and enlarging their UTC, and so on.

Links to the results of some community UTC assessments are provided below.


Approaches to Achieve UTC Goals

Once the assessment and goal setting process is complete, the next logical step is to develop an implementation plan that summarizes the approaches the community will take to achieve their UTC goals. In general, a UTC plan identifies the UTC goal and timeline, describes the relationship of canopy goals to local ordinances, regulations, and the community’s comprehensive plan, and outlines the specific strategies for achieving UTC goals, including identifying a timeline and responsible party. Each community must develop an approach to achieve UTC goals that considers their internal capacity and resources, political climate, and stakeholder needs. The range of strategies to achieve UTC goals includes:

  • Permanently protect priority forest tracts through acquisition, conservation easements or other method.
  • Prevent forest loss during development by adopting or amending site development regulations (e.g., forest conservation regulations, open space design, clearing restrictions) and zoning.
  • Maintain existing forest canopy by adopting regulations that restrict tree removal.
  • Increase tree planting during development by adopting or revising site development regulations such as landscaping and parking lot shading.
  • Reforest public lands, beginning with priority sites.
  • Encourage reforestation of private land by developing education, stewardship and incentive programs.

Links to some community UTC plans are provided below.


UTC Resources