Urban Tree Canopy

From the Natural Arlington Blog:

What is Urban Tree Canopy?

One of the assessments Arlington County uses to measure the success of our urban forestry program is through measuring tree canopy percentage, or Urban Tree Canopy (UTC) throughout the County. Virginia Tech defines Urban Tree Canopy as the layer of leaves, branches, and stems of trees that cover the ground when viewed from above. Urban tree canopy provides many benefits to communities including improving water quality, conserving energy, lowering city temperatures, reducing air pollution, enhancing property values, providing wildlife habitat, facilitating social and educational opportunities, and providing aesthetic benefits.

Tree canopy studies are usually performed using high-resolution satellite photo, and involves significant analysis by Geographic Information Systems technicians. Simpler methods of estimating tree canopy percentages do exist, such as iTree Canopy (http://www.itreetools.org/canopy/), but for comparable, scientifically-sound studies, UTC’s more accurate assessments are often used.

There are some limitations to urban tree canopy data, because this top-down data does not speak to what species of trees are in the community, the health of the trees, whether we have a vertically complete forest (trees, shrubs, grasses), or is an over-browsed unsustainable forest. Consistency between different methods of UTC analysis has also made comparison an issue, especially when used for scientific purposes, but abstractions can be used to create comparable data.

Studies performed

Arlington’s first tree canopy study was performed in 2008 by Virginia Tech. It showed us where our densest tree canopy was, and what areas needed extra focus. This study gave us a tree canopy percentage of 43 percent. With a target of 40 percent recommended by American Forests in areas like Arlington, we exceeded that goal. It also set a baseline to compare future studies. This study is available here:

2008 Tree Canopy report

Since 2008, another study was performed for the region, by Casey Trees and the University of Vermont. This study used the same resolution and similar methods to the 2008 study, and showed a significant decrease in tree canopy in Arlington County. Using this data, Arlington County shows about 40 percent tree canopy. You can find a map of the analysis here:

2011 Tree Canopy report

To truly identify trends in Arlington County on tree canopy, we will need to see future studies performed on the County. Recent studies have started for the entirity of the Chesapeake Bay (of which Arlington County is part), and this will include state-wide tree canopy assessments. It’s likely new data will be gleaned from this study to develop stronger trends. More information can be found here:

Chesapeake Bay Tree Canopy Goals

The trend in tree canopy

A reduction from 43 percent to 40 percent is a significant decrease for such a short period of time. From closer analysis, it is clear that no one source of impact (development, invasive plants, deer, disease) is at fault. It appears that all factors are involved in the reduction of canopy. Cross-analysis was performed with intensity of development, parks prone to deer browse and invasive plants, and areas with disease-prone trees, and it seemed that all issues were the cause here.

Some civic associations were hit particularly hard between the two analyses, such as Alcova heights, Arlington Ridge and Lyon Village. Some increases in neighborhoods in the Northwest were noted, but they were largely offset by surrounding losses. A map of changes in tree canopy can be found here:

Change in tree canopy 2008-2011

How is Urban Tree Canopy data used?

Data is only useful if it helps us communicate an issue and plan for the best outcome in projects. Our tree canopy data has been used with:

Planning for protection: Communicating where contiguous, high value forest patches are in the County can help drive park planning, road construction, and other facility needs. Using UTC data in planning was used successfully to educate about the losses incurred by individual projects, and to drive conversations of prevention. Showing which civic associations show below average tree canopy can help communicate the need for tree protection for both public and private projects.

Outreach: Some neighborhoods with low tree canopy have been targeted by outreach from our tree canopy fund partners to help residents become aware of programs for planting trees on their projects. This has led to strong ties with communities otherwise not typically involved in natural resources management, such as the affordable housing community.

Potential tree canopy and intelligent planting: A more positive way to look at tree canopy data is to identify possible planting spaces in the County. This allows the Parks department and other organizations to find areas where the most impact can be had for planting. Large reforestation projects tend to be easier to manage than individual street trees or plantings, and this data can help us identify potential locations for this.

Other analysis

UTC analysis gives us a great idea of the general quantity of tree cover, and can help show trends in tree canopy in the County. However, its limitations don’t give us a full understanding of the function and value of our urban forest. Some future projects are being considered right now, which will help us get a more comprehensive idea of the makeup of our natural world:

iTree Eco: Staff hopes to perform a 200-plot statistical survey of trees throughout the County, which will give us an idea of the species makeup and sizes of trees in the County. More information on this program is available here: http://www.itreetools.org/eco/.

Preliminary study results from Washington, DC’s iTree Eco study can be found here, to show what kind of data this study produces: Casey Trees iTree Study

Finding new spaces to plant and preserve: Many natural areas can be expanded in parks, along rights-of-way, without impeding existing use, and improving our environment. Using Tree canopy data can help us identify where these areas may be. Reducing mowed areas can help reduce maintenance cost and carbon emissions, and make our parks more sustainable.

Bringing in social factors into the analysis: The City of Baltimore, as well as other pilot studies in the country, have analyzed how tree canopy correlates to wealth, social capital, and other social factors, such as age, family size and ethnic background. Using these kinds of analyses, these communities have improved their outreach in areas with low tree canopy, and significantly improved support for improving the natural side of their cities. Detailed census data exists for Arlington County, and this may be a part of the Urban Forest Masterplan update.

More at the following link:

Comparing socio-economic factors and tree canopy

Resources

Arlington County Forest Health: Arlington’s tree canopy

United States Forest Service document on tree canopy: http://www.nrs.fs.fed.us/urban/utc/

Urban tree canopy studies in Virginia: http://gep.frec.vt.edu/va_utc.html

iTree Canopy: http://www.itreetools.org/canopy/

iTree Eco: http://www.itreetools.org/eco/