About Knoxville Trees

Learn more about Knoxville's urban forest.


Why Trees?

Role of Trees in Knoxville

Why all this planning and effort for trees?  Is this the best use of our time?  Consider the role trees play in Knoxville (and in every city):

Human Health

Trees have a significant impact on the human health and livability of a city.  This occurs through lower heat levels, lower air and water pollution, stronger community, better mental health and reductions in noise.  

Trees Alleviate Heat Stress

Heat stress has been proven to cause significant public health problems and even mortality. In fact, each year, more Americans die from extreme heat than all other natural disasters combined (i.e., hurricanes, floods, tornadoes, lightning). Urban trees are widely accepted as one of the most effective long-term solutions to reducing the effects of urban heat islands. Properly placed mature tree canopy can lower overall ambient temperatures by 20° to 45°F (EPA 2015).

Trees Reduce Air Pollution

Trees reduce or can completely remove many components of street-level air pollution, including carbon dioxide, ozone, nitrogen dioxide, sulfur dioxide (a component of smog), and small particulate matter (i.e., dust, ash, dirt, pollen, and smoke). Ozone and particulates can especially aggravate existing respiratory conditions (like asthma) and create long-term chronic health problems (American Lung Association 2015). Trees reduce nearby indoor air pollution by more than 50% (Maher et al. 2013) and reduce rates of childhood asthma (Lovasi et al. 2008). 

More neighborhood tree canopy is associated with better overall health, including lower rates of obesity, high blood pressure and asthma (Ulmer et al. 2016). Recent studies have also shown an association in total health care costs in areas with more canopy (Becker et al. 2019), with individuals in higher canopy areas paying on average $374 less every year in healthcare costs (Van Den Eeden et al. 2022).

Trees Buffer Against Noise and Pollution

Pollution and noise from busy roadways and rail lines can create unhealthy and undesirable conditions for those living nearby (ALA 2015). Buffers of trees can significantly reduce both noise and pollution. A 100-foot-wide, 45-foot-high densely-planted tree buffer can reduce highway noise by 50% (NC State 2012).
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Trees improve Mental Health and Create Stronger Communities

While less quantifiable, the tree benefits related to community building are no less important than other services. Trees have been shown to have a calming and healing effect on ADHD adults and teens (Burden 2008). Increased access to trees has also been linked to reduced rates of mental illness like anxiety and depression (Rajoo et al. 2021), increases in creativity (Yu & Hsieh 2020), better test scores amongst students (Sivarajah et al. 2018), and even higher rates of quality sleep (Astrell-Burt & Feng 2019).

Trees Reduce Water Pollution

Trees intercept, absorb, and slow rainwater, all of which play a major role in reducing the amount of contaminated stormwater that enters sewer systems and reducing the threat of floods. Based on recent research, urban trees may be even more effective at managing stormwater than we originally estimated, as single trees (like those planted in an open yard) appear to transpire at nearly 3X the rate of a similar tree located in a forest setting (Ponte et al. 2021). This estimates that the removal of a single tree increases stormwater runoff by an average of 1,585 gallons (Coville et al. 2022), and even more for trees that hold their leaves year-round (Seitz and Escobedo 2008).

Walkable Communities

Tree canopy cover is vital to a walkable community. According to the Federal Highway Administration, urban tree canopy along streets have been shown to slow traffic, helping ensure safe, walkable streets in communities The buffers between walking areas and driving lanes created by trees also make streets feel safer for pedestrians and cyclists.(U.S. Department of Transportation 2015).   Driver stress levels have also been reported to be lower on tree-lined streets, contributing to a reduction in road rage and aggressive driving (Wolf 1998a, Kuo and Sullivan 2001).
 
Energy Savings

Trees provide energy savings by reducing cooling and heating costs, both through their shade as well as the release of moisture through transpiration. Trees properly placed around buildings can reduce air conditioning needs by 30% and can save 20–50% in energy used for heating. Computer models devised by the U.S. Department of Energy (2018) predict that the proper placement of only three trees can save an average household between $100 and $250 in energy costs annually. Even properties not directly adjacent to greenspaces can experience energy saving benefits from trees, with buildings 500 feet away from park spaces still experiencing significant cooling effects in summer, reducing the demand for cooling energy consumption by 43% (Toparlar et al. 2020).

This is especially important in low income communities, where household have a higher energy burden (larger percentage of monthly budget is spent on energy).

Carbon Storage

Trees are constantly removing and storing carbon dioxide from the atmosphere. Most of the carbon dioxide (CO2) in the atmosphere comes from human activities that involve the burning of fossil fuels. High levels of CO2 result in climate change, which has resulted in more frequent and severe storms, droughts, and other natural stresses across the world in recent decades. An average tree in the United States can capture about 1,200 pounds of carbon dioxide per over its lifetime; the cumulative effect of all US trees results in capturing 17% of our nation’s carbon dioxide emissions annually (Daley 2022).

Successful Business Districts

In multiple studies, consumers showed a willingness to pay 11% more for goods and shopped for a longer period of time in shaded and landscaped business districts (Wolf 1998b, 1999, and 2003). Consumers also felt that the quality of products was better in business districts surrounded by trees and were willing to pay more (Wolf 1998a).
 
Property Value

The U.S. Forest Service estimates that the presence of street trees increases adjacent home values by an average of $9,000 (Donovan & Butry 2010).
 
Wildlife Habitat / Ecosystem Health

As smaller forests are connected through planned or informal urban greenways, trees provide essential habitat to a range of birds, pollinators, and other wildlife that feed on insects (Dolan 2015). Recent studies show that bees in areas with higher tree canopy have lower levels of toxins related to pollution (Barbosa et al. 2021), and trees can be an essential source of early pollen for these important pollinators (Honchar & Gnatiuk 2020). Higher density (and diversity) of trees is also associated with higher bird density, including natives and rare birds of prey (Heggie-Gracie et al. 2020 & Mirski 2020).

Associated with Lower Crime

Trees have been shown to contribute to a decrease in crime. A study in Baltimore found that a 10% increase in tree canopy was associated with a roughly 12% decrease in crime (Troy et al. 2012). In an experimental study, newly planted street trees were strongly correlated with a reduction in violent crimes, an effect especially prominent in neighborhoods with lower median household income (Burley 2018).

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City-Managed Trees

Street and Park Trees in Knoxville

The City of Knoxville maintains an inventory of over 25,000 public trees, including street and park (landscaped areas) trees. This information is collected by physically visiting each tree on public land and collecting multiple data on that tree and is stored and regularly updated in a GIS-based tree management software (see images of this system below). This information is also available to the public via this online public tree inventory viewer. Inventory data is also critical to manage trees, and is essential for planning and budgeting for annual tree care and planting.

There is limited information or assessment data available on city-owned natural areas and woodlands in Knoxville.





The City Forester manages these trees by dividing them into five management zones (see below).  Each year, one zone is proactively pruned and provided with an additional care necessary, and inventory data is updated as appropriate. This kind of "cyclical, proactive care" is considered a national best practice.





With complete inventory data, it is possible to examine the diversity, size and condition of Knoxville public trees.

 
Diversity

Pests and diseases often target one genus or other grouping of trees.  To avoid significant losses in the future, it is important to have a diverse population of trees. There is a general rule for how much of one family, genus and species we should have:
 
Family: No more than 30% from any one Family.
Genus: No more than 20% from any one Genus.
Species: No more than 10% of any one Species

In Knoxville, the public tree population is very diverse (no threshold is exceeded), as shown in the charts below.  This means that the public tree population is well suited to deal with pests or diseases that may emerge.




Age Distribution

In Knoxville, there are more trees in the younger categories, in large part due to the increase in tree planting over the last decade. However, there are much fewer mature trees than is considered ideal. There should be 20% in the "Maturing" stage, and 10% in the Mature stage. In Knoxville there is only 8% and 6% respectively (see charts below). 

Larger trees provide more community services, as they intercept more stormwater, sequester more carbon, and remove more air pollution. As such, ensuring our young trees make it to maturity, as well as preserving our large trees should be a high priority when possible. 
 
Mature trees provide exponentially more to a community. For example, A large healthy tree (over 30" in trunk diameter, or DBH) can remove 70 times more air pollution in one year than a small healthy tree (8" trunk diameter, or DBH). 

Note: Size of trees is not complete accurate indication of tree ages, as different species grow at varying rates and have varying mature sizes.  However, it is close enough to provide an accurate picture of approximate distribution of ages.



Size / age data on public trees.


Condition of Public Trees

Detailed understanding of the condition and risk potential of all publicly-owned trees is critical.  It is an important indicator of risk to citizens and where tree work is required, but also provides a glimpse into future canopy.   The majority of public trees in Knoxville are in fair or better condition.

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Knoxville Tree Canopy Cover

How do we measure tree canopy?

The amount of tree canopy in a community is measured from aerial imagery, and described in a "percent of land covered" number.

Tree canopy data is collected through a urban tree canopy (UTC) assessment.  This is a computer-based analysis of the amount of covered by five land cover types - Trees, Low Vegetation (grass, shrubs), Impervious Surfaces (roads, buildings, etc.), Water and Bare Soil (usually agriculture fields or construction areas).  

In 2020, the City of Knoxville partnered with Trees Knoxville and other community organizations to conduct a comprehensive Urban Tree Canopy Study. The study focused on both Knox County and the City of Knoxville to look at changes in tree canopy between 2008 and 2018. The map shows a picture of the results of the UTC (land covers) in the downtown areas of Knoxville. This comprehensive review explored urban tree canopy in many different geographic scales including neighborhoods, census tracts, watersheds, and parks. The information used from this study will help guide planning processes for urban forestry moving forward.





In Knoxville, 38% of the land is covered by trees when viewed from above (as of 2018). 

This is down from 40% in 2008.  

Just over 700 acres of tree canopy was lost between 2008 and 2018. Most of this loss occurred on residential private land.





What should Knoxville tree canopy be?

There is no standard quantity of tree canopy for every city, as every community is different.  Communities differ in the number of people living there, how densely it is built, how much residential land there is compared to commercial and industrial, what the natural environment is like, and more. 

Tree canopy data on nearby cities is provided for informational purposes only. 

There is currently no tree canopy goal set in Knoxville.


Tree canopy in Knoxville neighborhoods.

How much tree canopy is there where you live? Is tree canopy equitably distributed across Knoxville?  This is something that is important to examine, as trees play such an impactful role in Knoxville, lowering heat levels, providing cleaner air, better water quality and better human health (read more about the role of trees here).
 

Tree canopy cover in Knoxville neighborhoods range from 5% to 63%. Of the 60 neighborhoods in Knoxville, 25 fall below the citywide average of 38%.





Change in Tree Canopy. The change in canopy between 2008 and 2018 can also be examined.  Tree canopy gains occurred in 19 neighborhoods, losses occurred in 41 neighborhoods.






How much tree canopy does your neighborhood have?  The entire city of Knoxville has been measured for the amount of land in 5 categories:

• Tree Canopy Cover
• Low Vegetation (everything green except trees - lawn, shrubs, fields, etc.)
• Impervious Surfaces (hard surfaces that repels rain - roads, buildings, parking lots, etc.)
• Water (lakes, streams, rivers)
• Bare Soil (usually construction sites, empty agricultural fields, quarries, etc.)

So how much canopy cover is in your neighborhood?  And how has it changed between 2008 and 2018?  Click here for a readable table (PDF) of land cover by the 60 Knoxville neighborhoods.


Is there a correlations between heat levels and tree canopy? Tree canopy data can be overlaid with the findings from the Knoxville Urban Heat Watch campaign in which local organizers and volunteers collected thousands of temperature and humidity data points in the morning, afternoon, and evening on August 27th, 2022.  The chart below shows that areas of high canopy are 5-10 degrees cooler in the mornings and evenings.   This is important to note, as heat levels have a direct impact on respiratory and other human health conditions.  Considering that trees also remove up to 60% of the street level air pollutants (exhaust, dust, other chemicals) as well, trees have a significant impact on human health and must be available to every neighborhood. 





Impact of Redlining on Tree Canopy.  Tree canopy is being examined by areas that were redlined in the mid 1900s (watch this UT video to learn more about the discriminatory housing and lending practices of the FHA and Home Owner Loan Corporation in general, and in Knoxville specifically). The chart below right shows percent canopy in each HOLC district, each dot represents a neighborhood.  There were only three neighborhoods in the A category, and all have at least 50% canopy or more.  The rating categories B through D have tree canopy covers that go as low as 5%.  More analysis is planned.





More information on Knoxville Canopy

More analysis is planned for Knoxville's tree canopy, and will be shared on this website.  Until then, below are resources to learn more on your own.

You can use this web-based viewer to explore tree canopy cover in Knoxville from the recent study. 

If that is too complicated or technical for your taste, you can view the full report in PDF format here and geek out to all things trees as much as you like.  Two versions of the report were prepared - one that is county-wide, and the other that provides data on tree canopy just within the City of Knoxville boundaries. 

County-Wide Tree Canopy Report [PDF] goes over the finding of the study for both Knox County and the City of Knoxville.

City of Knoxville Tree Canopy Summary Assessment [PDF] 

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Trees & Climate Change

Knoxville Trees and Climate Change

In broad terms, there are two parts to dealing with climate change: 1) slowing climate change itself, and 2) working to adapt to climate changes already here.  Trees are keys aspects of of both efforts.  Learn more here about dealing with climate change and the roles trees play in those efforts, then take a look at the expected impact of climate change on Knoxville's trees themselves.
  

1. Slowing Climate Change Overall (also called "mitigation")

These are efforts to reduce the cause of climate change to slow or stop the effect altogether.  This is called "mitigation," and is most commonly undertaken through efforts of communities to reduce greenhouse gas emissions over time.  

Knoxville has emission reduction goals in place.  It is also important to note that trees absorb carbon in the atmosphere every single day through the process of photosynthesis.  They take in carbon in the air through the leaves, make sugars for food, and emit oxygen back into the air.  This process of taking carbon out of the air is called "carbon sequestration."  Trees also store carbon over their lifetimes in their tissues (wood).  If that tree is taken down and chipped up or burned, that carbon is released. 

Take a look at what the existing trees in Knoxville do already today:
Figures taken from 2021 Urban Tree Canopy Assessment Knox County and City of Knoxville Report.

COUNTYWIDE (Knox County)

- Carbon Storage:  5.5 million tons over the trees' lifetimes. A service worth $946 million.
- Carbon Sequestration: 100,000 tons each year.  A service worth $17 million each year.  

CITYWIDE (City of Knoxville)

- Carbon Storage:  767,000 tons over the trees' lifetimes - A service worth $130 million. 
- Carbon Sequestration: 4,000 tons each year - A service worth $3.6 million each year.

This all means that any losses in tree canopy will have an effect the amount of greenhouse gases in the air, and contribute to the increase in speed and severity of climate change in Knoxville.
 
City of Knoxville's "Mitigation" Efforts

Climate change is real and intensifying many challenges we already face in Knoxville, like extreme heat, extreme storms, flash flooding, air pollution, and pests. Reducing Knoxville’s emissions footprint is crucial to combating climate change, improving our ability to bounce back from disasters, protecting our health, and being smart with taxpayer dollars.

Learn more





2. Adapting to Existing Climate Change Efforts (also called "adaptation")

The second piece of dealing with climate change is figuring out ways to adapt and adjust to its effects (high heat levels, severe storms, flooding, drought) already here.  This includes finding ways to lessen the impacts on humans and ecosystems in general.

Trees are a significant part of this work, considered by many as one of the most effective and cost efficient ways for dealing with heat and heat stress in communities.  Urban trees are widely accepted as one of the most effective long-term solutions to reducing the effects of urban heat islands. Properly placed mature tree canopy can lower overall ambient temperatures by 20° to 45°F (EPA 2015). Human mortality (death) from heat stress related issues kills more Americans each year than any other natural disaster.

Learn more about the role trees play here.


Impact of Climate Change on Knoxville Trees

While tree canopy is a key piece of climate change resilient, they themselves are not immune from the effects of climate change.  They can suffer from the effects of climate change - warming temperatures, changes to precipitation, increases in threats from pests and diseases - can mean that trees currently thriving in Knoxville may not thrive in 100 years. 

The USDA Climate Atlas tool is a model that can be used to predict which trees may be impacted by the effects of climate change. While some species are predicted to face declines throughout the Knoxville area (including 23% of Knoxville's current street tree population), other trees are anticipated to fare better over the next 100 years. Additional, new species may even move into this area.

Check out this chart below for the full list of predicted changes to Knoxville's trees.



24% of public inventoried trees expected to fare better over the next 100 years due to climate change.

• Willow Oak (5% of public trees today)
• Common Hackberry (4%)
• Southern Magnolia (2%)
• American Elm (2%)
• Eastern Redcedar (1.3%)
• Boxelder Maple (1.2%)
• American Sweetgum (l.0%)
• Black Cherry (1.0%)
• River Birch (0.9%)
• Black Gum (0.7%)


23% of public inventoried trees expected to decline over the next 100 years due to climate change.

• Red Maple (7% of public trees today)
• Sugar Maple (7%)
• Eastern redbud (4%)
• Silver Maple(2%)
• Tulip Poplar (1.4%)
• Serviceberry (0.5%)
• Eastern Hemlock (0.5%)
• Black Locust (0.4%)
• Virginia Pine (0.4%)
• Scarlet Oak (0.3%)


Tree Species that may find new habitat within the Region.

• Florida Maple
• Black Hickory
• Black Ash
• Swamp Tupelo
• Ashe Juniper
• Slosh Pine
• Bluejock Oak
• Laurel Oak
• Live Oak
• Gum Bumelia
• Cedar Elm

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References

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Barbosa, M., Fernandes, A.C.C., Alvez, R.S.C., Alves, D.A., et al. 2021. Effects of native forest and human-modified land covers on the accumulation of toxic metals and metalloids in the tropical bee Tetragonisca angustala. Ecotoxicology and Environmental Safety 215: https://doi.org/10.1016/j.ecoenv.2021.112147

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