A City of Trees

Assessing Urban Tree Canopy with GIS & Remote Sensing for Chattanooga, Tennessee

Key Take Aways

  • Urban forests represent half of the landcover types for Chattanooga, but density of forest area varies spatially and is declining.
  • Associations between the amount of urban forests and variables such as median income, population density, and urban heat islands exists in Chattanooga neighborhoods.
  • It is crucial to protect and enhance existing urban forests for health, sense of place, urban ecosystems, to combat heat island effects, and climate resilience.
  • Precision GIS and Remote sensing are crucial for analyzing forests and other land cover types. Future land cover assessments will need to be ran to track changes to urban forests over time.
  • Protection of urban forests depends on stakeholder wants and goals and likely can’t happen without partnerships and collaboration among residents, non-profits, and local government.

The Benefits of Urban Tree Canopy

Urban forests which include all trees, shrubs, lawns, and other vegetation types provide numerous ecosystem services to urban residents. Trees provide clean air, climate resilience, ecosystem refugia, outdoor recreation, and a sense of place. Research suggests that urban forests can also help offset greenhouse gasses produced by cities, lowering a municipalities total carbon footprint. Air quality modeling has also identified links between air pollution and increased temperatures as the density of urban tree canopy and forests declines.

Source: Institute of Chartered Forests

Neighborhoods with fewer trees are more likely to contain "heat islands" where the average surface temperature is higher than that of the rest of the city. These neighborhoods are more likely to suffer from stronger heat waves during the summer leading to more heat related illnesses, such as heat strokes. Fewer trees and more urban heat islands are also associated chronic illnesses related to inactivity, respiratory illnesses, and lower mental health. A growing body evidence also suggest that more forested urban neighborhoods tend to be richer, safer, and overall, healthier.

A Changing City Scape

Today approximately 3% of the earth is classified as urban, 50% of the world’s human population live in cities, and 50% of earth's surface has been altered by humans. Urbanization & agriculture are the leading causes of forest loss globally and Chattanooga is no exception to forest loss.

The animated map above shows the growth of developed areas in red and loss of urban forest in green. Since 1984, Chattanooga has seen a 134% increase in developed land.

Chattanooga, Tennessee is nestled between the Cumberland Plateau and the Blue Ridge Mountains along the banks of the Tennessee River. A city of 181,000 people is renowned for its proximity to world class outdoor recreation, fast internet, and relatively low cost of living. Chattanooga has also been identified as the  sixth fastest warming city  in the United States in a recent study. The Scenic City has come a long way since Walter Cronkite famously declared it the most polluted city in the country in 1971. City leaders and citizens have invested heavily in transforming it to the bustling urban adventure town that it has become known for. With its recent popularity, Chattanooga is seeing rapid expansion. An analysis of LandSat Imagery classified into thematic land cover maps revealed that since 1984, Chattanooga has seen a 43% reduction in its urban forest and a 134% increase in developed land.

This map shows heat severity, land surface areas that exceed Chattanooga's mean surface temperature. The redder an area is, the hotter it is compared to the rest of city. These areas are referred to as heat islands. Source: Trust for Public Land, 2021.

Little research attention has been given to the relationship of urban forests, also referred to as urban tree canopy (UTC) and socioeconomic variables for Chattanooga, Tennessee. The City of Chattanooga and  Green|Spaces , a local sustainability non-profit, are working together to create an updated forestry management plan for the city. A large part of that plan requires an accurate and high resolution land cover map. These and other stakeholders are interested in understanding the relationship and impact of urban forests on local neighborhoods. The results of this study aim to help understand these relationships for Chattanooga and inform future projects related to tree planting and management, reduction of prevention and reduction of heat islands, neighborhood place making, park planning, and overall improvement of life in the urban landscape.

Mapping Urban Tree Canopy with Remote Sensing & GIS

 The University of Tennessee at Chattanooga's IGTLab  was tasked with producing the highest resolution land cover map possible for stake holders to understand the distribution of land cover and how it relates to neighborhoods in Chattanooga. UTC spatial data scientists turned to the Planet Skysat imagery program for  multispectral imagery , imagery which contains color values beyond the wavelength spectrum visible to the human eye. Over the course of late summer and early fall of 2021, Planet tasked its satellites over Chattanooga collecting 1000s of high resolution images providing the research team with 50 cm resolution imagery, among the highest resolution currently available. The latest imagery and remote sensing analysis tools found in Esri's ArcGIS Pro were deployed to derive land cover information from these high resolution images. A machine learning method known as  object based image analysis  was used to classify the Skysat images into land cover classes representing forest, non-forest vegetation, developed land, bare earth, and water. With an accuracy rate over 90%, the result is the most accurate and highest resolution land cover map ever produced for Chattanooga.

Images of raw Skysat Imagery classified into thematic land cover data. Left: Coolidge Park. Right: Chattanooga National Cemetery.

Exploring Chattanooga's Urban Forest Canopy

The resulting land cover map derived from the imagery is the most accurate and highest resolution ever created for the city, at 50 cm resolution with an accuracy rate of 92%. Zoom in on the map below to explore how land cover varies across the city. The map is broken in the following classes:

Forest, Non-Forest Vegetation, Developed, Water, Bare Earth/Soil

The resulting 50 cm resolution Land Cover Map of Chattanooga.

Left: 30 Meter Resolution National Land Cover Dataset vs Right: 0.5 Resolution Land Cover of the Walnut Street Pedestrian Bridge. Note, the dark areas are shadows which were removed in post processing steps.

Other high quality land cover information products exist such as the National Land Cover Dataset, but they tend to generalize forests at large scales and are more suitable for regional level spatial analysis. The project needs for Chattanooga's urban forestry plan require neighborhood and parcel level detail allowing city foresters to identify tree planting opportunities on locations such as road and sidewalk buffers, city owned vacant lands, and public parks.

While forest and urban tree canopy cover comprise approximately 49% of the total land area for Chattanooga, the distribution and density of forest and urban tree canopy are not even. By summarizing land cover to 100 acre hex bins and calculating the rate of forest cover as a percentage, the percent tree canopy was measured and shown on the map below. This tells us which areas have denser urban tree canopy than others.

Percent forest canopy. Dark green areas have a higher density of forest, lighter colored areas are less dense.

Applying this same methodology to neighborhoods gives an understanding of which parts of the city have more forest than others. Use the interactive chart and map below to explore the percent of urban canopy by neighborhood.

Chattanooga neighborhoods displayed by percent forest canopy.

Urban Tree Canopy and Demographics

When the urban tree canopy data highlighted above is overlaid with demographics and other variables for Chattanooga, patterns and relationships begin to emerge. This study found a positive statistical association between median income and educational attainment to percent canopy and a negative association was identified between percent canopy and minority populations. A strong negative correlation between percent urban tree canopy and heat islands was also identified as shown in the map below. In other words, there is evidence that suggests as urban tree canopy increases, so does median income for neighborhoods and as percent canopy decreases, heat severity increases.

Use this swipe map to compare percent forest canopy to urban heat islands (source: Trust for Publich Land, 2022.)

The map below shows census block groups where urban tree canopy and median incomes are low and high.

Urban Tree Canopy and Median Income by Neighborhood & Census Block Groups. Use the interactive legend to filter neighborhoods by median income and percent forests.

Final Thoughts

The protection of urban forests ultimately depends on the needs and wants of residents and will need to be monitored overtime to assess urban tree canopy goals and land cover change over time.

With forest cover representing half of the land cover types for Chattanooga, it is important to celebrate how much urban forests remain in Chattanooga. It is also important to maintain the existing canopy and increase it in neighborhoods where it is low. This will help maintain "tree equity" for the city ensuring that all residents have access to the benefits that urban forests provide. This will help maintain a sense of place, the overall quality of life, climate resilience, and help keep the "Scenic City" scenic.

The research was conducted by the Univeristy of Tennessee at Chattanooga with support from the Lyndhurst Foundation.

Charlie Mix-GIS Director, Dr. Azad Hossain-Associate Professor, William Stuart-GIS Technician & Master's of Science in Environmental Science Candidate, & Nyssa Hunt-Assistant GIS Director. Contact: charles-mix@utc.edu

Images of raw Skysat Imagery classified into thematic land cover data. Left: Coolidge Park. Right: Chattanooga National Cemetery.

Source: Institute of Chartered Forests

Left: 30 Meter Resolution National Land Cover Dataset vs Right: 0.5 Resolution Land Cover of the Walnut Street Pedestrian Bridge. Note, the dark areas are shadows which were removed in post processing steps.