Street Tree Canopy
Investigating street tree canopy cover in Charleston subdivisions
Access to tree canopy cover or "tree equity" has become a popular topic with outlets like The New York Times and National Geographic publishing articles outlining disparities in green-ness based on socioeconomic status and/or racial composition. This wonderful Tree Equity Tool by American Forests examines tree canopy coverage by census block group with all the related demographic data.
In contrast, this study focuses on tree canopy cover over public right-of-way (streets and sidewalks) by subdivision. Since the public right-of-way is the means through which one navigates a particular area, canopy cover over streets and sidewalks contributes more to how a neighborhood "feels."
This methodology also uses a high resolution (one-meter) land cover map to determine tree canopy rather than the 30 meter land cover map available through the National Land Cover Database .
Historically, public recognition of street trees within Charleston's urban landscape dates back to 1750, when a new street law proclaimed:
“That it shall and may be lawful for the owner or proprietor of any lands fronting any of the streets in the said town to plant such trees with the consent of the said commissioners but not otherwise as they shall think convenient and also to enclose such trees to prevent their being destroyed.”
The earliest street trees were intended to create a barrier between unpaved carriageways and brick-laid pedestrian footpaths. When the City of Charleston was officially incorporated in 1783, city council assumed authority over streets, including responsibility for the trees lining them.
(" Street Trees in Early Charleston: Fountains of Air and Shade ", Nic Butler, Charleston County Public Library)
Image: " A sketch of the operations before Charleston, the capital of South Carolina ", (1780) Library of Congress
Current interest in urban tree-scapes is less superficial; focusing on reducing greenhouse gas emissions through carbon uptake, lowering surface temperatures caused by "heat islands", and reducing stormwater runoff and flooding . According to American Forests , tree canopy should cover 43.3% of US urban areas.
So where do we need more trees?
This map shows the percent of street area covered by tree canopy.
The percentages were determined by creating a 50 foot buffer on either side of each street centerline (in order to include sidewalks), dividing the resulting area by subdivision, and calculating the percent of tree canopy covering that area.
Numbers displayed in a larger font size represent subdivisions with more mileage of streets.
For example, Wagener Terrace has around 11 miles of street, with 46% street tree canopy coverage.
Across the Ashley River, Charles Towne Landing (actually a park, not a subdivision) has a whopping 76% canopy coverage, but only about 2 miles of street.
Thus, subdivisions with a low percentage displayed by a larger font size number could use more trees along the streets.
Click on a subdivision for more info
To determine where more trees should be planted, lets filter out all neighborhoods that meet American Forests' target tree canopy cover of 43.3% or more.
Remember, the displayed percent is the tree canopy over the street, not total subdivision canopy.
These neighborhoods have less than 20% street tree canopy coverage.
Since trees take years to mature, newer neighborhoods likely have trees planted, but the canopy cover is not yet fully realized. These neighborhoods have less than 20% street tree canopy coverage and, on average, buildings older than ten years.
Note: Average year built was summarized by subdivision from County parcel data. Year built is not available for every parcel, therefore some older subdivisions are not represented.
For example, Carolina Bay has only 14.46% street canopy cover but average construction (and therefore tree planting) occurred in 2013, signifying that this neighborhood's tree canopy would expand as the trees continue to mature.
Is it hotter in these subdivisions with less tree canopy?
Here are the same 20% street tree subdivisions over afternoon temperature data collected from the NOAA HeatWatch campaign.
Click on a subdivision for average morning, afternoon, and evening temperature.
(Refresh if the temperature layer doesn't load)
In July, 2021, volunteers drove around the city with heat sensors mounted on their cars or bikes to measure how the temperature varied from one area to the next. They did this in the morning, afternoon, and evening. The afternoon (pictured) showed the widest range of temperatures, with low tree canopy areas experiencing higher temperatures.
Since temperature variations are affected by more than just the trees along the streets, here is the morning temperature plotted against the subdivision-wide tree canopy and impervious surface percentages.
Morning Temperature Range
Note the positive correlation between impervious surfaces (asphalt, concrete, buildings), and temperature; and the negative correlation between trees and temperature-- more trees keep temperatures lower.
The afternoon temperature range is significantly higher so while some areas appear to "cool", they are actually hotter than the lowest morning temperatures.
Afternoon Temperature Range
And here's the evening temperatures
Evening Temperature Range
Highly impervious areas (with little tree canopy) stay warm even after the sun goes down.
But how does tree canopy relate to equity?
These neighborhoods have less than 20% street canopy coverage, buildings older than ten years, and a median household income below the Charleston median of $68,438.
Note: Since median household income data is collected by the census at the block group level (not neighborhood), these values were determined using an aggregating Enrich Data tool. Census block groups don't necessarily align with neighborhoods so median household income may be skewed in either direction.
Here are the same neighborhoods overlaid on the CDC's Social Vulnerability Index census tracts that are identified as experiencing moderate to high social vulnerability.
More equity data in Tree Canopy Tools app
This map shows subdivision-wide tree canopy vs. median household income.
Here's the same subdivision-wide canopy vs. percent of households whose income is below the federal poverty level.
And here it is vs. percent African American
There is a fairly weak negative correlation (when one goes up, the other goes down) between subdivision tree canopy coverage and percent African American...
but when we look at just the subdivisions on the peninsula, the relationship becomes stronger.
So, where are trees currently being planted?
Around 100 new trees have been planted in the Eastside neighborhood since 2019. The current street tree canopy cover is 19%, but that number will increase as these young trees mature.
A few more have been planted in Hampton Park and Marion Square.
...and where could more trees be planted?
In order to meet our goal of sequestering 112 MtCO 2 e (that stands for Mega-tons of Carbon Dioxide equivalent) by 2025, the City of Charleston Climate Action Plan recommends planting approximately 1,500 trees on public property, including rights-of-way, parks, and other City facilities.
Another goal is to increase tree plantings on private property.
This interactive Tree Canopy Tools application allows users to identify areas that may be suitable for planting new trees. The potential planting area (orange) was created by excluding existing canopy, building outlines, edge of pavement, and athletic fields. This application also includes more subdivision equity data.
You can help!
Most of the possible tree planting area is on private property. Residents, businesses and organizations are integral to increasing tree canopy and can find information on how to plant trees here .
Happy planting y'all!
