Ensuring Sustainable Development with Tree Canopy Assessment in Wake County, North Carolina

Wake County, North Carolina, is one of the fastest growing counties in the US—by 22,000 people a year and projected to continue. With significant ongoing infrastructure development expected over the next five years, the county recognized the need for sustainable growth. It also knew that one major factor in ensuring this sustainability was reversing the county’s declining tree canopy. 

Trees provide critical community infrastructure, help clean the air, filter the water, and mitigate the impacts of climate change. Each year, Wake County trees remove 11,022 tons of pollutants from the air, absorb 414,710 tons of carbon dioxide, and intercept 8.1 billion gallons of stormwater. The ecosystem service capacity of the county’s tree canopy was measured and valued using i-Tree, a software suite made available by the US Forest Service in collaboration with the Davey Tree Expert Company. The total value was estimated at $3.2 billion, combining carbon storage, air pollution removal, carbon sequestration, and stormwater capture benefits. 

Trees also provide less quantifiable benefits, such as potentially increased property values, shade and cooler air in the summer, energy conservation, improved physical and mental health, and increased biodiversity and habitat.

The Wake County Planning Department saw what a vital resource its trees were and set out to mitigate the dissipating canopy. When the American Rescue Plan Act (ARPA) became available, the department decided it was the perfect opportunity to secure funding for a project to guide and support mitigation and planting efforts on both county and municipal levels. As the project aligned very closely with two of the county’s current goals—community health and vitality and growth and sustainability—it was an obvious choice for the county to award the funding.  

With funding secured, the Wake County Planning Department partnered with a consultant, Davey Resource Group (DRG), to perform a remote sensing analysis to identify the land-cover composition of the 857 total square county miles segmenting 12 core municipalities. Using vintage imagery made available by the US Department of Agriculture’s National Agriculture Imaging Program (NAIP), DRG employed ArcGIS software and remote sensing methods to identify different types of land cover, including impervious surface tree canopy, water bodies, vegetated areas, and barren land.  

DRG used ArcMap and ArcGIS Pro to extract and edit land-cover information for image analysis. It used ArcGIS Spatial Analyst to calculate information from raster data. DRG was able to share data with Wake County agencies via ArcGIS Online, giving them one authoritative source of data that they could review and analyze together. 

DRG performed a second land-cover analysis using 2010 NAIP imagery to measure the temporal change of Wake County tree canopy. The data then allowed DRG to measure and compare Wake County’s tree canopy over the 10-year span from 2010 to 2020. During that decade, Wake County lost 11,122 acres of tree canopy. The loss represents a 3.6 percent decrease in the total amount of countywide canopy and a 2 percent decline in canopy cover relative to overall land cover. 

One of the main goals of the geographic information system (GIS) analysis is to help identify planting locations throughout the county. Potential Planting Areas (PPAs) are from land areas within the Bare Soil and Vegetation land-cover types. Because not all pervious land is realistically suitable or feasible for planting trees, these possible planting locations are further analyzed by filtering them through an exclusionary layer of no-planting areas. The layer includes utility easements, access easements, public rights-of-way, recreational facilities, and active agriculture or horticulture sites. The analysis identified a total of 404,879 PPAs totaling 82,460 acres.

Once identified, PPAs were individually assessed across social equity and environmental impacts. Analysis was conducted using the Social Equity Index data provided by Wake County and health data gathered from the Centers for Disease Control and Prevention's (CDC) PLACES  study. The environmental assessment identified areas in greatest need of tree canopy to mitigate the consequences of environmental impacts such as stormwater, soil erosion, soil permeability, and floodplain proximity. Targeted tree placement can help reduce these effects.

A GIS-based tree placement model was then used to determine how many trees could potentially fit within each PPA. The model differentiates between tree size at maturity (large, medium, and small), giving preference to large-growing trees and utilizing spacing commonly suggested for a landscape setting. 

The completed Wake County Land Cover Analysis & Tree Canopy Assessment analysis was delivered to the municipal planning departments via PDF reports, and it immediately generated interest. Previously, planning departments didn’t have a way of identifying, measuring, or providing a metric for what is occurring with the development throughout the county. As Bill Shroyer, GIS manager for the Wake County Planning Department and technical lead for the project, explained, “It was very nebulous to a lot of our local communities as to how to approach what they're seeing, and now that they have the report, they understand the loss—and a lot of it occurred in areas that they wouldn’t have expected.”

Shroyer also used ArcGIS StoryMaps to deliver all the information available in the report more interactively. “I wanted to be able to create an engaging story with interactive maps that kind of gave the reports a little more life than just being very static. Several of the maps even allow our community folks to search their address and get an idea of where their homes fit within the analysis.”

The information has led to collaboration among various planning departments as well as other internal and external agencies. For instance, it led to a collaboration between the Wake County Planning Department and the North Carolina Department of Transportation on a planting project for a cloverleaf interchange.

Another approach the department took to disperse this critical information more widely was through an application called  TreeKeeper Canopy, an extension of the TreeKeeper® Software Suite of tools. Created by DRG using ArcGIS REST APIs and leveraging data published via ArcGIS Server, the mapping app provides the tools, data, and resources to guide urban forest management and reforestation efforts. Users can conduct priority planting analyses through criteria weighting and get insights on tree canopy projects and ecosystem benefits. As Shroyer explained, “If a municipality or a nonprofit or a parks program identifies their area as being a priority, they can get a report as to what it would take to establish planting in that area.”

Will Ayersman, GIS Manager for DRG’s Urban Tree Canopy Assessment projects, added, “[the application] allows communities to utilize the tools to plant with a purpose which can include addressing heat island effect, stormwater reduction, or social equity. Purposeful planting with a targeted approach increases awareness within the community and is also beneficial when applying for additional funding to support the planting programs within Wake County.” 

The reports and maps continue to be consumed as development continues, and the plan is to provide an update every five years. However, now that the baseline work has been done, the department can more easily provide updates to address specific concerns it is getting from the public via community engagement.