Authored by: Gregory Cooper, Anne Liberti, and Michael Asch
Supervised by: Nicolette Cage, Nicholas School of the Environment, Duke University
The scope of this project sought to understand the present state of the urban forest by examining the current canopy through a historical lens. Historical planting efforts shed light on why and where trees are and are not located. Assessments of recent plantings, current canopy cover, and extent of invasive species in parks will offer insight for the management of the urban forest. The resulting analysis will guide the City of Durham to determine ideal planting sites for new trees to maximize environmental and social benefits with a recommendation for policy change in the existing planting protocol.
The Urban Forest is defined as all trees within the boundaries of a community and is described by the US Forest Service as a forest ecosystem that provides a variety of services to residents (USDA)
In the 1930s, a federal program known as the Home Owners' Loan Corporation (HOLC) created maps within Durham and across the United States depicting neighborhoods based on their credit risks which, generally followed racial lines. These maps have become know as "redline" maps; they colored communities with the highest loan default risk red; these were usually black neighborhoods. The potential relationship between the tree plantings of the 1930s and the racial composition depicted by the HOLC maps are explored in the assessment of current canopy cover in the City of Durham.
History
The history of Durham’s urban forest is rooted in the early twentieth century and attributed to some notable figures in Durham’s history. The Olmsted Brothers were contracted by James B. Duke of Duke University to design the new West Campus landscape. Their tree species selection and location of plantings on West Campus continue to inspire the City of Durham in its design of public spaces and parks. In 1930, Clarence Korstian arrived in Durham as the first dean of the School of Forestry at Duke University. At the same time, the City of Durham was exploring the possibility of starting an urban forest and created a tree commission in 1931. Korstian, who had been involved in the City beautification committee, was named the tree commissioner in 1933. He worked on drafting ordinances guiding the planting and care of trees planted along streets and in other public places. These trees are what has grown to become the mature canopy seen along Durham’s streets today.
The City of Durham is an older city located in the central part of North Carolina. The current state of Durham’s urban forest is an aging and declining canopy of willow oaks (Quercus phellos) and water oaks (Quercus nigra). These trees are being replaced by planting three saplings for every large tree removed. The Durham City-County Environmental Affairs Board recently released a document estimating that 1,600 new trees will need to be planted annually to offset the loss of the aging oaks (Durham EAB 2015). Furthermore, the city has a goal to maintain canopy cover at 40% for the next 20 years (Durham EAB 2015). Therefore the planting of new trees will be a high priority in the coming year. This project seeks to give guidance in the selection of tree planting sites as well as species selection to the City of Durham.
Method
To characterize the current canopy in the City of Durham, a canopy cover assessment was conducted along with an analysis of recent plantings and investigation of city parks. The canopy cover assessment revealed that overall the City of Durham has 56% tree cover, and the city maintained rights-of-way (ROW) have 28% tree cover. Among neighborhoods, there was the variability of tree canopy within the ROW ranging from 0% up to 74%. This analysis was also conducted based on the historic HOLC map and found that current tree canopy cover within the ROW of the most affluent neighborhoods was 65%, while the most impoverished areas had a 21% canopy cover. Tree plantings between 2007 and 2014 were analyzed for composition and distribution. Of the nearly 2,500 trees planted, it was found that approximately half of the species were native, and the other half were exotic. These trees were planted across 33 neighborhoods, with Trinity Park receiving the most trees (569) within this time period. Through surveys of a sample of city parks, parks were shown to have a high occurrence of invasive herbaceous species and ash trees.
Urban Canopy Cover of Historic HOLC Redline Maps
The focal area of this particular study is the City of Durham and its urban forest, which compared to the analysis of three other cities within the United States. Utilizing US Census blocks and their percent non-white demographic distributions, this study aimed to find relationships between urban canopy cover and racial composition. The HOLC map of Durham was assessed with the intent of finding historical roots in current urban forest inequities. This analysis will explore the following hypotheses:
- Will there be less canopy cover in areas of lower HOLC grade?
- Will the canopy cover be lower in rights-of-way areas with lower HOLC grade?
- How do current demographics within the original HOLC zones correlate with canopy cover?
Through the investigation of these hypotheses, this analysis intends to describe the current distribution of canopy cover through historical lens, which reflects racial composition in physical space over time, dating to when existing mature trees were planted.
Analysis
A set of multi-criteria decision analyses (MCDA) were constructed to guide the City of Durham in identifying future sites for planting, and areas to focus efforts on park maintenance and reforestation using a variety of environmental metrics. The MCDA’s result in prioritization lists to be used by the City of Durham to allocate future efforts efficiently.
The final recommendations of this project are for the City of Durham to:
- Reassess the protocols for tree plantings to create an equitable distribution.
- Reevaluate the native/exotic species composition used for street tree plantings.
- Use the MCDAs for tree plantings and parks to develop an action list for future projects.
Results
The City of Durham was found to have an area of 283 square kilometers (including unclassified pixels), which varies by the real sector of 280 square kilometers by 0.72% (Table 1). The City of Durham overall was found to have a canopy cover of 56% or 158 sq.km, while its ROW was covered with 28% canopy (11 sq. km).
When looking at the neighborhood level, on average, they were very representative of the City of Durham as a whole with 56% canopy cover and a slightly higher canopy cover in the ROW with 29% (Table 2). Approximately 23% of the canopy cover of the entire City was found in the study neighborhoods, and around 1% of the total City canopy was found within the ROW in these neighborhoods. The distribution of canopy cover among the communities shows that generally, less canopy cover is within the downtown areas with higher canopy cover in the outlying communities (Map 3 & Map 4).
Discussion
The results of this analysis generated a comprehensive list of street segments within the City of Durham that can be used to target future planting sites throughout the City. This analysis can be combined with other information such as the willingness of the various neighborhoods to participate in planting plans, as well as identifying streets within areas that historically have not been involved in planting projects. This analysis sought to maximize the goals of the City of Durham in reducing stormwater runoff, as well as to improve air quality within the most susceptible areas that can be achieved through tree planting.
Conclusions and Recommendations
This project was designed to aid the City of Durham in taking steps in replanting a diverse, resilient, and equitably distributed urban forest. One first needs to know what they are working with to help plan for the future; this included looking at the present canopy, recent tree plantings, and parks throughout the City. To garner an understanding of the canopy distribution, one had to consider the history of the City of Durham. Knowledge of the spatial structure of the canopy, prioritization through MCDAs can be put in place to create a structured planting plan for the City. Working towards this end required multiple analyses, on varying scales, and a literature review.