Durham Tree Canopy Assessment 2017

Summary

Durham has 36,600 acres of tree canopy, covering 52% of the City. This compares favorably with many urban areas. There is a statistically significant inverse relationship between tree canopy and surface temperature providing heat island mitigation. All land uses have relatively high canopy cover, but as most of the vast majority of the land in the city is residential, a canopy strategy for that land use is critical to achieving the tree canopy cover goal.

Durham has a substantially higher percent tree canopy than most other comparable cities in the USDA Forest Service’s database. It also has room to establish more tree canopy.

HOW WAS THE TREE CANOPY MAPPED?

Tree Canopy derived from high-resolution imagery and LiDAR. Advanced artificial intelligence algorithms were applied to the imagery and LiDAR data to automatically extract tree canopy and other land cover features, a process that was then followed with a detailed manual editing process to correct for any errors. The end result of this mapping process was the most accurate accounting of Durham’s tree canopy ever conducted.
Lidar (A) and imagery (B) were used to map tree canopy (C) and land cover (D) for the entirety of the city.

DOES DURHAM’S TREE CANOPY MITIGATE THE URBAN HEAT ISLAND?

Existing tree canopy (left) and surface temperature (right) summarized by 1000-foot grid cells.

For this study, we obtained thermal imagery from the Landsat satellite to derive surface temperature. We then divided the city into 1000 by 1000 foot grids cells, summarizing the percent existing tree canopy and surface temperature for each grid cell. There is a statistically significant inverse relationship between tree canopy and surface temperature, pointing to the likelihood that tree canopy helps to keep the city cool.

Surface temperature in relation to tree canopy. Each circle represents a 1000-foot grid cell and is color coded based on the impervious surface percentage.

HOW IS DURHAM’S TREE CANOPY DISTRIBUTED BY LAND USE?

We summarized tree canopy for each and every property parcel in the city. Then grouped them into ten generalized classes.
Percent existing tree canopy and total tree canopy area in each generalized land use type.

Unlike most other communities, the percent existing tree canopy in each land use class is remarkably consistent. Typically, industrial and commercial areas have substantially lower tree canopy than residential, recreational, and wild areas, but this is not the case for Durham. In terms of overall control of tree canopy, it is the city’s residents that are the clear majority owners. The future of the city’s tree canopy rests largely with Durham’s residents, who control the majority of the tree canopy.

HOW MUCH TREE CANOPY AND HOW MUCH ROOM TO PLANT TREES IS WITHIN EACH WATERSHED?

All watersheds have close to 50% tree canopy or slightly above. Maintaining this amount of tree canopy will help insure that Durham’s rivers and streams are functional ecosystems for years to come.

Tree canopy metrics for the portions of watersheds that fall within the city boundary

HOW CAN THE TREE CANOPY ASSESSMENT HELP DURHAM DECIDE WHERE TO PLANT TREES?

Issues of environmental justice arise when tree canopy is unequally distributed based on factors such as income or ethnicity. Planting initiatives can help address these issues. We summarized tree canopy for each Census block group. The Census block group highlighted in the above figure has only 11% existing tree canopy, far below the city average. This area has a high diversity index (the likelihood that two persons randomly sampled are of a different race), lower than average income within the city, and the presence of young children is high compared to the average. In addition, there has been some recent construction in that area. The combination of these factors may make it an excellent candidate for a street tree planting initiative. Using the tree canopy data, we know that 29% of the ROW is vegetation without tree canopy, indicating that there is room to establish new street trees.

HOW CAN THE ASSESSMENT HELP DURHAM DECIDE HOW MANY TREES TO PLANT ANNUALLY?

In order to maintain the present level of tree canopy cover in road rights-of-way, Durham will have to plant at least 500 trees annually through 2040.

  • Base case (no planting program): approximately 50% reduction
  • 500 trees planted annually: approximately break-even
  • 1,000 trees planted annually: greater than 50% increase
  • 1,500 trees planted annually: more than double.

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