City Trees is a premier publication focused on urban + community forestry. In each issue, you’ll learn how to best manage the trees in your community and more!
Issue link: https://read.dmtmag.com/i/701610
32 City Trees When I began my PhD program in the summer of 2011, emerald ash borer (EAB) was well established in Pittsburgh, Pennsylvania (see Figure 1). Ash was a large and important canopy species within the city and EAB was spreading quickly. Pittsburgh is a city that is prone to landslides, and Pittsburgh's urban forest plays a significant role in stabilizing the city's hillslopes by reinforcing soil and removing excess water. There was some concern that the loss of ash from Pittsburgh's urban canopy could lead to increases in landslide susceptibility. With the help of two Pittsburgh urban forest NGOs, Tree Pittsburgh and TreeVitalize, I carried out a project to assess the impact of the loss of ash trees on Pittsburgh's slope stability. To accomplish this task, I used geographic information systems (GIS) to model changes in the risk of slope failure following the loss of ash trees. Specifically, I modelled Pittsburgh slope stability with the SINMAP (Stability INdex MAPping) equation. This model is built on the key factors of slope stability includ- ing: soil density, soil water transmissivity, rainfall rate, and Figure 1. Dead ash trees in Pittsburgh, PA. Aerial image from Google Earth; ash tree and EAB galleries photos by Erin Pfeil- McCullough slope angle (see Figure 2 for all properties). The model uses these characteristics to classify forested pixels (segments) into 1) stable areas that would not fail, 2) moderately unstable areas that might fail, and 3) unstable areas that would fail as a result of a specified rainfall intensity (see Figure 3 for an example of the output). The soil property most affected by the loss of ash was cohesion, as the loss of root cohesion should be substantial. Cohesion is the tendency of a soil to stick together under stress. Tree roots further increase cohesion by anchoring soil in place and removing excess water. Figure 2.A) The SINMAP equation and the landscape char- acteristics that determine slope stability. B) The application of SINMAP in ArcGIS using gridded spatial data. In the equation, FS stands for "Factor of Safety," whic is a ratio indicating slope stability. Figure 3. This is a hillshade (shaded relief) of a DEM (Digital Elevation Model), with a SINMAP landslide model output over- lain on top. Mount Washington and North Perry are identified in the image. Yellow pixels are considered quasi-stable and the red pixels are considered unstable and likely to fail under the given rainfall conditions in the model.