By J. Morgan Grove and Jarlath O’Neil-Dunne
Newton Square, PA (August 1, 2011)- An important part of urban tree planting campaigns is measuring and evaluating current urban forests so that realistic tree-planting goals can be set. The term “urban tree canopy” (UTC) is used to describe the layer of leaves, branches, and stems of trees that cover the ground when viewed from above. Scientists now use various aerial and satellite reconnaissance methods to measure and evaluate the UTC. Northern Research Station (NRS) scientists have developed techniques to analyze and prioritize UTC data so that urban planners and parks departments can focus their efforts on locales that benefit most from tree planting.
Although aerial and satellite images can now show amazing detail, there are substantial shortcomings to using reflected light in mapping the UTC in urban areas-trees can be totally obscured by building shadows. A technology similar to radar is now being used instead to overcome these limitations-LiDAR (light detection and ranging). Unlike most aerial and satellite imaging systems, LiDAR sensors emit their own energy (near- infrared light) in the form of laser light, which is not sensitive to shadows. In addition, LiDAR data can provide estimates of height of features to within a few centimeters. Using advanced processing algorithms, NRS researchers are thus able to extract accurate and detailed estimates of the current tree canopy as well as find new locations to plant trees.
LiDAR is now about 20 years old, and has been used like radar by state troopers to catch speeders! Around 2005, NRS researchers-Morgan Grove (research forester at the Baltimore Ecosystem Study) and Jarlath O’Neil-Dunne (a geospatial analyst who is jointly funded by the U.S. Forest Service NRS and the University of Vermont’s Rubenstein School of Environment and Natural Resources)-developed process protocols for using LiDAR to inventory and analyze the UTC. Their first city was Baltimore, MD, and the team has completed UTC assessments for over 50 cities and towns, encompassing over 300 communities, mostly in the mid-Atlantic and northeastern states.
UTC estimates made with LiDAR rather than reflected light are considerably more accurate. Individual trees cumulatively can contribute substantially to a city’s overall tree canopy. In Philadelphia, previous estimates that ignored small trees put the total tree canopy at 10 percent of the city’s total land area. The newer and more advanced techniques developed by NRS scientists and their university collaborators, which can accurately detect smaller patches of canopy, revealed twice that amount, or 16,884 acres of existing tree canopy.
Urban Tree Canopy Analysis Helps Urban Planners With Tree Planting Campaigns