Canopy in flux: 40 years of tree cover change in Philadelphia

Authors: Lara Roman*, USDA Forest Service, Indigo Catton, University of Pennsylvania
Topics: Urban Geography, Land Use and Land Cover Change, Anthropocene
Keywords: urban forest; urban tree canopy; aerial imagery; urban development; shrinking cities
Session Type: Paper
Day: 4/3/2019
Start / End Time: 8:00 AM / 9:40 AM
Room: Congressional A, Omni, West
Presentation File: No File Uploaded

Many cities have set ambitious goals to increase urban tree canopy cover. By looking to the past, we can better understand the rates and processes of tree cover change. This presentation shares findings from a study of tree cover change in Philadelphia, Pennsylvania (US) from 1970 to 2010. Using aerial photography, we laid 10,000 points across the city to interpret land cover. The goals were to 1) document tree cover gains/losses, 2) assess transitions between land cover classes and tree cover stability, and 3) carry out those analyses for the entire city and various geographic units within the city, including parks and planning districts. Across the entire city, tree cover increased: 19% in 1970 to 23% in 2010. Most tree cover gains were transitions from lawn to tree. Yet tree cover was far from stable: only 12% of points were classified as tree across all decades. Protected open space had large canopy gains: 50% in 1970 to 64% in 2010. Planning districts which gained substantial canopy include residential areas that were developed relatively recently, as well as older neighborhoods with population loss and vacancy. We posit several possible explanations for tree canopy gain: urban form of recent residential developments, planting programs starting in the 1970s, emergent tree cover on vacant lots, and cessation of mowing in park lawns. This study represents long-term analysis of tree cover change, using a time window long enough for trees to show substantial growth, and demonstrating the continually fluctuating nature of urban tree canopy.

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