Understanding Street Tree Species Composition Shifts Over Time

Urban forests are among the most dynamic and essential ecosystems in the age of the Anthropocene, where land use is ever evolving to serve human needs. Cities have adopted strategies such as increasing canopy cover via tree planting initiatives to combat the negative impacts of development. If properly distributed and maintained, these organisms promote a wide range of benefits, from cooler temperatures to happier residents. Achieving a healthy street tree community, for example, requires regular assessments and diligent management to understand forest characteristics and species-specific implications on the vigor of trees and the surrounding environment. While data for one planting season or a static inventory provides a snapshot in time of urban forest structure, often addressing acute or temporary solutions for areas of concern, analyzing trends in planting records over time can shed light upon management decisions of the past, present, and future. In this study, I show that street tree planting initiatives in Philadelphia, PA have shifted focus from larger, non-native species to a variety of medium- and small-stature species. Using street tree planting records from both a public agency and a non-profit program, I identified a transition from planting almost exclusively large species, as high as 99.17% of plantings in the 1950s, to a more diverse and evenly distributed community of small-, medium-, and large-stature trees in the 2000s, with relative planting abundances of 21.77%, 44.14%, and 31.43%, respectively. Further, I found that native plantings increased by as much as +95.74% in later years. Quantifying the species composition paradigm of street tree systems can provide insights as to how and why these susceptible communities are changing, and how these changes may affect ecosystem service potential.