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Monitoring and Modeling Changes in Street Tree Communities Over Time
Robert Fahey, PhD
University of Connecticut
Street trees have a hard life, and some studies have indicated high mortality and turnover in street tree communities because of factors such as poor growing conditions, removals related to development, and damage (both intentional and unintentional). Urban forestry stakeholders are often focused on increasing the number of street trees through planting programs, yet ongoing mortality could undermine these efforts. Understanding the factors that affect street tree growth and longevity is essential to planning and promoting the future urban tree canopy.
In a recent study funded by TREE Fund in partnership with the US Forest Service, we investigated street tree population dynamics, including tree mortality, new plantings, growth, and health, in Philadelphia, PA and New York City, NY. We re-inventoried >3000 street trees across 161 randomly-located, fixed-length street segment monitoring plots that were originally established in 2015-2016 and re-inventoried in the summers of 2021-2023.
In Philadelphia, mortality for street trees was 3.67% per year, which is high compared to other studies on tree survival across urban forests as a whole. We found that a mix of tree characteristics (e.g., vigor, size), site characteristics (land use, impervious cover, demolition events), and neighborhood characteristics (e.g., poverty rate, unoccupied housing rate) all affected street tree survival. Mortality and planting patterns shifted the species composition of Philadelphia’s street trees away from large-statured shade trees toward small-statured ornamentals. These changes are likely to affect the benefits that street trees provide to urban residents, such as shade and pollution removal, and their role in the urban ecosystem in general (e.g., carbon storage potential).
This research study was funded in-part by TREE Fund's 2019 Asplundh- Hyland R. Johns Endowed Research Fund Grant Program.
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