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Puget Sound Region: Tree Species Vulnerability Assessment

As the climate changes over the 21st century, the Puget Sound region’s urban forest will be impacted by changing temperatures and precipitation regimes, leading to implications for the people who depend on its ecosystem services.

This report summarizes climate change projections for the Puget Sound region and provides an assessment of tree species vulnerability in the region. We used projected shifts in plant hardiness and heat zones to understand how tree species of interest are projected to tolerate future conditions. We also assessed the adaptability of planted trees to stressors such as drought, flooding, wind damage, and air pollution, as well as environmental conditions such as shade, soils, and restricted rooting using “modification factors”—an adaptability scoring system for planted environments.

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Key Vulnerability Assessment Findings:

  • The Puget Sound region has been warming at a rate of about 0.4°F per decade since 1960, and the average temperature is projected to increase by 5.0°F to 8.6°F by the end of the century.
  • Precipitation in the region has been increasing by over 0.5 inches per decade since 1960 and is projected to increase by 2.1 to 3.2 inches by the end of the century.
  • By the end of the century, the Puget Sound region is projected to shift from hardiness zones 8–9 to zone 9 completely, and from heat zone 2 to heat zone 3  or 6, depending on the climate change scenario.
  • Over 150 tree species were evaluated, and 27% were rated as having high adaptability, 59% were rated as having medium adaptability, and 14% were rated as having low adaptability.
  • Heat and hardiness zone suitability was analyzed, and tree species vulnerability was summarized.
    • Considering just heat zones, most of the assessed tree species fell into the low-moderate vulnerability category (57%), followed by low vulnerability (26%) and moderate vulnerability (17%) under both low and high climate change scenarios.
    • Considering both heat and hardiness zones, most of the assessed tree species fell into the moderate-high vulnerability category (34%), followed by low-moderate (25%), moderate (18%), low (14%), and high (9%).
  • Tree species vulnerability is just one factor to consider when making urban forest decisions, and biological, economic, organizational, and social factors were also examined.
  • These projected changes in climate and their associated impacts and vulnerabilities will have important implications for urban forest management, including the planting and maintenance of street and park trees, equity and environmental justice efforts, and long-term planning from partnerships to green infrastructure.

How to cite this report

Rutledge, A.; Brandt, L.A.. 2023. Puget Sound Region: Tree Species Vulnerability Assessment. White Paper. Houghton, MI: U.S. Department of Agriculture, Northern Forests Climate Hub. 61 p. https://doi.org/10.32747/2023.8054016.ch

 

Acknowledgments

Many individuals contributed to the writing, reviewing, and editing of this document. We are grateful to the municipal foresters, park district representatives, natural areas managers, and individuals from private, nonprofit, academic, and governmental organizations who helped provide feedback on the tree species vulnerability section of this report. This work would not be possible without their interest and feedback. This work was funded in part by the USDA Forest Service Urban and Community Forestry Program in partnership with American Forests. We are grateful for the continued collaboration with American Forests to develop resources to enhance the ability of urban forests to adapt to climate change. Photo credit: Seattle Sunset Marina Harbor by Frank Ravizza