Puget Sound Region
Authors: Rutledge Annamarie , Brandt Leslie (Leslie Alyson) ,   USDA Northern Forests Climate Hub,
Published: June 2023

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. The 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 compared with the 1971-2000 historical average. 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 compared with the 1971-2000 historical average. 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 (RCP4.5) or 6 (RCP8.5), depending on the climate change scenario. Of the evaluated tree species, 27% were rated as having high adaptability, 59% were rated as having medium adaptability, and 14% were rated as having low adaptability. Given that the hardiness zone range is projected to remain within the historical (1980-2009) range, we considered both heat zones alone as well as heat and hardiness zones. Considering heat zones only, 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. The vulnerability ratings remain the same between low and high climate change scenarios because all assessed tree species are considered suitable under both sets (low and high) of heat zone projections through the end of the century. 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%). The vulnerability ratings are the same between low and high climate change scenarios because the projected hardiness zone is the same under both scenarios through the end of the century. The vulnerability of individual species is not the only factor to consider when making urban forestry decisions, and this assessment also contains species diversity and human health as additional factors. 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.