Study shows climate change vulnerability of trees used for urban forestry in Puget Sound

A study by an Evergreen State College student shows the climate change vulnerability of several trees used for urban forestry in the Puget Sound region.

In a presentation to the Tumwater Tree Board on November 13, researcher Amita Devarajan explained that the study looked at how different tree species are projected to tolerate future conditions while assessing the adaptability of trees to climate-related stressors.

In conducting the study, Devarajan compiled several studies to analyze the vulnerability of 180 trees based on two factors: urban adaptability and suitability to heat and cold.

Regarding urban adaptability, Devarajan scored each tree species based on existing published methods. This factor considered stressors such as drought, flooding, wind damage, and air pollution, as well as growth requirements like shade tolerance, soil needs, and ease of propagation.

According to the study, 27% of urban tree species in the region have high adaptability, 59% have medium adaptability, and 14% have low adaptability.

The other factor assesses the suitability of the trees to the projected heat and hardiness zone in the Seattle region. Hardiness zones, which range from 1 to 13, are determined by the average lowest temperature in a geographical area over 30 years while heat zones, which go from 1 to 12, are determined by the number of days above  86°F in an area.

In determining the vulnerability of a tree species, species had low vulnerability if they could thrive in the region’s projected zones while having high adaptability. In contrast, a species would have high vulnerability if it could not succeed in the region’s future heat and hardiness zones, while also having low adaptability.

The study found that most urban tree species in the Puget Sound region have “moderate-high” vulnerability.

Devarajan noted that species with “moderate-high” to high vulnerability include crabapple, Japanese maple, Japanese cherry, paperbark maple, red oak, Japanese snowbell, silver birch, katsura tree, rowan, and common hawthorn.

Meanwhile, tree species with low to moderate vulnerability include red maple, Norway maple, cherry plum, Callery pear, and big-leaf maple.

Devarajan told the commission that her study should not be relied on in totality as there are additional considerations when picking what tree to plant. The researcher added that most of the trees assessed in the study were not native to the Pacific Northwest.

Devarajan, who interned at the city’s Water Resources and Sustainability Department, also cross-referenced the trees in Tumwater’s draft street tree list to show the climate change vulnerability of several trees that the city is looking to use. The list would be brought to a future joint meeting of the city’s Planning Commission and Tree Board, according to documents prepared for the meeting.