Vancouver eyes climate-adaptive trees for future

Local tree experts are planting ahead for the area’s future generations. Literally.

July was the hottest month on record worldwide, and some climatologists predict intensifying summers will continue. The onset of high temperatures for long periods is threatening regional trees that aren’t adapted to these conditions.

“Our palate of trees is going to have to change,” said Charles Ray, Vancouver’s urban forester.

The city of Vancouver has assembled a list of climate-adaptive trees, both coniferous and deciduous, native and non-native, that are likely to persist amid intensifying summer conditions. However, just because these trees are drought- and heat-tolerant doesn’t mean they don’t need water or are immune to extreme weather, Ray said.

Extreme heat causes tree leaves to close their stomata, pores that allow the plant to cool itself, and causes the tree to slow its growth to conserve resources. A lack of water during droughts can lead to nutrient deficiency and weaken a tree’s immunity, making it susceptible to pests and disease. Eventual symptoms can look like leaf scorch, wilting and dying twigs.

To compensate for trees’ climate sensitivity, municipalities source plants from nurseries that are adapted to Southwest Washington’s climate — perhaps buying stock from the Willamette Valley rather than Puget Sound.

Lessons learned

The Pacific Northwest’s trees were subjected to record-setting triple-digit heat in June 2021, leading to visible tree scorch on Western red cedars, Western hemlocks and various foliage. It was an uncontrolled test of trees’ thermal tolerance, and turned swaths of verdant forest to stands with splintered reddish-brown patches.

Damage resembled harm caused by heat generated from wildfires, which was thought to be the result of drought and a tree’s inability to stay cool as its water supply evaporated. Researchers later found that, in addition to drought, extreme heat can hinder tree development and increase mortality.

Introducing species native or adaptive to hot climates is imperative for ensuring a stable urban canopy, especially as heat waves become more common, Charles said.

Vancouver’s western and southern facing trees experienced browning during the heat dome. Mature stands endured, while some young emerging trees died. Western red cedars and Western hemlocks were among the hardest hit species and continue to be across the region, while heat-tolerant trees, such as giant sequoias, Douglas firs, oaks and ponderosa pines are more resilient.

However, if climate trends continue, remaining hotter and drier than average, temperate native trees will be at risk of increased stress and mortality.

Action at home

A successful canopy begins with planting appropriate trees in the best location, typically in lowland areas where sun protection and water are abundant, Charles said.

Landowners may consider planting heat- and drought-tolerant trees on their property, but maintaining existing stands is just as critical for the urban canopy.

A key to minimizing a tree’s stress during hot weather begins with proper irrigation, which goes deep into the soil. Watering the ground’s surface can result in shallow rooting, which exposes the tree to drought damage.

New Day Arborists, a Ridgefield-based company that works with local municipalities, recommends using arborist chips, material leftover from tree removal or pruning. The soft and spongy layer mimics organic matter and, as it accumulates, provides a buffer between the soil and air, effectively slowing evaporation and maintaining soil moisture, owner Jeff Day said.

Similarly, leaving organic material in place to decompose will enhance water and nutrient uptake. This can be as simple as raking fall leaves under a tree canopy or contributing to its understory. Concerns include aesthetics and increased flammability, though Day said the leaves’ ecological benefits are innumerable.

Urban forests aren’t born, they are made. While cities expand, tree canopies are shrinking, but the onus to care for this green infrastructure isn’t solely on the government

“We need all development projects, as well as private property owners, planting large climate-forward species,” Ray continued, “not small ornamental trees that do not provide the ecosystem and public health benefits.”