Difficult to determine which tree species will be most climate resilient

Scientists agree: more trees in cities are better. Trees soak up carbon dioxide and provide shade as temperatures rise. But debate remains about which trees to plant.

Oregon’s annual average temperature has increased by 2.2° F since 1895 and could increase by 5° F by 2050, according to the United States Department of Agriculture.

While native trees and shrubs provide homes for insects, birds and other animals, those homes may not survive the heat of climate change.

“There is an ongoing scientific debate about whether planting trees outside their historic native ranges can improve climate resilience or create unintended risks,” David Grandfield, Portland Clean Energy Fund’s Regenerative Agriculture and Green Infrastructure program manager told Street Roots.

While planting native species is the emphasis in forests and other natural area restoration projects, urban forestry planting programs offer the opportunity to experiment.

“The focus is not on strictly planting native trees, but on selecting trees that are resilient under future climate projections for the Pacific Northwest,” Grandfield said. “Some of those trees are native, many are not, but all are chosen with long-term health, canopy cover and climate resilience in mind.”

According to Vivek Shandas, a researcher and geography professor at Portland State University, emerging studies don’t provide a final answer on this debate.

“I’m interested in continued planting of different species — non-native and native — so that we can create a highly species-rich landscape, and continue studying the pros and cons of such efforts during uncertain shifts in our climate systems,” Shandas said.

This conversation is rooted in history and a changing tree landscape.

“Since cities have existed, residents have looked to non-native trees to fill the landscape,” Shandas said. “As colonial processes occurred in the U.S., native trees were replaced with European varieties, and a larger conversation among restoration ecologists is about bringing back more of the Indigenous landscapes through planting evolutionary adapted species.”

Diversity in the canopy

Scientists gather data on how different species thrive during storms, heat waves and general temperature increases, Shandas said. Planting native as well as non-native trees over time will give scientists opportunities to evaluate the importance of each.

In some cases, urban foresters are removing native trees from the mix.

In Vancouver, across the Columbia River from Portland, the list of approved trees will no longer include western hemlock, western red cedar and bigleaf maple. The city recognizes that native trees are extremely important for pollinators, according to Andrew Land, urban forestry specialist for the city of Vancouver. However, there are several reasons why non-native trees might even be more appropriate.

“The hot, dry summers Clark County and much of the Pacific Northwest have experienced in recent years are harming some native tree species,” Land told The Columbian. “Forestry experts and environmental groups say western hemlock, western red cedar and bigleaf maple are dying off due to the changing climate.”

Other native trees are also in trouble. The Oregon white oak is in decline, largely because of infestations of the Mediterranean oak borer, a beetle that carries fungi that block water conduction and cause the trees to wilt, the Oregon Department of Forestry reported in 2024.

Oregon white oaks can support as many as 200 species of birds, mammals, amphibians and reptiles — some of which rely solely on Oregon white oak to survive.

“Oaks savannah was once abundant throughout the Willamette Valley but is in rapid decline,” the Bird Alliance of Oregon told Street Roots.

But Oregon white oaks are still on the Friends of Trees list and thrive in sunny locations.

“Urban forests forever and a day have been comprised of many non-native as well as native tree species,” Jenn Cairo, Portland Parks & Recreation city forester and urban forestry manager, said. “In a forest there are no sidewalks, car doors, or other things that make it hard to be a tree.”

Non-native species are a significant part of every American city’s urban forest. A great example in downtown Portland is the South and North Park Blocks.

“The biggest trees in the park blocks are American elm trees, big canopy trees,” Cairo said. “They are not native. They were planted over 100 years ago and came from the eastern U.S. to here.”

Lisa Tadewaldt, owner of Urban Forest Professionals, said it’s important to use native species where it makes sense. A tree may do well in its native environment, but that may not mean the species does well in the city. With evolution, plants adapt and become resistant to environmental factors. Technology and human influence also play a part.

Parallel to city efforts, nonprofit organizations are working to preserve native trees and plants for urban forestry. The Oregon Bird Alliance and Columbia Land Trust have created the Backyard Habitat Program to help build native habitats in backyards and community spaces. The organization has planted about 60,000 native trees on nearly 15,000 urban sites to date. The National Wildlife Federation provides native tree seedlings through its Trees for Wildlife program to partner organizations throughout the U.S.

Tadewaldt said she is happy that the city is working again with Friends of Trees. Her company used the Friends of Trees method for professional tree planting and she has volunteered with the organization for 20 years as a planting crew leader.

“One good thing they are focused on is a variety,” Tadewaldt said. “Portland’s canopy is over-weighted on maples, but now there are none on the Friends of Trees list and city list. A lot of people want to plant popular trees, but they don’t want too much of one thing. We want native trees where we can but it can’t be everywhere.”

Predators and pests

Because of predators and pests that attack certain tree species, arborists in the Portland area involved with Friends of Trees and Urban Forestry agree on bringing a variety of tree species to median strips, backyards and parks — the places that comprise the urban forest.

One concern in the urban forest is the emerald ash borer, an insect that attacks ash trees and has been confirmed in Portland.

“Years ago, we removed ash trees from our approved planting list because we knew it would be here,” Cairo said. “Diversity in our urban canopy is critical. We know some of the pests and pathogens, so we want to avoid having too many of those at one time and losing health, heat mitigation and air quality benefits.”

Conversations about choosing which tree species to plant in a changing climate are complicated. Established trees cool concrete areas, but they first have to survive that hot environment before they can grow into mature shade trees.

Damien Carré, Oregon Tree Care owner and operator, has been working as an arborist with trees in the urban forest for 25 years and volunteers with Friends of Trees.

“We’re talking about heat islands and radiant heat from sidewalks,” Carré said. “The urban canopy has been diminished. When trees are draught or heat-stressed, thirsty trees send out stress phermones, which attract bugs.”

Experimentation helps. Diseases that attack trees have different effects in different regions so it can be hard to predict what will happen with some trees, Tadewaldt said.

“There is an Ironwood tree in front of Friends of Trees we recommend a lot to landowners because we’ve had good success with neglecting it,” Tadewaldt
said.

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This article appears in October 22, 2025.