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Smart trees hold climate clues

Researchers study urban trees to better understand environmental impacts

Smart Tree
Photo by Zach Putnam

Headlamps illuminate narrow swaths of urban forest in the darkness as researchers prepare for the morning climb.

Their day starts as early as 4 a.m. in a race to reach the top before first light. They hope to collect samples while the trees are as close to rest as possible and not actively photosynthesizing. Climbers will also check on the tree’s digital sensors to make sure they’re working. 

Douglas-fir needles and cones crunch underfoot as the climbers set the ropes, using a guide line to pull the orange climbing rope up and over the pulley at the top. 

Portland State University graduate and postdoctoral researcher Hannah Prather pulls on her climbing harness, the final piece of equipment, carabiners clinked together in a cascade of metallic tones. She is ready for her 100-foot ascent into the canopy. 

Prather is part of a team of researchers from Portland State, Reed College, Washington State University, and The Nature Conservancy studying the impact of climate change on urban trees. Todd Rosenstiel, biology professor and dean of PSU’s College of Liberal Arts and Sciences, helps lead the work known informally as the Smart Trees Collaborative.

The Smart Trees team uses a range of technologies to monitor the health of the urban tree canopy, a key resource for reducing the social and environmental impacts of our warming climate. The work continues to branch out in interesting ways to include multiple institutions. Portland State, along with its Digital City Testbed Center, is helping to lead the way. 

For the climate change experiments, Portland is a perfect testbed for future conditions in remote forests. 

“In many ways, the environment we live in in Portland is 50 years in the future for the rest of Oregon,” Rosenstiel says, “with greater levels of ozone, more drought, more heat waves and drier conditions."

“It becomes an amazing experiment in how trees will adapt.”

Or how they don’t. Many of the changes currently being monitored point to the greatly increased risks of wildfire throughout the Pacific Northwest.

One species, western redcedar, is dying off around the state, especially in cities.This group of researchers hopes to uncover ways to reverse the trend. The species is not only important to forests and urban landscapes, it also has a long history of use by indigenous people, for everything from traditional canoe making to salmon smoking. 

The loss of the western redcedar would have a significant impact, Rosenstiel says, and it might be a harbinger of what is to come for the rest of Oregon. He believes this issue has the potential to unite the timber industry and environmentalists because without action, there might not be an industry left. 

Portland is uniquely poised to show the impact of climate change on trees because it boasts  a rich urban forest. Trees reach 100 to 200 feet tall right in the middle of an urban center. Few other cities can offer a similar testbed for this kind of research.

To better understand what trees experience in the urban environment, researchers install sensors and gather remote data from drones and satellites about temperature, weather, humidity, drought stress, and other conditions from the base to the crown. In addition to western redcedar, they’re studying Douglas-fir and bigleaf maple. 

On this morning’s climb, Prather and her team are exploring Douglas-fir at Reed College. The species appears more adaptable when compared to western redcedar; understanding these differences could reveal key insights for the future. 

As soon she clips into the ropes, Prather starts her ascent. In less than a minute, she disappears into the branches. 

“The night climbs are exciting,” she calls down. “You can only see what’s right in front of you.”

Prather, who earned her PhD in biology at PSU, started climbing 10 years ago, after becoming a certified arborist following her undergraduate studies. She is now a postdoctoral researcher at PSU and Reed College, helping to lead the team’s instrumentation and sampling efforts.

Prather and her colleagues monitor Douglas-firs at three sites around Portland. The tree on this climb hosts a sophisticated weather station including  sensors that monitor wind speed, rainfall, temperature, humidity, and CO2. All the data is fed to a cloud-based storage platform that is accessible remotely — even via smartphone. This instrumentation offers the first look at what urban trees experience at the canopy level. 

The team also collects data in more traditional ways. On her climb, Prather bags needle samples from the top of the tree to assess plant water stress. She then checks the weather station and sensors, making sure the zip ties are securely attached, taking a quick break to admire the view of downtown from 100 feet up.

“There’s a really beautiful sunrise,” she says. “I might take my camera and snap a few photos.”

“It’s so beautiful.”

Portland treeline
Photo by Zach Putnam

 

By the time Prather’s feet touch solid ground, the birds start their morning songs and the sky turns blue. 

“Climbing is great, especially on mornings like this with an awesome view, but I see this as a tool to do my science,” she says. “It’s all for the data.”

She feeds the needles into a contraption called a pressure bomb that forces them to give up their held water. The more pressure it takes to expel this moisture, the higher the drought stress of the tree. They measure the results before first light as a baseline and then gather more samples during the heat of the day to compare the two. 

The results help researchers understand how the trees might adjust to some impacts of climate change. For example, Douglas-firs might be able to shorten the time they open pores on their needles to collect CO2 because there is so much of it in urban environments. The shorter duration also means losing less water, which becomes critical during times of drought. Other species, such as western redcedar, may not be able to make these types of adjustments as quickly. 

Researchers are also monitoring western redcedar around Portland using on-site measurements and remote sensing to better understand their health. In the future, the city might be able to use this data to develop management techniques, such as a watering schedule based on the trees' exact needs, the way some farmers use sensors to irrigate crops.

These physiological studies are only a sample of the Smart Trees projects. PSU researchers Idowu “Jola” Ajibade, assistant professor of geography, and Vivek Shandas, professor of urban studies and planning, are working with their Smart Trees colleagues on a $1.5 million grant from the National Science Foundation that will study how the history of redlining and structural racism continues to impact neighborhoods through tree-planting policies. The principal investigator on this grant is Aaron Ramirez at Reed College.

“Our goal is to understand how these practices produce inequities in human health and community resilience especially when dealing with hazards such as heatwaves and pollution,” Ajibade says.

Researchers will also engage urban planners and forest managers to discuss how an environmental justice lens and anti-displacement strategies can be used to increase the tree canopy in poor neighborhoods of color.

Longer term, the team hopes that Smart Trees will become a model for new and impactful ways to collaborate across cities, states, and institutions. Rosenstiel will soon lead the first summit on western redcedar decline, bringing researchers together from across the Pacific Northwest. Their findings could help ensure the survival of that tree species as well as others.

“It could take generations to recover what you could lose in a second,” he says.