Ghost Trees
¹û½´ÊÓƵ biology students have been scouring the Portland area for hybrid oak trees. Their journey could reshape the way we see our ecosystem.
With its labyrinth of banks, edges, ledges, rails, and stairs, Ed Benedict Park is a haven for skaters. Yet when Prof. Keith Karoly [biology] visited in the fall of 2020, he came for the trees—specifically, a row of Oregon white oaks (Quercus garryana) and a row of English oaks (Quercus robur), parted by a nearby street like star-crossed lovers.
By testing genetic markers, Karoly sought to discover whether the two oak species had hybridized. “We had visible acorns, we had first-generation seedlings growing in this area,” Karoly says. “It looked like the kind of experiment that 40 years ago, if we wanted to know this answer, we would have set it up that way.”
It was perfect—too perfect. Keith determined that every tree on the street was actually an English oak, and that an entire row of non-native trees had been mistakenly labeled as native by Portland Parks personnel. “Little bit of a scandal,” Karoly jokes.
Beyond their contrasting physical traits, the two species are very different for two types of tall, green, and acorn-sprouting trees. Oregon oaks have towered over the Pacific Northwest for centuries, while English oaks are so synonymous with Britain that they’ve been adopted as a national emblem.
Karoly is well aware of the ways non-native plants can damage their surroundings. “We often find that non-native [plants] don’t exactly provide the same kinds of resources to animals, they don’t have the same interactions with other plants, they may have different responses to environmental change,” he says. “But there is also, to be clear, a pretty straightforward knee-jerk response: ‘That’s a non-native, it doesn’t belong here, get it out of here.’”
Karoly has a different response: He wants to find out if Oregon and English oaks could “birth” a tree that bridged the gap between the native and non-native trees. Over the past five years, he has enlisted three thesis students in his search for hybrids, though he’s the first to admit that the results haven’t been what he hoped.
“The punchline—we have yet to find any individuals that we can definitely say are hybrid oaks,” Karoly says. “We still don’t have any evidence that the two species are able to hybridize.”
That hasn’t stopped Karoly and his students from looking for answers. The journey they have undertaken is a testament to the ¹û½´ÊÓƵ principle of intellectual inquiry for the sake of deeper understanding—and their efforts to untangle the morphology and genetics of rival oaks have led to a more nuanced understanding of our city’s ecosystem.
“What has been nice for me is the students were very interested in putting their own imprint on the project,” Karoly says. “If I had been the only one doing the work from the beginning, it probably would have taken a very different course.”
Morphological Matters
Oregon oaks can grow up to 120 feet and live for hundreds of years—like the Signature Oak at the Oregon Garden, which took root more than 200 years before Lewis and Clark first set foot in the Pacific Northwest.
By contrast, the English oak is inseparable from the European history of migration and conquest. Settlers brought them to New York in the 1700s, displaying an intense attachment to a tree that appears in Shakespeare’s plays and the story of Robin Hood (according to legend, the merry archer concealed himself inside a hollowed-out English oak).
“The Oregon white oak has been here for a long time and has a lot of cultural significance among Indigenous people in the Pacific Northwest,” says Ziqi Xie ’23. “And then there’s the English oak, which has a very high significance to the Europeans, before and during the settlement.”
The possibility of the two species hybridizing caught Karoly’s attention when it was raised on the Native Plant Society of Oregon’s listserv. Subsequently, Sam Aldape ’19 began studying oaks for an independent project, attempting to identify potential hybrids using genetic markers, which are DNA sequences with a known physical location on a chromosome.
Such markers can function as a “paternity test” for plants, albeit not without obstacles. “It can be really hard to get usable DNA out of oaks,” Karoly says. “They have a lot of compounds in their tissues, so when you’re trying to extract the genetic component, you get a lot of these other compounds that create problems for all the work we do in the lab.”
Studying genetic markers isn’t the only way to differentiate species. Morphology—the branch of biology that focuses on the form of living organisms—presents another possibility, one explored by Lenny Blair ’21 in her thesis, “Questioning Quirky Quercus: Examining Possible Oak Hybridization in Portland, OR.”
“I had taken courses at ¹û½´ÊÓƵ where I had done more hiking, versus being in the lab,” says Lenny, who grew up as an outdoorsy kid in rural Tennessee. “More of my interest just was with morphology and with the physical aspects of the plants.”
The pandemic provided a rationale for Lenny to focus on morphology. Rather than conduct long hours of lab work while wearing the requisite two protective masks, she was eager to conduct research that would allow her to spend time off-campus taking samples in parks across Portland.
Some scientists believe morphology can be misleading, given its emphasis on physical appearance. “If I had been able to force the issue, I might have tried to do the genetic markers from the start with Lenny,” says Karoly. “But she was very interested in the multivariate statistics side of things, and being able to tackle the project from a morphology standpoint.”
Amid the chaos of the 2020 election, Lenny and Karoly drove around Portland in a van, collecting samples everywhere from ¹û½´ÊÓƵ meadow to Lair Hill to Argay City Park. The morphological traits Lenny studied included the English oaks’ “shiny and dull” leaf adaxial (top) and the Oregon oak’s “short, hairy, dull, light green” leaf abaxial (bottom).
Morphology allowed Lenny to differentiate between species, but proved insufficient to detect hybrids. Yet the research she conducted—and the samples she collected—would inform the work of future ¹û½´ÊÓƵies.
“I remember feeling some bit of, I wish I had—not worked a little harder, but been better able to leave something that would allow for future directions,” Lenny says. “I definitely didn’t think that anyone would be doing the DNA aspects of the research so soon after I graduated.”
Comic-Con for Oak Lovers
In her thesis, Lenny included a “Future Directions with This Research” section, noting that since morphology alone wasn’t able to distinguish hybrids and parental species, returning to genetic markers was a logical next step. That’s the path that Lucy Montgomery ’22 followed with her thesis, “Identifying Oak Hybridization in Portland: A Genetic Approach.”
“I do have a hard time connecting with [science] sometimes,” Lucy says. “It’s so deeply in that academic sphere. But this work was really rooted in the parks and the Portland area—and also had some ecological implications for the parks.”
Lucy set several goals for her thesis, including successful DNA extractions and the testing of possible hybrids in parks and natural areas. “There are so many factors that go into choosing what plant goes where,” she says. “The English oak is an ornamental species, and I don’t want to shoot anyone down [for planting one], but you can’t make an informed decision if you don’t know what you’re planting.”
Using Portland Parks’ tree inventory, Lucy identified potential hybridization hot spots, including the aforementioned Ed Benedict Park. Unfortunately, she didn’t learn until late into her research that Portland Parks had mislabeled the oaks she had been sampling. “It was definitely a setback, for sure,” Lucy says. “It was a moment of reevaluation.”
Despite not discovering any hybrids, Lucy succeeded in identifying interspecies genetic variation and developing suitable markers. In 2022, she brought her research to the International Oak Society Conference in New Mexico—essentially, Comic-Con for oak lovers.
The Oak Society Conference attracts academics, private sector biologists, and general oak fans, some of whom were surprised that so many trees could be incorrectly identified by the parks personnel.
To Karoly, the mislabelings were less of a shock. “It’s not surprising,” he says. “A lot of that work was done by volunteering and [Oregon white oaks and English oaks] are very hard to tell apart.”
The Soul of a Tree
The story of the Oregon white oak and the English oak is the story of two cultures adapting to each other—a story that Ziqi Xie has not only studied, but experienced.
“My biology classes I took in high school were taught in Chinese,” says Ziqi, who grew up in Beijing. “There were some words I didn’t know [in classes at ¹û½´ÊÓƵ], but I wasn’t sure whether I didn’t know them because I missed some part of the cultural experience of being in the Pacific Northwest.”
When she began her thesis, Ziqi didn’t just bring the perspective of someone raised in another hemisphere—she brought the perspective of an artist. While her thesis would involve testing genetic markers, she added a visual element: a white-oak hand case containing branches, leaf prints, and oak gall ink writings.
“I was thinking of a 50/50 mix of art and biology,” says Ziqi, who was advised by both Karoly and Prof. Aki Miyoshi [art]. “Because artists who work with biology concepts, maybe they place more value on the artistic representation of those concepts. But I was more interested in how I can integrate them in a balanced way.”
That balance is felt throughout Ziqi’s thesis, “A Tale of Two Quercus: Storytelling of Evolutionary Genetics and Local Botanical History through Artistic Approach.” She provides a detailed account of the genetic markers she tested, but also images of the hand case, which make sometimes-abstract biological concepts tangible.
“The idea was putting together the body of the oak in another way,” Ziqi says. “The reason why I made the hand case was to emphasize the history of the oaks—their migration history. I was thinking about how to present this idea of migration in a small piece of art.”
Ziqi’s research didn’t uncover any hybrids, but it wrote an important new chapter in ¹û½´ÊÓƵ’s oak detective story by taking an interdisciplinary approach, even as she paid homage to those who came before (in a reference to Lenny’s thesis, a section of Ziqi’s is called “Quirky Quercus Morphology”).
Separately, Lenny, Lucy, and Ziqi each have a story to tell about their search for oak hybrids in Portland. Together, their work forms a single narrative how that search has united a trio of ¹û½´ÊÓƵ students—and how Karoly’s mentorship drew them to a common cause.
“Speaking with Keith, I always felt like I had someone in my corner,” Lenny says. “He gave me the perfect amount of independent work and being guided. There were weeks where we wouldn’t meet and it would just be me working, but I always knew if I had some pressing questions, I could email him or go to his office and we would be able to chat.”
All three of Karoly’s oak thesis students continue to study and work in the scientific community. Lenny earned a master’s degree in ecology and evolutionary biology from the University of Helsinki last June, Lucy monitors water quality for the San Francisco Estuary Institute, and Ziqi is pursuing her master’s in biology at Portland State University.
Karoly is considering taking his quest for hybrids beyond the Portland city limits—while preparing to submit a manuscript that collects his, Lucy’s, and Ziqi’s research on genetic markers.
Despite Karoly’s joke about the inability to locate any hybrids becoming a punchline, his students have shown that what is undiscovered can be an opportunity. “I think the implication of hybridization would be potential backcrossing—sort of a dilution of the garryana genetic hold,” Lucy says. “And I think the implications of no hybridization [in Portland]…I mean, there’s always a possibility.”
Whether that possibility exists or not, Karoly, Lenny, Lucy, and Ziqi have helped us to better understand not only the story of oaks in Portland, but Portland itself.
“I remember Keith telling me about one tree in particular that had seen so much of the city,” Lenny says. “An entire highway had been built next to it and all the other trees around it had been taken down, but this tree was able to remain. That was something that was really exciting: How much the city can change, but these trees, they stand, and they look on, and they’re still there.”
Tags: Alumni, Climate, Sustainability, Environmental, Research