What One Leaf Does: Samantha Hartzell on Measuring and Multiplying

How does the behavior of one organism, multiplied across trillions, produce consequences at the scale of an entire ecosystem? That question has organized the career of Samantha Hartzell, Assistant Professor of Civil and Environmental Engineering at Portland State University (PSU), since a biology course at MIT reoriented her. Penny Chisholm, the professor who taught it, had helped establish that a single-celled bacterium called Prochlorococcus, unknown to science until the late 1980s, was responsible for a substantial share of Earth's oxygen production. One species, invisible to the naked eye, shaping atmospheric chemistry. Hartzell had arrived at MIT interested in biological engineering. She left that course pursuing a different discipline and a single, durable question.

Her real interest in engineering was how "one plant or one organism multiplied it over all these trillions of organisms that create our environment-how the small fits into the big picture." As an undergraduate, Hartzell explored the question across different fields: "I really kind of wandered around quite a bit." She studied the fluid mechanics of disease transmission, did remote sensing work and applied for a summer Research Experiences for Undergraduates (REU) at Duke hoping to work in a fog-capture lab. When she did not get the position, another professor approached her with an opening, and she ended up studying photosynthesis. "I think it was more curiosity than problem solving," she says of those years. At Duke she met the advisor who would direct her doctoral work. She returned to MIT, finished her undergraduate degree in environmental engineering in 2015, then followed the advisor to graduate school. Two years in, the advisor received an offer from Princeton. "My whole lab group followed him up there." She completed her PhD there.

The scaling question traveled with her. At PSU, her research programs span geography and method but share a single architecture: measure the individual process, then determine what it means when multiplied. In Portland, she is modeling how specific plant species on green roofs capture stormwater, tracking how species selection on a rooftop changes aggregate runoff into the city's drainage system. In El Paso, Texas, she collaborates through a National Science Foundation (NSF) funded network to study how salt-laden irrigation water alters pecan orchards, building physiological models that link individual tree responses to orchard-scale outcomes and assess the long-term sustainability of alternative choices. A separate project investigates foliar water uptake: how trees absorb water through their canopy rather than their roots, and what that absorption means for forest-level water budgets.

Hatzell's lab is building its own instruments to do this work

We've been working on building our own probes from scratch," Hartzell says. "We found a method recently for a low-cost, low-power data logger. I think we've got some prototypes ready to go.

If they work, the ambition is an urban sensor network across Portland's street trees and green infrastructure. A single tree's water use, measured precisely, becomes a variable in a model of how an entire city manages flooding.

She applies the same logic to her students, though she arrived at it through a less comfortable route. As a freshman at MIT, she took her first calculus exam and failed. "I got an F, after the curve, everything. I failed the exam, and I got that red flag that I had to go talk to the TA to make a plan to do better." She ultimately reworked her study habits, but credits MIT's freshman pass/no-record grading system for creating an environment where learning experiences were welcomed rather than penalized. When she describes what kept her moving forward, she does not credit the policy. She credits something older.

"I had this fire, or this feeling, probably from my parents, how they brought me up. I had definitely a moment of doubt, but I had support and this feeling that I'm going do this and I'm going to figure this out." Her mother was a first-generation college student who became an accountant. Her father was an engineer. The confidence she carried into that failed exam was, by her own account, not something MIT gave her. It was something she brought. She is precise about why this matters now.

That kind of thing only works if you have already that belief that you belong and that you are going to succeed in the end. And a lot of our students don't have that built in.

PSU is not MIT. Its engineering students are often the first in their families to attend college. Many commute. The ambient belonging that residential campuses generate through sheer proximity, through study groups forming on dorm floors and cohorts eating together, does not materialize on its own at a commuter school. "We don't have that as much," Hartzell says. "That's still something I grapple with." Her department's and lab's responses are structural, not exhortatory. A new student lounge in the engineering department. Scheduled meeting times that create routine contact. In a new fall elective mixing graduate and undergraduate students, a PhD student emerged as a near-peer resource, someone closer in experience to the undergraduates than Hartzell is. Beyond telling students to believe in themselves, she is actively building conditions that make belief plausible.

I really like just watching the students grow over time," she says, "especially when I have a class of just entering juniors and then I see them again their junior spring, and then I see them again senior year, just how they're able to grow and build confidence, especially students that feel like they don't belong here.

What difference does one leaf make to a forest's water budget? What difference does a woman make in a male-dominated department? What difference does the first person in a family to attend college make? What difference does being seen make? Five years into her research and teaching at Maseeh College, Hartzell is still counting on what happens when you multiply.