Stanford professor spotlights evolutionary tree concepts with campus trees

From stately oaks to raggedy eucalyptus, trees of great variety stand proudly on every corner of the Stanford University campus. In fact, more than 400 diverse tree species call the campus home. This grand array of tree colors, growth patterns, sizes and more inspired one professor to illustrate concepts of evolutionary trees through characteristics of physical trees.

Noah Rosenberg, the Stanford Professor in Population Genetics and Society in the School of Humanities and Sciences, researches phylogenetics, the study of evolutionary trees and networks to deduce relationships between organisms. Specifically, he examines this field through a mathematical lens, also using statistics and computer modeling to draw inferences.

In an effort to make this complicated work more accessible, Rosenberg started the Stanford X-Tree Project, which includes a gallery of tree images from around campus. Each tree is accompanied by the name and description of the concept it represents, as well as the tree’s location on campus.

“There are many concepts of phylogenetics that are borrowed from botany and that are very literally represented on actual trees,” Rosenberg said.

The project began in 2018 after he noticed this resemblance for himself among many of the trees he walked by on campus. After sharing his discoveries with his lab members, a postdoctoral scholar in his group, Jaehee Kim, agreed to design a website to open his passion project to a wider audience.

The site launched while Rosenberg was teaching a phylogenetics class, so his students were some of the first to see the science communication project. “I used some of the examples to help clarify what I was teaching or just as comic relief interspersed with the mathematics,” he said.

As the audience for his project grows beyond the classroom, Rosenberg hopes that the X-Tree Project inspires viewers to learn more about the mathematics of evolutionary trees.

“More than a hundred years of attentive groundskeeping have turned the campus into a museum of mathematical phylogenetics,” he said.

Searching for the perfect tree

Rosenberg is constantly on the lookout for new trees to add to the X-Trees collection.

“When I’m walking around campus, I kind of have the mathematical concepts floating around in my head, looking around for trees that might illustrate them,” he said.

He explained that one time, he was thinking about ultrametric trees and stumbled upon some flaxleaf paperbark trees (Melaleuca linariifolia) during their flowering season. The trees’ somewhat hemispherical shape with white flowers around the surface of the hemisphere perfectly represented ultrametric trees, in which the “leaves” are all equidistant from the “root.”

“Sometimes, something is happening with the tree depending on what season it happens to be or what stage in the life cycle it’s in,” Rosenberg said. “Each aspect of a tree is a potential connection to a different phylogenetic concept, one that might not have been apparent, even after having seen that tree many times.”

The reverse also happens. Occasionally, Rosenberg comes across trees with intriguing characteristics and then contemplates what they could represent.

One such tree is a sugar gum (Eucalyptus cladocalyx) prominently located in the median of Campus Drive. The tree’s tangle of branches caught his eye many times over the years. Rosenberg finally concluded that it could depict the idea of a planar representation, in which the branches of a tree can be “untangled” so that they do not cross each other.

Even human-made alterations influence how Rosenberg views a tree and its potential.

Recently, construction workers put a fence around four Italian stone pines (Pinus pinea). To Rosenberg, highlighting them in this way was reminiscent of a phylogenetic quartet, in which algorithms look at groups of four lineages at a time to estimate all lineage relationships in an evolutionary tree.

Enthusiasm for X-Trees has even spread to Rosenberg’s lab members.

“Some of them have developed a bit of an observant eye for trees on campus,” he said proudly, noting that his trainees have contributed photos to the project.

A lifelong love of trees

A long-time nature enthusiast, Rosenberg has always carefully observed the trees around him.

“I was interested in trees before I got started in mathematical evolutionary biology,” Rosenberg said. “So I’ve tried to learn a lot of the trees on campus.”

His love of trees stretches far back, even beyond his earliest memories. According to his mother, “tee” – with the intention of meaning “tree” – was one of his first words as a toddler. She can still point out which tree in Milwaukee they were near when he exclaimed the word.

As a child, Rosenberg collected leaves and seeds from the neighborhood trees. He flipped through his copy of the National Audubon Society Field Guide to North American Trees, Eastern Region so often that today the book sits discolored and worn. The corresponding western region version traveled with him to Stanford when he began his graduate studies in 1997, though he credits the book Trees of Stanford and Environs and the Trees of Stanford website as the key to learning about the trees on campus.

Toward the end of his PhD in population genetics, Rosenberg gained enough knowledge to move into mathematical phylogenetics, which became a research focus in his current lab. His work today is thus a manifestation of a lifelong love of trees.

“This field’s link to trees was a natural draw for me,” Rosenberg said.