Plant Researchers Give Black Walnut Trees a Boost

Plant Researchers Give Black Walnut Trees a Boost
Black walnut trees are a familiar sight on campus.

Dotting the hills and fields of Cal Poly Pomona with their black trunks and dark green leaves, Southern California black walnut trees are an important part of the university's history. They are the campus' oldest living residents and a significant element of the region's native environment.

Over the years, residential and commercial developments have reduced the trees' population in the San Gabriel Valley, Chino Hills and Puente Hills areas. However, with more than 1,000 trees on campus, Cal Poly Pomona is one of the rare places in Southern California where walnut woodlands abound.

Biological Sciences Professor Ed Bobich and Biological Sciences Department Chair Frank Ewers, both plant biologists, and their students are studying the trees, their habitat and the germination process to aid the restoration of this native plant.

“This tree is the most common tree on Cal Poly Pomona's campus. This really is the symbol of our natural landscape,” Bobich says. “Our project is to know how to have a successful habitat for these trees and what are the ideal conditions.”

Their research has turned up some interesting results. Black walnuts are more likely to grow on the north side of hills, possibly because there is more water and less solar radiation, making it easier for seeds to germinate. However, as graduate student Brandy Wood discovered, established trees (more than 3 years old) on the south sides of hills are often more productive than trees on the north side.

Bobich and his colleagues also have observed that walnut trees can survive wildfires. In fact, most will thrive after a fire, growing two to four new trunks. The resprouting stems also grow faster and photosynthesize at a higher rate.

“Walnut trees could be the only native plant that survives frequent fires,” Bobich says.

At the Lyle Center for Regenerative Studies, 12 acres of land are dedicated to the restoration efforts, and about 400 black walnuts live in a flourishing ecosystem. The secret to their success is simple: “We really don't do anything,” says Juan Araya, a lecturer at the Lyle Center. “The land has been untouched for about 12 years. There are no cows, no cutting, no trimming, no mowing. We've left them to restore themselves.”

Over the years, the trees have rebuilt their own ecosystem. Old branches that have fallen provide homes for insects and animals. Squirrels pick and bury walnuts in the ground. And some of those nuts give birth to new trees, helping the forest to regenerate.

Bobich advocates for incorporating black walnut trees in residential landscaping, especially at homes in the Inland Empire. Not only do they require less water compared with other trees, they also provide plenty of shade and produce a delicious fruit.