Bay Nature magazineWinter 2020


Tree Detectives

The Northern California black walnut led scientists into a genetic mystery: is this a rare tree, or a common one?

January 6, 2020
black walnut tree
The gnarled, twisting black walnut led scientists into a genetic mystery: is this a rare tree, or a common one? (Photo by Richard Morgenstein)

Last April I followed Gretchen Hayes into the leafy shadows and woods along Las Trampas Creek in the East Bay. Pipevine swallowtail butterflies flapped like black handkerchiefs in the warming sun along the trail, from which we swiftly departed. Just a few steps into the foliage and we were surrounded by green, avoiding poison oak, and gingerly stepping across a creek. Hayes, a geomorphologist and veteran environmental consultant, soon zeroed in on the object of our odyssey: a gigantic Northern California black walnut (NCBW) tree. These walnut trees grow between roughly 20 and 75 feet high, with broad-spreading crowns of long, thin, and fluttery pinnate leaves, and their fruits are, well, a little larger than walnut size. The tree Hayes sought was enormous. Its trunk bent and turned in a most torturous perpendicular shape. Undoubtedly a survivor, the tree had a presence as settled and stolid as a tribal elder’s.

This particular tree was one of but a handful at the center of a mystery that’s been unfolding for more than 150 years. The story reaches not only into the past, but into a future that matters. A large cast of characters across the Bay Area have played a part—city officials, natural resource management agencies, NGOs—but Hayes has mostly been aided and abetted by Heath Bartosh, a native plant botanist, and UC Davis plant sciences professor Daniel Potter. Their painstaking work enabled others to determine whether the Northern California black walnut is rare. Or not.

At issue are the many handsome, tall, leafy trees lining Bay Area roads and creeks, inspiring names of places like Walnut Creek, where the trees provide beauty and shade. Like our iconic oaks, NCBW (Juglans hindsii) is a California native and evolved here in tandem with the climate, sheltering and helping to support many other species.Indigenous Californians likely cultivated the tree before Europeans arrived in California, making use of its nutmeats for eating and shells for playing games and making dye.    

The botanical mystery surrounding this species begins back in the 1850s, when Europeans eager to capitalize on the state’s amazing agricultural potential introduced a nonnative walnut to the landscape. The English walnut, Juglans regia, soon succumbed to an endemic soil pathogen. In the 1890s, a storied character from the world of plant breeding entered the picture. Looking to create a robust walnut for commercial purposes, Luther Burbank, “the plant wizard,” crossed regia with hindsii. Burbank thus created a walnut hybrid still cultivated as a rootstock today, an amazingly resilient plant he named Paradox. 

Now for the forensic backstory, the technical details which our story turns. Walnut trees are wind-pollinated. While many plants depend on bees, flies, bats, and other creatures to physically transport pollen from the flowers of one plant to those of another, walnuts are metaphorically tickled by the breeze. Pollen from a male flower’s stamens on one walnut tree lofts along the invisible currents of breezes. Much of it disperses or gets busy making you sneeze, but sometimes the long, feathery stigma of a female flower captures that pollen, and when it does, reproduction proceeds. Most species of walnut readily interbreed, and it can be near-impossible for a layperson to tell from looking at one whether it is pure anything or a cross of different species. From the time Europeans brought English walnut trees to California, the question of what tree was what species became confused. 

In 1908, famed botanist Willis Linn Jepson, who documented much of our state’s plant life in the early part of the last century, hypothesized that pure, wild NCBW (J. hindsii) could be identified with confidence in just three locations in California. He cataloged the locations of these trees, which eventually became the exemplars by which the question of J. hindsii purity were judged. Botanists later trying to sort out the confusion sowed by hybrids reasoned that only trees predating 1850—when walnut growers started planting other species—could be confidently called indigenous. The tree to which Hayes led me, along with those in Jepson’s two other locations, were eventually classified as rare by the California Native Plant Society and the California Department of Fish and Wildlife. In some counties, property owners seeking approval to remove or impact an NCBW tree have had to undergo costly and lengthy permitting and mitigation processes. 

These requirements weighed on the plans of Swanson Vineyards, a winery along the Napa River near Oakville. Hayes was well acquainted with management at Swanson. The vineyard had participated in a big project aiming to restore ecological functioning to 4.5 miles of the Napa River, an undertaking Hayes coordinated. With 30 other riverside property owners, Swanson converted some of its arable land back to riparian forest. Swanson owners also wanted to put a winery on land where an old home stood, but they couldn’t get a permit. Young walnut trees were growing there from nuts of older trees on the property. Napa County puts the onus on the landowner to determine whether on-site trees are genetically pure—and if they are, there is a costly process for removing them. Swanson owners, hoping to prove the trees were neither pure nor rare, enlisted Hayes, who had worked for years with private property owners on similar matters, to investigate. Hayes facilitated genetic testing, performed by UC Davis’ professor Potter. Nobody expected it, but the results showed the trees were pure. 

Hayes understood right away that the issue went far beyond Swanson Vineyards. Property owners, regional and state land managers, the agricultural industry, conservationists, and biologists would all benefit from knowing whether or not this pure stand on Swanson’s property was a rarity in itself. The existence of a surprise pocket of J. hindsii purity indicated that more walnut trees in the Bay Area might be pure too. Treating the Swanson case as a pilot study, Hayes set out to fundraise and cast a wide collaborative net in order to figure out the provenance of the NCBW in more Bay Area counties. 

Gretchen Hayes with black walnut tree
Environmental consultant Gretchen Hayes holds the leaves of an old Northern California black walnut tree growing in the East Bay. (Photo by Richard Morgenstein)

Hayes laughs like jingle bells, and she does it often. Looking for deep native plant expertise, she approached Heath Bartosh of Nomad Ecology in Martinez. “I told Heath [about] the issue of the NCBW,” Hayes trilled. “The look on his face! I could tell he was all in.” He proceeded to pull out a 1970s article that declared the rarity status of NCBW was hopelessly confused. Bartosh initially evinces something of the laid-back surfer dude of his youth—until he gets started talking plants. And indeed, his approach to the black walnut was “all in,” delving back into the mists of time and extending across the state.

“The fossil record shows this genus going back 10,000 years,” Bartosh said, citing findings about pollen cores from Clear Lake. “More recently, archaeological research uncovered walnut shells in 500-to-900-year-old campfire sites,” indicating indigenous Californians used NCBW as a food resource. Bartosh tracked down the NCBW trees Jepson referenced in his work. He also delved further into the historical collection records, locating the first specimens collected by Richard Brinsley Hinds. Hinds was a surgeon and botanist aboard the HMS Sulphur, which in 1837 traveled 150 miles up the Sacramento River from Yerba Buena Bay pursuing Britain’s exploration of the Pacific. “He brought a plant press along,” Bartosh noted, “as any good botanist would. At the time he observed the Sacramento beautifully belted with cottonwoods, willow, and black walnut, all native to California along the floodplains. He deposited the type specimens from Rio Sacramento, 1837, back home at Kew Gardens.” 

Hayes led Bartosh to the trees on the Swanson property and the pair went on a hunt to identify other trees for genetic testing. UC Davis’ Daniel Potter, with Bartosh and colleagues, published the results of their meticulous and detailed research in the plant journal Madroño in 2018. The study, titled “Clarifying the Conservation Status of Northern California Black Walnut (Juglans hindsii) Using Microsatellite Markers,” discerns the genotypes of 158 mostly wild J. hindsii trees from 10 counties in Northern and Southern California as well as one county in Southern Oregon, using DNA analysis. The results showed that 71 percent of the trees were “genetically pure,” a high number that led the authors to recommend that NCBW no longer be considered rare. Everyone was surprised. “We have many more pure trees than we thought,” said Hayes. “They aren’t ‘rare’ at all.” As of June 2019, the Northern California black walnut no longer has special protection. 

And yet, Bartosh told me, “despite the fact that individual trees are no longer considered rare, stands of Juglans hindsii that form communities along riparian areas are still considered sensitive” by both the California Native Plant Society and the California Department of Fish and Wildlife; the NCBW communities are “critically imperiled,” according to state and global rankings. “Just because there are a lot of trees doesn’t mean the natural habitat is plentiful,” Hayes noted. “The trees are endemic to five-to-seven-year floodplains.” 

Hayes showed me how the topography of Las Trampas Creek evidenced past flooding events, with gradual step-downs of the banks toward the water. “This is how a creek should look,” she said. “There’s room here for ebb and flow. When it floods, nutrients can settle down and foster plant life, which holds the banks in place.” Many streambeds and creek beds in Northern California are sharply incised. In heavy rains the water carves deep troughs into the banks. When NCBW trees are part of the riparian ecosystem, their roots hold the banks in place and allow those gradual step-downs in sediment to accumulate. “It’s my job to help restore function,” Hayes said, her face crinkling up again, followed by that dancing laugh. “And now I’ve got more rootstock to work with!” 

And with any luck, Bartosh said, these probes into the history of the NCBW will spur appreciation for a tree that has been of “continual significance to the human experience in California dating back to indigenous peoples.” He suggested that amateurs and professionals alike make use of observation platforms like CalFlora to record NCBW communities, “so we can continue to better understand the range of this uniquely Californian denizen.”

About the Author

Mary Ellen Hannibal is an award-winning environmental journalist and the author of Citizen Scientist: Searching for Heroes and Hope in an Age of Extinction.