6 minute read
The Call of the Wild Walnut
By Christina Varnava, Living Collections Curator
Last fall, on sunny, forested hillsides, staff from the horticulture team at Santa Barbara Botanic Garden spent the day climbing trees. The shady dome of large leaves kept the sun from our eyes as we sought out our quarry: walnuts. The walnuts can be tough to spot earlier in the season when they are still cloaked in their green outer husk, but at this time of the year we found plenty of seed to collect. We came back to the Garden with paper bags filled with dozens of them. However, this was not a snack for hungry botanists. We were in the field seeking seed of the Southern California black walnut (Juglans californica) to cultivate and add to the Garden’s Living Collection.
Southern California black walnut is a close relative of the domesticated English walnut (Juglans regia), but it’s uniquely adapted for our Southern California landscape. The trees don’t become large, though they can be up to 30 feet (9 meters) tall, and they have a deep taproot that helps them access hidden underground water sources. They are also resilient to damage from wildfires and droughts and able to resprout from the crown of their roots. This allows them to quickly reestablish when the aboveground parts are damaged. A walnut seed is considered a nut, which botanically speaking, is a type of fruit where the seed is contained in a hard outer shell that does not pop open when mature. The Southern California black walnut is smaller than English walnuts with a harder shell, but the nuts are considered quite delicious.
For the Love of Fruits and Nuts
True to its common name, the Southern California black walnut is a species endemic to Southern California. It’s found on hillsides and canyons along the coast from Santa Barbara County south to San Diego County. This species is now threatened, but it was once widespread across the region, especially in the hills of Los Angeles County. Large stands of Southern California black walnut make up the Southern California black walnut woodland plant community. Black walnut woodland is a highly threatened and fragmented plant community that has been lost to development and widespread grazing across Southern California. There are still a few large stands of black walnut woodland, but today Southern California black walnut is often found as a component of oak (Quercus spp.) woodlands. Wherever it grows, Southern California black walnut is a keystone species that provides food and habitat for dozens of other species.
We were in the field to seek out walnuts to add to the Living Collection at the Garden, both to help conserve the species and specifically as part of a project to create “ex situ” (off-site) repositories of crop wild relatives (or CWR). This project was an initiative of the North American Fruit and Nut Tree Crop Wild Relatives Working Group and funded by Botanic Gardens Conservation International and the United States Botanic Garden. CWR are plant species that are closely related to those typically grown for food or other uses. CWR are wild cousins of our domesticated plants, just as wolves are the wild cousins of domesticated dogs. The species of plants used in agriculture have been cultivated and selectively bred by humans, sometimes over the course of thousands of years. These plants are desirable for various traits, such as larger fruits with smaller seeds. A classic example of this process is that a single plant species of wild cabbage (Brassica oleracea) was selectively bred by humans to create a multitude of different vegetables, including cabbages, broccolis, cauliflowers, Brussels sprouts, collard greens, kohlrabies, and kales. We have this artificial selection process to thank for almost every kind of delicious fruit or vegetable that you’d find at the farmers’ market or grocery store. The trade-off though is that these selectively bred plants have a smaller gene pool than their wild relatives. Because of that, they are less agile at adapting to new environmental stresses such as warming weather, prolonged droughts, diseases, or different rainfall patterns. Anthropogenic climate change is here, and many scientists are concerned that the species that we rely on for food will not be well adapted to the new normal. California will continue to experience higher temperatures and dwindling groundwater supplies, and our crops will need to adapt.
Building Resiliency for the Future
One way that we can start to address this is through plant breeding. Researchers who hope to add resiliency and other traits back into the gene pool of important crop species can start by cross breeding them with their wild cousins. To do that effectively, researchers need access to reproductive material for these species, and that’s where botanic gardens come in. The living collections of botanic gardens can provide easily accessible material to use for cross breeding, which can help to create new, more resilient varieties of crop plants that are better adapted to our changing climate.
The most practical way to conserve the genetics of fruit and nut species ex situ is as a part of the living collection of botanic gardens, like the one at Santa Barbara Botanic Garden. This is for a variety of reasons. First of all, because fruit and nut trees take years to mature, living collections are an ideal way to make reproductive material from these trees more easily accessible for researchers. There’s no need to start from scratch and wait 10 years for the walnuts to mature if the plants are already grown up at the botanic garden. Additionally, walnuts cannot actually be stored as seed (aka seed banked) long term since they very quickly become rancid if not cultivated. This is true for many of the fruit and nut species that are important sources of food.
Coming Soon to the Garden’s Living Collection
Now that we have added these walnuts to the collection and shared material with other institutions, Southern California black walnuts are a little more available to researchers, whether they are looking to breed a new variety of walnut or restore a black walnut woodland. The walnut seed we collected last fall has been carefully grown by our Living Collection Nursery staff into a beautiful stand of trees. We are planning to plant these this winter, so keep an eye out in the Garden for these wild Southern California black walnut trees.
