When considering all the wildlife around the world that are in critical need of conservation, you probably aren’t thinking about your neighborhood oak tree. You wouldn’t be to blame for this oversight—after all, oak trees are a customary symbol of strength, resilience, and life; these are trees that exude stability, not vulnerability.
Oaks have been cultural and spiritual icons for thousands of years, considered sacred by societies like the ancient Greeks, Vikings, and Indigenous Peoples around the world. Today, oaks are still firmly—and perhaps, surprisingly—present in our everyday lives. We find oaks on countless logos and within the names of the streets and cities we call home. The word “tree” itself originally derives from drus, the ancient Greek word for oak. And with lifespans ranging from decades to thousands of years long, oaks are fundamentally part of our communities, growing alongside the generations of our families.
Oak trees are robust—literally. The Latin word for oak, robur, is the root of the word “robust.” (Pictured: island scrub oak)
There are about 450 species of oaks worldwide, distributed mostly across the northern hemisphere, and they occur in various shapes, sizes, and habitats. But despite their wide distribution, oak trees are in trouble. Approximately one-third of all oak species are threatened with extinction, and in the United States, about one-fifth (16 of 91) of our native species are threatened. The effects of climate change, including extreme temperatures, droughts, and floods, are some of the leading threats to oak trees globally. Habitat loss and modification, invasive pests and pathogens, and overharvest of acorns are also among the major threats.
Losing oak trees isn’t just a cultural sadness, it’s also a tremendous ecological loss. Oaks support more species of wildlife than any other plant genus on Earth—over 900 species of moths and butterflies alone depend on oak trees for food and shelter. These trees also provide ecosystem services that benefit billions of people around the world. They clean our air, stabilize our soils, and they are some of the most effective tree species at absorbing and storing carbon from our atmosphere. In short, oak trees are keystone species: they have an irreplaceable role in supporting the balance and function of their environments. Without oak trees, entire ecosystems could collapse.
The national tree of the United States (selected by popular vote!) is the oak tree. Native oaks are found in 47 of the 50 United States. (Pictured: island oak)
San Diego Zoo Wildlife Alliance is committed to conserving wildlife around the world, starting with species in our regional backyard—and this certainly includes our native oak trees. Our Horticulture and Plant Conservation teams, together with dedicated partners in the US and Mexico, combine leading expertise, cutting-edge innovation, and an unwavering passion for our local flora in a world-class program to protect California oaks and their role in keeping wildlife, people, and our planet healthy.
Our Local Oaks
California is home to approximately 20 species of native oak trees, and the primary oak hotspot in the state is San Diego. With at least 10 recognized species, plus several natural hybrids and varieties, San Diego County is home to more oak taxa than any other county in California.
Our oak conservation program at San Diego Zoo Wildlife Alliance is focused on six species of conservation concern that are found in San Diego and in the broader region of Southern California and Baja California (Mexico). These efforts are a close collaboration with multiple partners, including the Global Conservation Consortium for Oak, The Nature Conservancy, and San Diego Botanic Garden. The six focal species in our program are Nuttall’s scrub oak (Quercus dumosa), island scrub oak (Quercus pacifica), Cedros Island oak (Quercus cedrosensis), island oak (Quercus tomentella), Santa Cruz Island oak (Quercus parvula var. parvula), and Engelmann oak (Quercus engelmannii).
Habitat loss is one of the major threats to oaks. (Pictured: Engelmann oak)
Besides needing to contend with human-driven threats, there is another significant hurdle in our path towards conserving these trees: all oak species are categorized as “exceptional,” meaning that their acorns cannot be cryopreserved (frozen) in seed banks because they do not survive the necessary drying, freezing, and/or thawing conditions. This, in turn, means that we must find other ways to preserve oak genetic diversity for the long term.
Working in the field, in the lab, and on grounds at the San Diego Zoo and San Diego Zoo Safari Park, our teams and partners have developed a creative, multi-layered approach to save our oaks.
Take a Hike
Our work with oaks begins in the field. We trek to find wild populations of trees in their native landscapes; this includes sites like Santa Catalina and Santa Cruz Islands (Channel Islands), Otay Mountain (San Diego), and Ensenada (Baja California). These treks call for sure-footed adventurers, as the trees are often located in rugged, steep landscapes that are only accessible by hiking in.
The hike is tough, but the view isn’t bad. San Diego Zoo Wildlife Alliance senior horticulturists Dario Berrini and Greg Bluffin log the location of an oak perched at the top of a mountain on Santa Catalina Island.
We log the locations of the wild oaks we find so that we have a record of existing trees, and so that we can cross-reference these locations with data about the environmental conditions of those sites at other points in time. With permission from land managers, we also collect acorns, leaves, and small cuttings.
Wild oak tree populations are shrinking, so it is imperative to find trees and collect samples while we still can. Collecting these samples is like taking a living snapshot of oak genetic diversity. Because genetic diversity underpins species health and long-term sustainability, these samples form a foundation for future conservation options.
Small cuttings contribute to conservation in big ways. Joseph Ree, Ph.D., postdoctoral associate on San Diego Zoo Wildlife Alliance’s Plant Conservation team, works with cuttings to develop and apply technologies in the lab that help preserve oak tree genetic diversity.
Going For Groves
The samples we collect in the field are brought to the San Diego Zoo. Our team germinates the acorns in our nursery, and when ready, we can plant the resulting seedlings on Zoo or Safari Park grounds. We share seedlings with partners at other botanic gardens as well, for added safekeeping.
The cuttings we bring back from the field can also be propagated, generating clones of the source tree that serve as “backups” of wild trees. We can root the cuttings either in our greenhouse or under controlled conditions in our lab. This process in the lab, called micropropagation, involves placing small tissue segments in a growth medium to produce multiple new trees in vitro. Micropropagation requires specialty equipment, expertise, and facilities; the San Diego Zoo is one of just a handful of botanic gardens in the United States equipped with this capacity.
Oak seedlings are carefully tended in our nursery for up to a few years, until they are ready to be planted in the ground.
It’s worth noting that different plant species each require tailored formulas of growth media, so the medium we use for our local oak trees was custom developed by our team. Like the trees grown from acorns, the propagated clones could be planted and shared. Additionally, these clones are a source of knowledge: they serve as a regenerating study model that allows us to investigate oak genetics without having to deplete our limited supply of acorns.
The goal of propagating new trees and planting them at safe sites is to create a network of conservation groves that together form a genetically diverse, sustainable assurance population. Doing so converts the living snapshot of genetic diversity into a living reservoir of genetic diversity. Someday, these groves could serve as source populations for restoring oaks to their native landscapes.
Growing entire trees is currently the most effective tool in the oak conservation toolbox. However, because it isn’t always feasible to plant groves when space and resources are limited, our team is also at work to create alternative solutions.
Rewind To Move Forward
In the lab, our goal is to preserve oak genetic diversity using methods that do not depend on putting whole trees in soil. One of these methods is somatic embryogenesis.
Somatic embryogenesis, or the generation of embryos from non-reproductive (somatic) tissues or cells, is not a new approach for plant science, but it is a rare approach for oak species. The process starts by placing a section of somatic tissue, such as a tiny piece of an oak branch, in tissue culture medium. Inducing a series of natural biochemical reactions under just the right conditions, we stimulate these branch cells to “rewind” their development and revert to being stem cells. We then stimulate these cells to transform into precursor embryo cells, culture these into embryos, and eventually grow the embryos into plants.
Growing oak plantlets in vitro ensures that they are protected from factors such as infections, hungry insects, and extreme weather events that threaten their survival in the wild.
The advantage of this method is that, unlike acorns or other types of oak tissues, the precursor embryo cells are resilient to freezing and thawing and can therefore be banked for long-term safekeeping. So, while trees in the soil are a living reservoir of genetic diversity, cryopreserved cells become a living stronghold of genetic diversity. Our team has recently developed this process for Nuttall’s scrub oak and successfully banked 18 genetically unique somatic embryos in our Wildlife Biodiversity Bank. To the best of our knowledge, this is only the third oak species for which a somatic embryogenesis protocol exists. We hope to adapt this process for each of the species in our oak conservation program.
Live and Let Leaf
Oaks have been on Earth for more than 50 million years, and they have silently shaped our ecosystems and our cultures. Today, oaks are facing a silent extinction. But hope for oaks is far from lost. By merging a diversity of expertise, exploring innovative approaches, and collaborating with shared knowledge and resources, conservation programs like the one at San Diego Zoo Wildlife Alliance are supporting security for these trees in the present and for the future.
Around the world, oak trees have long been upstanding members of our communities—together, we can make sure they still feel welcomed.
You can be an oak ally! Our team participates in California OakWatch (an initiative to collect local oak tree data)—and you can, too. Those in California can download the free iNaturalist app, search for “California OakWatch” in the Projects tab, and upload photos of oaks that you find around you. (Not in California? No problem! Users anywhere in the world can upload images of their own local oaks, and more wildlife, to other regional projects in iNaturalist.)
