By Rachel Clemons, Restoration Specialist
The soft crunch of fallen leaves is comforting beneath my boots. The air feels crisp and clean. My eyes wander up through the dark green canopy, and I pause to delight in the abundant sounds of life: the bright chirp of the western bluebird, the quick tapping of the acorn woodpecker, a gentle breeze passing through leaves. Two squirrels chase each other up a winding tree branch, where a wood rat’s nest is precariously built. A deer hesitantly pokes its head out of a blackberry patch. From the smallest beetle to the gentle jackrabbit to the fearsome mountain lion, all life in the woodland is intertwined with the trees that turn the land into a home.
Perhaps the real world embodiment of The Giving Tree, the tall and benevolent oak not only provides a shaded reprieve for the tired hiker, but is also unparalleled in its ability to grant food, water, shelter, and protection for wildlife. But like many wild things, our mighty oaks are threatened by a rapidly changing world. What can history tell us about the importance of these noble giants? What perils do they face? And most importantly, how can we protect and preserve them so that our children may one day stand in their shade?
The Heart of the Forest
Worldwide, there are over 450 species of oak trees and shrubs (600 including hybridized varieties), which together, make up the genus Quercus (1). They thrive all across the Northern Hemisphere—in Asia, Europe, North Africa, and the Americas. Mexico hosts over 160 oak species, 109 of which are endemic, making it the world’s greatest center for oak diversity (1). China ranks second, with over 100 species, and the United States comes in third with over 90 species (around 60 endemic) (1).
Alongside the hickory family, the oaks once ruled the eastern woodlands of the United States from the Atlantic coast to the Great Plains, the largest range of any North American deciduous forest (2). Historically in California, oak woodlands and savannas set the stage for an intricate display of food webs involving hundreds of vertebrates, thousands of native insects, and dozens of keystone plants. Today, over 300 golden state species directly depend on 19 different kinds of oak trees (3). A fallen log becomes a salamander’s home, a single leaf makes a meal for an insect, branches high above act as bird sanctuaries, and abundant acorns feed the masses.
Densely nutritious and neatly packaged, the acorns from California oaks provide a staple food source for deer, wild turkeys, squirrels, raccoons, rabbits, and dozens of bird species. Humans, too, have relied on them for over 4,000 years (4). California’s indigenous people dedicated much of their time to harvesting, processing, storing, and cooking acorns. Acorns were collected in the fall, leeched to remove bitter tannins, laid to dry in the sun, and stored in large granary baskets. Upon preparation, they were cracked out of their shells, peeled from the thin, paperlike skin around them, and pounded into a flour. The Tamyen Ohlone people of the Santa Clara Valley used this flour to make variations of mush, bread, or soup, which rounded out a nutritious diet of wild onion, carrots, blackberries, and trout. Today, oaks and their life-giving acorns are a sacred connection to history, culture, and sense of place for California’s native communities.
Great Loss in an Era of Progress
During colonization of the 18th and 19th centuries, early Mexican and American towns across California were nestled into the shade and beauty of oak woodlands, a legacy revealed by towns like Oakland, Thousand Oaks, Oakley, Oakville, and Paso Robles (Oak Pass). But the settlers’ affinity for oaks did not beget an attitude of stewardship. Massive agricultural growth at the turn of the 20th century coupled with rapid urban development led to the disappearance of the majority of oak woodlands across the state.
Silicon Valley’s story is no exception. Early explorers were so impressed with the vast expanses and diversity of oaks, they deemed the area “Llano de los Robles”, or Plain of the Oaks (5). Even at the turn of the 20th century, farmers and wealthy travelers in the South Bay admired the beauty and dominance of the valley oak (Quercus lobata) and coast live oak (Quercus agrifolia). Most of them—as many as 99% in some areas—were cut down to accommodate more farmland, vineyards, and burgeoning cities (5). Moreover, European settlers drastically altered any remaining natural landscape by introducing non-native plant species, accelerating grazing regimes with livestock, and suppressing controlled burns that were historically (and highly successfully) conducted by native tribes.
Several decades passed as the valley’s agricultural land slowly metamorphosed into a bustling metropolis of technology, innovation, and concrete. With urbanization came the street trees. The magnolias, the ginkgos, the beloved palms, selected for their aesthetic appeal and easy-to-maintain canopies but decidedly lacking in habitat value. Silicon Valley’s non-riparian native oak canopy cover decreased from 80% in 1850 to just 4% in 2016 (5). Oak-associated species dropped from 13% to less than 1%, a staggering loss of native herbaceous and shrub ground cover (5). The incidental oak can be spotted along today’s streets, backyards, and open spaces.
More Dangers, Big and Small
Tiny but powerful creatures by the name of Phytophthora compound the human assault on oaks. While many species of Phytophthora affect and kill oaks, one of the most well known is Phytophthora ramorum, the pathogenic water mold that causes Sudden Oak Death (SOD). Since its first appearance in California in 1995, it has devastated oak populations across the state. Though it’s not entirely clear how it arrived here, researchers hypothesize it originated somewhere in Southeast Asia and was unintentionally introduced through the global gardening trade (6). The genus Phytophthora also includes P. infestans, the infamous cause of the late 19th century Irish Potato Famine. Similar to P. infestans, P. ramorum degrade’s a plant’s phloem and inner bark to produce trunk “cankers” that bleed sap. It may also restrict water transport. Leaves turned yellow or brown often precede the death of individual branches or the entire crown. In Silicon Valley, SOD primarily impacts the coast live oak (Quercus agrifolia), black oak (Quercus kelloggii), shreve oak (Quercus parvula), and the tan oak (Notholithocarpus densiflorus). However, well over 100 plants act as hosts for the pathogen, and few control mechanisms exist beyond early recognition and proper disposal of infected plant material (6).
Climate change will likely threaten the historic range of several California oak species, further complicating conservation and restoration efforts. One study by UC Santa Cruz found that potential ranges of two California endemic oaks, the blue oak (Quercus douglasii) and the valley oak (Quercus lobata), may shift considerably northward and shrink by 59% and 54% of current ranges, respectively (7). Our modern challenge is to meet these abysmal statistics with positivity and progress. The good news? It’s already happening.
Sowing the Seeds of Resilience
In addition to planting and protecting various oak species throughout Grassroots Ecology’s 20+ stewardship sites, our restoration team at Pearson-Arastradero Preserve in Palo Alto has taken special interest in the park’s next generation of oaks. The preserve’s canopy cover has been diminished over time, and while there are many mature oaks in the preserve, younger saplings are often browsed by hungry, overpopulated deer—preventing them from reaching maturity and reforming the canopy. Rather than investing time, resources, and money into growing and installing new oak trees, we asked ourselves, “why not leverage what mother nature has already given us?”
In collaboration with the City of Palo Alto and hundreds of volunteers, Grassroots Ecology has placed cages around 500 existing oak saplings throughout Arastradero to lend a helping hand on their journey toward adulthood. Focusing our energy on well-established saplings makes our job a lot easier. All are locally native, self-selected in a location appropriate for success, and do not need irrigation since they have already formed deep enough roots to survive independently. After caging, all we must do is observe. Each of the trees are geolocated to monitor health and growth over the project duration. This fall, students from The Girls Middle School and Palo Alto High School visited Arastradero to check up on our oaks. Their results are promising: over 80% of their 250 tree sample are alive and growing, and most have little to no evidence of herbivory. Over the next several years, we’ll be able to remove the cages from the trees that have grown large enough to withstand grazing and survive on their own, a rite of passage into maturity.
Grassroots Ecology has stepped up its Phytophthora-fighting game, too. Over the past several years, the native plant nursery in Foothills Park has partnered with researchers and land managers to identify and implement best practices for minimizing the spread of deadly Phytophthora species that affect oaks as well as other native plants.
Strict sanitation measures have been enacted. Anyone who enters the nursery must sanitize their shoes with 70% isopropyl alcohol solution, and the same procedure must be repeated upon entering any of the greenhouses where plants grow. All surfaces are steam sterilized, potting soil is pasteurized on site, all tools are cleaned with alcohol, and all plants are kept on metal or plastic tables at least 3 feet above ground, the “splash zone” of dripping water. Each “clean house” is surrounded by a barrier cloth and sits on a bed of gravel to ward off standing water. Shoe and tool sanitization practices have been incorporated into planting work days across all restoration projects, as well. These prevention initiatives have not come cheaply, but the widespread collapse of oak and other plant populations could be far more costly.
The global implications of climate change are enormous and may even feel immobilizing, but research suggests that effective land restoration can make a significant impact at the regional level. For example, alongside the coast redwood, the coast live oak far and away outperforms other, non-native street trees in terms of carbon sequestration. By 45 years of age, a single tree will sequester 325 Kg of carbon annually, more than double that of the London planetree, triple that of the sweet gum, and nearly tenfold that of a ginkgo, all common street trees in Silicon Valley (5). Simply stated, oak trees support a healthier climate, which in turn supports the preservation of more oak trees. By revegetating degraded lands, preserving biodiversity, and building green infrastructure, Grassroots Ecology is investing in the positive feedback loop of climate resilience. Since 2012, we have sequestered an estimated 140 tons of carbon through native plantings and tree caging.
Re-Oaking the Valley
Conserving our wild oaks and open spaces is crucial, but it may only be half the battle. With half of the world’s population now residing in cities, a growing interest in urban ecology has emerged. Researchers, city governments, and community organizations now contemplate the ecological role that oaks play in the “urban forest.” As many of Silicon Valley’s urban trees, planted 50-75 years ago, near the end of their lifespan, the time to launch an oak comeback is fast approaching (5). In their new report, Re-Oaking Silicon Valley, scientists at the San Francisco Estuary Institute (SFEI) outline the opportunity to increase biodiversity, reclaim cultural heritage, and improve human health by re-incorporating elements of oak woodland ecosystems into urban spaces.
In addition to carbon sequestration, the coast live oak also outperforms common, nonnative urban trees in runoff reduction, which helps reduce flooding and erosion while improving water quality in local creeks and the Bay (4). Furthermore, oaks may be better slated to withstand the effects of climate change. Many of our urban trees come from wetter regions, yet oaks have evolved to be drought-tolerant. As we anticipate warmer weather and scarcer, more erratic rainfall, might it make sense to return to our roots, so to speak?
The report proposes practical solutions based on scientific evidence that maximize the ecological benefits of re-oaking. For example, a single mature oak surrounded by a cluster of 20 others, all within 15 to 20 acres, could support an entire family of acorn woodpeckers (8). It’s an idea easily employable for neighborhood associations and scalable for cities. SFEI re-oaking principles are already being utilized by Google, who funded the report. The company has transformed their campus landscaping by planting valley oaks less than 500 feet apart to encourage pollen dispersal and safe wildlife movement (8). They’ve also partnered with the City of Mountain View, HP, LinkedIn, Life Sciences, and others to install more than 150 native oaks and a pedestrian path in the Charleston Retention Basin, a Mountain View wetland (9).
A Hopeful Future, in Bloom
In the coming decades, local governments, private corporations, and ordinary citizens will determine the sense of place and ecological resilience of Silicon Valley in the 21st century and beyond. In light of all the challenges discussed here, it’s easy to feel overwhelmed. At times we may feel powerless and unable to effect real, lasting change. But we owe it to the oaks —and the many creatures that depend upon them—to respond with action instead of indifference. The key is to think locally and start small. Here are five ways to make a tangible difference:
Plant a locally native oak tree in your backyard, or become a steward of an oak sapling and help it reach maturity by watering it regularly and caging it to protect it from herbivory.
Purchase locally native plants from nurseries following best Phytophthora management practices. Give extra support to small-scale or family businesses, for whom implementing these practices may be a massive logistical and cost burden.
Keep a boot brush and a bottle of isopropyl alcohol solution (70% or higher) in your car or at home, and clean your boots after you go hiking. This will minimize your risk of transporting Phytophthora between different parks or preserves.
Educate your friends and family about the beauty and importance of oaks. Start by learning to identify one oak species, an ecological benefit it provides, and one interesting fact about it. Share what you learned with someone during a hike or walk through your neighborhood.
Support re-oaking efforts in your city or neighborhood by joining a Grassroots Ecology work day. We work with over 12,000 people annually to restore native species and implement re-oaking principles in our local open spaces and urban areas. We also work to address our region’s most troubling invasive plant issues and educate the public about climate change.
During my walk through the woods, I reflect on the importance of my work and the sacredness of this space where a diversity of life still has room to grow. My gaze drops to the forest floor, and I spot a valley oak acorn, a beacon of hope and perseverance. It reminds me of its ancestors, artfully scattered across golden hillsides and deeply rooted in cool, dark canyons; enduring across our lands with ubiquity despite threats from all directions.
Perhaps there is still time, I think to myself as my hands dig a small hole in the earth, to reclaim our sense of place in the woodland. The acorn is small but feels smooth and sturdy when I pick it up and place it in the ground with care. Within it lies the foundation of entire ecosystems, a fracturable lifeline to so many. What will become of its future? I gently cover it with soil and continue on.
Dragani, Rachelle. “How Many Types of Oak Trees Are There?” Sciencing, Oct. 18, 2018, sciencing.com/many-types-oak-trees-there-5347784.html.
“Restoring the Bounty of North America's Native Woodlands.” Mast Tree Network, Nov. 2009, www.mast-producing-trees.org/2009/11/native-oaks-of-north-america/.
Oak Woodland Conservation Workgroup. “California's Oak Woodland Species.” Oak Woodland Management, University of California. ucanr.edu/sites/oak_range/Californias_Rangeland_Oak_Species/
“California Indian Acorn Culture - Background.” National Archives: Educator Resources, National Archives, 15 Aug. 2016, www.archives.gov/education/lessons/acorn.
San Francisco Estuary Institute-Aquatic Science Center, 2017. Re-Oaking Silicon Valley: Building Vibrant Cities with Nature. Publication #825, San Francisco Estuary Institute, Richmond, CA. Version 1.1, August 2017. https://www.sfei.org/projects/re-oaking#sthash.SeyvddYg.dpbs
What Is Sudden Oak Death? California Oak Mortality Task Force, www.suddenoakdeath.org/.
Kueppers, Laura M, et al. “Modeled Regional Climate Change and California Endemic Oak Ranges.” Proceedings of the National Academy of Sciences of the USA, vol. 102, no. 45, 8 Nov. 2005, doi:10.1073/pnas.0501427102. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1283413/
Wong, Kathleen M. “Re-Oaking Silicon Valley.” San Francisco Estuary Partnership, June 2017, p. 3. http://www.sfestuary.org/wp-content/uploads/2017/04/EstuaryNewsJun2017-v10pages.pdf
“Ecologically Focused Landscapes Are Coming to Life on Google Campuses.” Google Sustainability, sustainability.google/projects/urban-ecology/.