PALO ALTO, California (Landscape News) — Silicon Valley today sprawls from San Jose north towards San Francisco, a landscape dense with suburban housing, commercial corridors and office parks. But just over a century ago, valley visitors instead encountered woodlands and savannas and great numbers of oak trees. Indigenous people relied on acorns from these trees as a dietary staple and Spanish explorers referred to the area as Llano de los Robles or Plain of the Oaks.
Such oak trees are now few and far between in Silicon Valley, but a recent report by the San Francisco Estuary Institute (SFEI) explores strategies to re-introduce them in ways that benefit urban conservation and provide resilience to climate change. The report also aims to serve as a guide for similar projects in other urban environments.
Using science, historical data, tree inventories, and contemporary landscape analysis, SFEI determined there is both a need and a role for more oak trees.
Ecologically, the SFEI report finds that as a species native to Silicon Valley, oaks have evolved and adapted over time in ways suitable to the local environment. Oaks are therefore better able to handle drought, heat, and wildfires than other non-native trees as the climate becomes drier and hotter. As a native tree — one that once thrived on hot, open ground — oaks have built-in resilience to temperature extremes and can help offset global warming.
In one example, oaks retain more water from heavy rains than other trees and are unlikely to need irrigation during long dry seasons as non-native trees do. At the surface, expansive shade from oak trees helps cool ground temperatures.
SFEI also found that over a 45-year period coastal live oaks sequester more carbon than other more common trees in the area. This aligns with other findings about the role of large trees in filtering pollutants and mitigating climate change.
Oaks too are naturally suited to support the Silicon Valley’s wildlife, particularly native birds and insects, and serve as a keystone species for area-ecosystems.
“You can’t swap out an oak and replace it with another tree and expect it to have the same ecological function,” Erica Spotswood, an applied ecologist with SFEI, said ahead of International Day of Forests, this year focused thematically on urban forests. “Oaks play a special role in the ecosystem.” She cited the high tannin levels found in oak leaves that local species such as insects have adapted for as they co-evolved.
In the report, SFEI says more oaks can be planted over time to supplement Silicon Valley highly diverse contemporary treescape. Most oaks were removed long ago, initially for vineyards and fruit orchards in the 19th century, and later during the post-World War Two building boom, which included the nascent high-tech industry, and early computer chip and semi-conductor manufacturing left behind the largest number of Superfund toxic waste sites in the United States.
SFEI estimates just 4 percent of street trees in the communities of Mountain View, Palo Alto and Cupertino are native oaks compared to levels of about 80 percent in the past.
“Silicon Valley has gone from about 20 tree species 150 years ago to more than 400 species introduced from all over the world,” Spotswood said. “Many of those trees are from more temperate climates and don’t do well with drought and heat stress.”
Many of these trees, both in Silicon Valley and across California, are also roughly 50 to 75 years old and will soon need to be replaced. Replacement could bring drawbacks such as pests and pathogens, and conflicts between humans and wildlife drawn to oaks and their understory. Space too is needed to house the large trees.
But the need to replace the older trees also provides an ecological opportunity and a chance to provide the often featureless Silicon Valley a more distinctive, scenic natural look. SFEI scientists point out that one of the most common trees in Silicon Valley, the London plane, is also the one of the most common trees in cities as diverse as Buenos Aires and Paris as part of a more homogenized urban landscape.
Silicon Valley-based Google, which helped fund SFEI’s project, has already taken note and is incorporating more oaks into two Silicon Valley campuses and into its plans for a new campus. Apple’s new headquarters in Cupertino has incorporated oaks, as well.
Says Spotswood, “Silicon Valley doesn’t need to add more trees, but this is a chance to re-integrate oaks that are at the root of its ecology back into the urban forest.”
RESOURCES
Benefits of Urban Trees, Food and Agriculture Organization of the United Nations
Urban Trees and Climate Change, Canopy
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