From Tiny Acorns to Mighty Oaks: Rebuilding Oak Woodlands in the Napa Valley

Oaks are a keystone species that can help preserve our watershed

Before the land was cleared for orchards and vineyards, stands of oak trees dominated the valley’s landscape.

Blanketing the foothills and shading riparian landscapes, oaks are an iconic part of the Napa Valley image. Napa Valley has the greatest density of oaks of any other county in California: thirty-three percent of the county is covered by oak woodlands. Four species of oak are most common in Napa Valley: Valley oak (Quercus lobata), Coast Live oak (Quercus agrifolia), California Black oak (Quercus kelloggii), and Blue oak (Quercus douglasii). These trees have evolved to adapt to the hot, dry seasons of California, requiring little water once established in cycles of drought. Resilient in fires, thick bark protects the trees from low-intensity and medium-intensity burns and their ability to sprout new growth from the base of their trunks helps them rebound from the ashes. Less intense fires can actually promote oak survival by increasing the availability of nutrients in the soil and killing harmful pathogens. For many years, indigenous people such as the Wappo of Napa Valley used fire to manage oak woodlands. Prescribed (purposefully ignited) grass fires help the oaks thrive and decreases the chances of larger, more destructive fires occurring. Fire can lso improve the vitality of the oaks, creating higher yields of acorns.

Oak trees are a critical food and habitat source for wildlife. Biodiversity hotspots, oak woodlands are home to over 300 vertebrate (bird, mammal, reptile, and amphibian) species, 1,100 plant species, 370 fungal species, and 5,000 arthropod (insect) species. Organisms that have a large effect on a broad range of species are considered keystone species because their presence provides positive benefits for a wide range of flora (plants) and fauna (animals). Dense canopies provide shelter for birds, leaf litter and fallen branches provide shelter for terrestrial (ground) animals, and acorns provide food. In return, birds and other animals disperse the acorns across the landscape so new oaks can sprout and grow. The shade of the oaks lowers the temperature of the ground and adjacent bodies of water, helps to retain soil moisture that helps plants in the understory grow and keeps creeks cool for fish and amphibians. Fallen leaves decompose (break down) to add valuable nutrients to the soil and provide food for creatures in the rivers and creeks. 

A critical function of oaks in the valley is erosion control. From leaves to roots, oaks work to slow the flow of water and prevent sediment and contaminants from entering creeks and rivers, which can be harmful to water quality and fish. The broad canopies capture the rain as it falls from the sky, the drops temporarily held in the tree as they tumble and drip from leaf to leaf before hitting the ground. Leaf litter and branches below the tree filter the water before reaching the soil, helping to prevent flash flooding and runoff. Oak woodlands can capture 20-30% more rainfall than grasslands, delaying peak flows in rain events to help the environment absorb the rainfall and reduce flooding. The retention of moisture in the soil promotes groundwater recharge. Slowing down the water helps it trickle down into the ground for storage instead of rushing away downstream. The roots of the oaks stabilize the soil and hold it in place as it becomes saturated. The roots’ symbiotic relationship with fungi in the soil helps create cohesion and stability, preventing fine sediment and nutrients from washing away.

Oaks also help clean the air & soil by absorbing and storing harmful toxins through their leaves and roots. The roots absorb contaminants in the soil such as heavy metals, pesticides, and fertilizers so they don’t become part of the water supply. Oaks absorb ozone and other airborne particulates (small particles) through their leaves, helping to improve the air quality by lowering levels of pollution and returning pure oxygen to the atmosphere. As the leaves ‘breathe’ in carbon dioxide from the air, they store the carbon in their trunks and branches, sequestering (storing) carbon in the atmosphere that contributes to global warming. The soil below the trees is also better suited to sequester carbon because of its high content of organic matter. Sequestering carbon helps offset the carbon dioxide levels resulting from the use of fossil fuels, helping to reduce overall temperature levels and pollution in the environment. 

Oaks make a remarkable impact on the air and water quality in our watershed. The more scientists discover about the contributions of oaks in the environment, the more communities are beginning to realize the benefit of replacing oak woodlands that have been lost to agricultural and urban development, fires, and disease. Researchers are also discovering the benefits of oak woodlands adjacent to vineyards: the ecosystems associated with the oaks also help reduce root rot and vine disease, as well as provide habitat for predators that drive away pests such as insects, rodents, and birds that harm the fruit and vines. Oaks have begun to play a big role in sustainable vineyard management as growers look to decrease erosion and use of pesticides on their properties.

In the Napa Valley, the number of mature native oak trees is estimated to have reduced from 45,000 in the early 1800s to less than 1,000 today. Re-oaking the valley involves careful planning to make sure that acorns and new oak seedlings are planted in optimal locations for their survival. Replanting species in the same area they once grew after a fire is one strategy that has proven to be successful. Another is planting oaks within close range of established oaks of the same species. This makes it more likely the optimal conditions for the survival of oak trees are present, while also expanding existing stands of oaks and the wildlife corridor. Currently, the Napa County Resource Conservation District (Napa RCD) works with volunteers in the community to collect acorns and monitor plantings to study and expand efforts to replant oak woodlands. Acorns are collected by community members and sorted by species before they are planted in local parks and near vineyards. Volunteers can sign up to help monitor re-established oaks in local parks, while the Acorn to Oaks program allows local schools to participate in the effort while teaching students about oak ecology. Napa RCD also works directly with land owners to develop re-oaking plans specific to their properties. 

Oaks represent the health and future of the watershed.  Replanting oaks helps our economy by promoting sustainable vineyard practices and enhancing the beauty that attracts tourists. Re-oaking helps our ecosystems by expanding food souces and habitat for plants and animals and helps improve water quality by reducing erosion and contaminants in local rivers and streams. Planting more oaks positively impacts the health of everybody living in the valley by removing contaminants from the air and soil, providing shade to reduce temperatures, and sequestering carbon in the atmosphere that contributes to global warming. As you stroll through your neighborhood or drive through the valley, keep your eyes open for these magnificent trees and appreciate all the wonderful things they do to improve our quality of life. Maybe even consider becoming part of the effort to re-oak Napa Valley and improve the community for generations to come.

Check us out! Books and resources available from the Napa County Library:
1). From Acorn to Oak Tree by Emma Carlson Berne
2). The Life Cycle of an Oak Tree by Ruth Thomson
3). See it Grow: Oak Tree by Joyce L Markovics
4). Oak Tree by Gordon Morrison
5). Dot & Jabber and the Great Acorn Mystery by Ellen Stoll Walsh

Go deeper! Explore these great resources online:
Napa County Resource Conservation District Acrons to Oaks Video
Napa County RCD Oak Montoring Team
Napa RCD Acorns to Oaks Kit (curriculum)
A Day in The Life of an Oak Tree

Click on the images below to learn more: