Nuts About Oak Trees

“Mighty oaks from little acorns grow”
(From old proverb)

I love to stroll under the shade of majestic Valley oak trees in the oak woodland riparian habitat along the Stanislaus River at Caswell State Park in Ripon. I visualize Lakisamni Yokut women (the indigenous people who lived in Stanislaus County area for millennia) as they gathered acorns.

Vital Roles

One of the oldest and widespread trees on Earth, oaks appeared about 65 million years ago, long before humans. Once a common feature of California's landscape, today they inhabit a fraction of their former terrain. Playing a vital role wherever they're found along California's riverbeds, foothills, and valleys, they provide food, water, and shelter for approximately 350 vertebrate species including mammals, birds, amphibians, and reptiles, and can also support over 250 different species of insects and arachnids.

Early humans built their homes, created tools, built shelters and ships from oak wood. Oak galls were used to make dyes, writing ink, and tan leather. Today we use its strong wood to construct furniture, flooring, cabinets, and wine barrels. If you like truffles, thank oak trees, since truffles have a close relation with the roots of oak trees. Truffles are almost impossible to grow. Instead, truffle farmers plant oak trees, hoping to create favorable conditions conducive to the growth of truffles.

California Oaks

Thriving in our hot, dry summers and cool, wet winters, oaks can be found in 52 of California's 58 counties. Of the 600 species found worldwide, there are 20 species of native oak in our state, with three species being the most common in the Central Valley:

• Valley oak (Quercus lobata) – One of California's iconic species, it is the largest oak tree found here, living up to 300 years. The Valley oak grows where there is a water table within reach of the roots, often near creeks and rivers. They grow quickly, reaching 20 feet in 5 years and up to 60 feet in 20 years. A deciduous tree, it's distinguished by deeply lobed shiny green leaves and long, narrow acorns.  
• Interior live oak (Quercus wislizenii) – An evergreen tree, growing up to 25-80 feet tall, it is found in hilly or mountainous areas, near creeks and streams, living up to 200 years. It can produce two types of leaves at the same time, one with a serrated edge and the other with a smooth edge, and produces small, thin acorns.
• Blue oak (Quercus douglasii) – A deciduous tree found in the hot, dry foothills, it grows to be 20-60 feet tall with blue-green leaves which vary in size and shape. With a lifespan of 200-500 years, their acorns are fat and stubby.

Oaks tolerate fire due to their thick, furrowed bark and tough leathery leaves. During wildfires, the larger oaks in areas cleared of fuel may scorch, but rarely burn completely. Damaged trees will resprout from the root crown.

Acorns

Oak trees produce inconspicuous greenish female flowers in the spring and are wind pollinated. Each acorn contains one seed nut enclosed in a tough shell coated in tannic acid which can protect them from fungi and insects. Mature acorns typically drop during September and October. Oaks have unpredictable cycles of boom and bust. Boom times, called “mast years,” occur every 2-5 years with smaller acorn crops in between. Masting takes energy, so oaks grow slower during those years.

An oak tree can produce millions of acorns during its lifetime, but only one in 10,000 acorns grows up to be an oak tree. Acorns are highly nutritious, carbohydrate-rich, and were a diet staple of the Californian indigenous people. Mammals and birds who eat acorns include the Acorn woodpecker, Yellow-billed magpie, California ground squirrel, and Mule deer. However, acorns are toxic to dogs and horses.

The indigenous people called the California scrub jay the “gardener bird” because of its propensity for caching thousands of acorns and not eating all of them, which helped replenish and expand oak forests.

Oak Galls

Have you ever observed “oak apples” in oak trees? Oak galls are created by tiny female Cynipid stingless wasps about the size of a grain of rice. The parasitic wasp inserts its egg into an oak twig, causing a reaction by the tree, a swelling of woody tissue which surrounds and protects the egg, providing shelter and food for the hatching larvae. The larvae eventually eat their way out of the gall, dropping to the ground. Galls come in many sizes, shapes, and colors, which are specific to the wasp and oak species. Some galls are as tiny as sesame seeds and others as large as an apple. The galls do not seem to harm the tree, though oak trees with a lot of galls tend to produce fewer acorns.

A favorite oak gall of mine is “jumping galls,” the size of poppy seeds, round with a dot in the center. Some years large numbers of them drop and litter the ground and sidewalks. The galls “jump” each time the larva moves inside. You can see this in action in this YouTube video: https://www.youtube.com/watch?v=0VI7USm4J5I

Oak Challenges

With their spreading branches casting deep shade, mature oak trees take up a lot of space, so are not ideal for the average residential garden. However, if you have the space and patience for the time it takes to grow, an oak tree makes a stunning garden centerpiece, attracts a variety of wildlife, and requires little care with virtually no water in the summer once established. Oaks may look indestructible, but are sensitive to soil compaction, overwatering, or altered soil levels. It is recommended to limit plantings under an oak since they do not like soil under the canopy to be disturbed.

Today, oak trees face many challenges. Disease, drought, and fire can all destroy oak seedlings. Young oaks are stepped on by grazing animals or run over by machinery. Full grown oaks are often damaged or killed when new homes, roads, stores, or businesses are built. Sudden Oak Death is a disease caused by the pathogen Phytophthora ramorum,discovered in Mill Valley in 1995. Causing a rapid color change from green leaves to brown in infected oaks, it has killed thousands of live oak, black oak, tanoak, and Shreve oak in 14 California counties. Climate change is also putting pressure on oak trees.

To maintain a forest or woodland, each oak tree needs to produce just one replacement tree in its lifetime. You can help regenerate California oak habitat by caring for an acorn seedling and protecting it from harm while it grows into a mighty oak.

All photos by Denise Godbout-Avant unless otherwise noted.