By: Diana Sherwood
“Every plant is a story. An oak tree is a highly dynamic system. Though now, in early autumn, it may not look like a lot is happening in a tree, across oak trees, dramatic changes are taking place. In this story, begin to explore the oaks overhead in autumn.”
An oak tree is a highly dynamic system. Oaks are one of the oldest and most widely spread Northern hemisphere trees on the planet. With, now, 461 species worldwide, oaks range in size from knee-high evergreen shrubs to magnificent deciduous (leaf-dropping) trees.
Natural landscapes dominated by oaks at one time covered more than a third of California. The abundance of oak trees allowed California’s indigenous inhabitants to achieve one of the highest population densities in the Americas north of Mexico. Oakwood has been an integral part of many communities’ material cultures, and acorns gathered, a staple of many diets.
But oaks are not rare to California. Oak’s Latin name “Quercus” derives from two Celtic words: quer meaning “fine” and quez for “tree.” These fine trees have long been revered in cultures wherever they grow.
From stories of Eden, to the Ancient Greeks, Romans, Celts, Slavic and Teutonic tribes, oaks have been honored among trees. In each case, as here, oak has been sacred, storied, and associated with ancestors and deities.
Oaks improve air quality by storing carbon dioxide. It has been observed that one tree can absorb up to 10lbs. of air pollution in a single year. Oaks feed, support, and shelter hundreds of creatures—many more than many trees—insects, spiders, owls, birds, and bats. And an oak can live for hundreds of years.
Some have theorized that plant foods containing high quantities of starch were essential for the evolution of the human phenotype during the Pleistocene. If so, among Oaks that feed so many, their fruit likely fed early human evolution too. And acorns are nutritious with the nutritional breakdown of the acorn of a white oak containing more than 50% carbohydrates, also water, fat, and fiber.
In California, 19 species of oak remain. Of these, nine are trees, five deciduous—about to lose their leaves in this season (or, at other times, in response to extremely dry conditions), and four evergreen. “Live oaks” get their name because they remain green and “live” throughout the winter when other oaks have lost their leaves.
Each oak species is unique with specific characteristics, physical traits, growth form, leaf shape, bark texture, and acorns, based on the specific conditions in which it stands. Will it drop its leaves; endure cold, winds, or repeated natural fires in California? Native plants are remarkable in their marvelous adaptations to exactly where they stand. As part of a 56 million year evolutionary history with evidence of oaks shifting and diversifying with shifting land masses, every oak’s story is specific and deep. But whether losing leaves as a Valley oak will, or holding them, as does Coast live oak, as the days grow shorter and nights longer, both are preparing for dormancy.
Trees derive their energy from sunlight. Though now, in early autumn, with shifting light, it may not look like a lot is happening in a tree, especially among those who do not lose leaves, nor perhaps because of our sometimes mild winters, across oak trees, dramatic processes are taking place. Within an oak, a massive redistribution of its resources has begun.
In some trees, where conditions suggest it, triggered by light and signaled by hormones, trees begin to break down pigments and nutrients in their leaves to store over the winter. The result is a change in the color of the leaves. Once all the nutrients have been extracted, the tree, not wind, will begin to shed its leaves to conserve water and energy.
But trees are not simply responding to the cooler temperatures. They use a much more sophisticated method than temperature alone to sense the changing season. In their own way, they can “see.”
Just as we have photoreceptors in our eyes, oaks have their own. An oak is able to differentiate between red and blue, and even see in wavelengths that we cannot. Phytochrome, a chemical pigment in oak’s leaf cells, senses red (and far red) light. And it is phytochrome that is able to measure the hours of sunlight and darkness, and work as a sort of light-activated switch. As nights grow longer, phytochrome signals to the tree that autumn has begun.
Among trees that do not drop leaves, like Coast live oak, the tree is still entering dormancy. A botanist explained it to me this way: In Coast live oak’s dormancy, with less sunlight, leaves grab less energy and processes slow. Though hormones signal changes, leaves remain until, late in the spring, the tree will do it all at once. Within weeks, Coast live oak will shed old leaves while putting on new and beginning to bud with flowers. Coast live oak takes a while to get rolling in the spring unlike some other trees. In breaking it, the tree nods to its dormancy.
But shed leaves or hold them, among all oaks—or anyway those old enough to reproduce, generally at 20 years old or older—in fall, oaks also release their offspring into the world. And the acorn, like many seeds, is a masterpiece. Solidly protected from the elements, inside each seed capsule is not only the genetic code to make a new tree, that may someday be hundreds of years old, each acorn comes packed with energy and nutrients. What we call the “meat” of the acorn is actually the new tree’s “sugary” first leaves, which the baby will use to emerge before putting on true leaves. An acorn has all that it needs, though only one in 10,000 will become a tree…
So, right now, in each oak, resources are pulling in, receding into a vast root system, deeper than the oak is tall, and wider than it is wide, below. Some have said if you could take, unravel, and lay end to end the roots of a 15-year-old oak, it would stretch five miles. Add the filament-like mycorrhizal fungi attached everywhere to oak’s roots and it might span half Earth. This deep “tree beneath the tree“ that you do not see is really the tree’s winter home. And here, even with less of the sun’s energy, the oak will store much of its energy and nutrition through the coming months, with the help of mycorrhizal fungi continuing to extract water and minerals from the soil. The vast, latticed network of roots is also oak tree’s stronghold connection to earth; roots evolved to keep the oak standing year after hundreds of years through whatever may occur above when less actively collecting energy—bitter cold, wind, drought, even fire—deep below. And this begins the story of we’aashar, one of the many Tongva words meaning “oak tree,“ these magnificent beings millennia older than we, in their home… though at present, an estimated 31% of the world’s oak species are threatened with extinction.