A Mast Year? The Form and Function of the Iconic Acorn

  

Photo: Acorn from a blue oak, or blue oak hybrid (Quercus douglasii). Note the stout, rounded body of the acorn and the well-fitted cap.

Who doesn’t love an acorn? Who can resist bending over to pick up a fresh acorn newly fallen on the ground, looking at it carefully, tucking it into a pocket?

Photo: Acorn from a blue oak (Quercus douglasii), or blue oak-valley oak (Quercus lobata) hybrid. Note the slightly longer, tapering, pointed shape and smaller cap.

Recently a friend wrote to me about noticing the first acorns on the driveway – ripe acorns, harbingers of the changing season. Looking out my kitchen window the day just before this note, I had noticed the abundant acorns on the blue oaks in my garden. The day after this note, another plant friend wrote enthusiastically that he had collected acorns from some notable oak species from around the area – oracle oak (being a cross between two species), Brewer's oak and Tucker’s oak, among them – some of which seem to hardly ever put on a good acorn crop. It looks like it could be a mast year for acorns in Northern California.

Photo: A handful of dried acorns from a variety of oaks, including valley oak (Quercus lobata), blue oak (Quercus douglasii), live oak (Quercus wislizeni) and black oak (Quercus kelloggii).

You may have heard the term “a mast year” for acorns. “Mast” is defined in botanical terms “as the nuts or fruits of trees and shrubs, such as beechnuts, acorns, and berries that accumulate on the forest floor, providing forage for wildlife and people.” The phrase “a good mast year” refers to a year in which there is a heavy crop of "wild nuts."

Photo: A California oak. This everyday kind of scene in interior Northern California roots us literally, psychologically and visually/aesthetically to this place we call home.

One of the things I love about California is its oaks – its mighty oaks, its sprawling oaks, its scrub oaks, its enduring oaks, its diversity of oaks, and all the life and variation of function and form that is incubated, nurtured, fledged and otherwise supported in the arms of this abundance of oaks. They are the back bones of much of our regional diversity of life.

Photo: Acorn of an oracle oak (don't you love that name? A naturally occurring cross between an interior live oak and a black oak.)

One of the things I love about my home garden is its backbone of native oaks – blue oaks, blue oak crosses, interior live oaks, and a valley oak. One of the many things to love about any oak is the particular form of its acorns.

Photos above and below: Acorns from the black oak (Quercus keloggii), note the scaly rather than sculptural cap texture as well as the cap to body ratio and proportion.

Each oak has its own signature acorn. As symbolic and loaded with literal and metaphoric meaning a symbol if ever there was one, an acorn epitomizes the magic of those moments in life when beauty of form meets well-engineered function. (One of my children's favorite stories as small people was a book entitled "Miss Suzy" about a grey squirrel who lived in a red oak. She had acorn cap bowls and cups and she would sing a song that went: "Oh, I love to cook and I love to bake, I think I'll make an acorn cake," sweet testimony to the many many uses of oaks and acorns in our cultural iconography).

An acorn’s job is to ensure the reproduction of its oak. And each acorn is designed a little differently for the task. On an acorn, each part of the package plays a role: the cap (cupule), the shell (basically two layers, the pericarp being the outer layer and the testa, which covers the flesh inside being the inner layer, and the cotyledons, the plumule and the radicle, which together form the embryo of the potential oak tree. Each part of the acorn cap and nut is designed to protect and sustain the young embryo until just the right conditions of light, temperature, humidity and sometimes even acidity are met in order to successfully germinate. (Diagram above from wikipedia.)

Photos above and below: Acorns from valley oak (Quercus lobata), note the sculptural texture of the cap and the clasping nature of it on the relatively long, slender, more pointed nut.

Oaks are monoecious – meaning they bear their male and female flowers on the same tree. They are also wind pollinated. Seasonal temperature extremes, wind, rain, drought and other environmental stresses such as fire or pests can affect a year’s acorn production. A large crop one season can affect the reserves of a tree’s nutrients, causing them to produce a smaller crop the next year.

Photo: A deciduous California oak in winter.

Over 800 species of oaks exist worldwide, with the center of diversity being in Mexico, home to more than 200 species. More than 20 species are native to California. Each of our native oaks has an ecosystem or niche in in the California landscape: valley oaks prefer the deep more riparian soils of the valley floor and riparian corridors, the blue oaks are legion in grasslands on shallow soils, the black oaks like the cooler temperatures and soils higher up in the foothills, growing happily alongside ponderosa pines, the canyon live oak can be found in steep cooler canyons of the region.

Photo: Acorns lining a young branch on a non-native scarlet oak (Quercus coccinea).

In terms of seed dispersal mechanisms, oaks and acorns rely on gravity or biological dispersal – meaning that the acorns are heavy enough to fall on the ground and germinate or that animals or people transport them to a place in which they might successfully germinate. Although most of the animals that pick up an acorn eat it, birds such as acorn wood peckers, blue and/or scrub jays and squirrels that "scatter-hoard" acorns in caches in trees for future use, make it possible for some of the acorns they collect each season to germinate and grow on. Acorns germinate on different schedules, depending on their place in the oak family.

Photo: Acorns from a non-native, evergreen cork oak (Quercus suber). Note the distinctive slender fringe/spikes covering the cap.

Because of the wide number of oaks we have in our region, over hundreds of years natural hybrids, crosses and even cross backs have occurred and so not every blue oak or blue oak acorn looks exactly the same. Further, in each tree and its own line of offspring/relatives, there can and will be similarities of variation - like all of your family have freckles, or bow legs, etc. But still, there are some notable characteristics in the form of acorns from each species.

Photo: An acorn woodpecker prepares to cache another acorn in his cache-tree snag.

Very few acorns from any oak make it to germinating into a seedling. Before they can become trees, most acorns serve as important sources of protein-rich food for wildlife, insects, fungi, and humans. Some oaks (from the so-called red oak group) produce acorns that mature in one season, while others (from the so-called white oak group) produce acorns that mature in two seasons – in which case the weather conditions of two springs ago are what impacted the acorns we are seeing now.

Photo: Acorn from the canyon live oak (Quercus chrysolepis) with its lovely golden hued caps.

All of these oaks also provide the abundance of acorns that we enjoy the sight of each fall, whether it’s a mast year, or a lean year.

And, as Ralph Waldo Emerson reminded us: "The creation of a thousand forests is in one acorn."

Photo: Brewer's oak and oracle oak acorns collected for propagation by Mike Thiede.

For further reading on the lives of our California oaks, the eating of acorns or the growing of native oaks in your home garden - try these resources:

California's native oaks are under tremendous pressure from expanding land development and other human environmental impact. As plant-propagating friend, Mike Thiede points out, despite the fact that most acorns become food, it’s easier to grow an oak tree from an acorn than from a cutting or a graft. This might be a good year to give it a try!

"The Life of an Oak, an intimate portrait", by Glenn Keator, Illustrations by Susan Bazell. 1998, Heydey Books and California Oak Foundation.

"The Oaks of California", by Bruce Pavlik, Pamela Muick and Sharon Johnson. 1993, Cachuma Press.

"Sacramento Valley Feast, 2nd Edition", by Wolfgang Rougle. Self-Published. For copies go to Lyon Books in Chico, Enjoy the Store in Redding, UC DAvis Bookstore or Davis Food Co-op in Davis. More information email Wolfgang at: springfedfarm@yahoo.com.

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To submit plant/gardening related events/classes to the Jewellgarden.com on-line Calendar of Regional Gardening Events, send the pertinent information to me at: Jennifer@jewellgarden.com
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In a North State Garden is a weekly Northstate Public Radio and web-based program celebrating the art, craft and science of home gardening in Northern California. It is made possible in part by the Gateway Science Museum - Exploring the Natural History of the North State and on the campus of CSU, Chico. In a North State Garden is conceived, written, photographed and hosted by Jennifer Jewell - all rights reserved jewellgarden.com. In a North State Garden airs on Northstate Public Radio Saturday mornings at 7:34 AM Pacific time and Sunday morning at 8:34 AM Pacific time. Podcasts of past shows are available here.

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5 Responses »

  1. I absolutely love the solidly strong, yet stately gracefulness of the Valley Oak. I always watch for them where they have been preserved in some of the fields as I drive south on I-5.

  2. What a great reference article - I loved the detailed photos of the various acorns and that there were leaves shown with them for better identification out in the field.....nice job! I'd like to hear more about propagation from Mike - does he use pots or just direct plant? When I've had acorns get a start in a pot, they seem to immediately grow a very long tap root - seems like you'd almost have to direct-plant?

    • I will ask Mike about how he starts them. You are right - they do put down a tap root first, and I have seen them started in nursery rows in the field to be dug later, but I have also seen them started in those very long, narrow pots as well - I will try to add some info on this. Thanks for bringing the point up.

  3. I was just thinking the other day that there seemed to be a lot less acorns this year. (This is eastern foothills ~2500')

    Those cork oaks seem to be quite rare - that would be a good one for re-planting...

  4. Good article by the way, great details.

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