What Is This Thing? Bur oak acorn cap

May 10, 2018

"What is this thing???"

We often hear this question from friends and family in relation to natural "treasures" found in the landscape. Sticks, leaves, flowers, fruit, fungi, lichens, moss. You name it, somebody has likely brought it to BRIT for identification at some point (or emailed us a photo).

This time we feature the crazy, gargantuan, monster acorn caps from the bur oak tree (Quercus macrocarpa).

acorn caps
RAWR! Monster caps!


Bur oak distribution
Bur oak distribution in Texas. Adapted from digital version of "Atlas of United States Trees" by Elbert L. Little, Jr. U.S. Geological Survey.

Bur oak is native to the central and eastern US, including most of the middle swath of Texas, top to bottom. This fast-grower typically likes an open, limestone or chalky clay habitat and is adapted not only to fire and drought but also to extreme cold and flooding. You can find it in the prairies and savannas as well as along waterways. So basically it's a super champ of trees, which is why it's common in cultivation.

And as you can see above, THE CAPS OF ITS ACORNS ARE WEIRD, mostly because they're just so big. Bigger than we're used to seeing relative to what we think of as lil' ol' acorns. But they're also weird because of the shaggy ornamentation encircling the cap. This is a feature that can vary over the tree's range. Caps in the southern portion have long fringe hairs while others at the far north of its range are much smaller and barely have any shag at all. The caps seen above have what one might call "average shag." The shagginess and the size of the cap are the reason for one of the tree's other common names: mossy-cup oak. To me, because the size of the cap often dwarfs the little acorn attached, it resembles one of those big Russian fur hats (which Google tells me is called a "ushanka"...think George Costanza's "leave behind" hat).

man in ushanka hat
Just your average megafamous hip-hop mogul sporting an acorn cap (coined it!). (Credit: everyskyline, Wikimedia)

There's another oak, Quercus lyrata (overcup oak), that the bur oak can sometimes be confused with—their ranges overlap a bit in East Texas, they can hybridize, and overcup oak also has an acorn cap that wraps much of the way around the acorn (hence its common name).

Bur oak (Q. macrocarpa) vs overcup oak (Q. lyrata)
Bur oak (Q. macrocarpa) vs overcup oak (Q. lyrata). Adapted from Flora of North America.



bur oak caps
Bur oak (Quercus macrocarpa) acorn caps. The size can be pretty variable within a single tree, but they're all pretty bulky compared to other species.


bur oak acorns
Bur oak (Quercus macrocarpa) acorns.


Bur oak (Q. macrocarpa) vs. English oak (Q. robur)
Bur oak (Q. macrocarpa) vs. English oak (Q. robur).
bur oak vs Texas live oak
Bur oak (Q. macrocarpa) vs. Texas live oak (Q. fusiformis).

I mean, LOOK at the size of these things! You know that scene in Honey, I Shrunk the Kids where the kids are tiny and running around in the yard, and they happen upon a crumb from an oatmeal creme pie, and it's as big as a house? That must be what it's like for a squirrel to eat these acorns! But they're not just a treat for the wee animals. American black bears also consider them worthy snacks, breaking off branches to get to those still attached to the tree. 

giant creme pie
This is my dream, y'all.


Bur oak line drawing
Bur oak leaves and acorns (Illustrated Flora of North Central Texas).


bur oak craft
Ornament made from a bur oak acorn cap. (Thank you, littles!)

So why the big cap? Why the big fruit? The Latin name macrocarpa translates to "large fruit." The acorns of bur oak are the largest of all the native oaks. But making fruit this big is energy-intensive; it's expensive to the tree. So bur oaks use a strategy called masting (="synchronous and intermittent reproduction") where large nut crops are produced only every few years in order to overwhelm seed predators, making more than could be eaten in a single season and hoping that some will survive to become new trees. Many oaks do this. Serendipitously, the MinuteEarth channel on YouTube just posted a new video on this very topic (When Trees Go Nuts - YouTube).

This doesn't exactly explain why individual bur oak acorns are BIGGER than acorns from other oak species, but the evolutionary advantages of any large-fruited plant are somewhat related to the idea of masting: fruit/seed predators are given more than they can possibly consume in total, hopefully leaving some fruit and seeds intact to germinate and contribute to the next generation. Think of the strategy of making few big watermelons versus hundreds of little grapes. That may be what's going on here. Why do you think the acorns are so big?

Excerpt from Shinners & Mahler's Illustrated Flora of North Central Texas (1999):
Quercus macrocarpa Michx., (large-fruited), BUR OAK, MOSSY-CUP OAK, PRAIRIE OAK, MOSSYOVERCUP OAK. Large tree; nuts and cups (3-6 cm wide) largest of all nc TX species. Stream bottoms, lower slopes, upland woods; usually in at least moderately drained places; in or near limestone areas; se and e TX w to West Cross Timbers and Edwards Plateau. This species is well known for its large acorns and thick, fire-resistant bark.

Links to more info on the species...


So keep bringing us your weird and wonderful treasures! We'll do our best to turn them into teaching moments that benefit us all. Happy botanizing!



Leave A Response

Restricted HTML

  • Allowed HTML tags: <a href hreflang> <em> <strong> <cite> <blockquote cite> <code> <ul type> <ol start type> <li> <dl> <dt> <dd> <h2 id> <h3 id> <h4 id> <h5 id> <h6 id>
  • Lines and paragraphs break automatically.
  • Web page addresses and email addresses turn into links automatically.

Related Articles

The Living Herbarium: Instructions for Life

Article originally published in The Leaflet (April 2014) by Brian Witte, PhD, BRIT Research Associate (Disclaimer: The technical aspects of this article are dramatically simplified in the interests of communicating with an audience entirely unfamiliar with molecular biology. Send me an email ( bwitte@brit.org ) if you would like a deeper explanation.) We like to repeat, loudly and often, that there are over 1 million plant specimens in the Philecology Herbarium at BRIT. It’s a nice, big, round number, and it sounds cool when tour groups come through. What if I told you that as imposing as that number sounds, the real number is closer to a thousand billion (1,000,000,000,000) plants*? The goal of a herbarium is to preserve plants. The ideal specimen, in many respects, has all the essential...
Read More >

The Living Herbarium: Many Hands Make Godzilla

Article originally published in The Leaflet (May 2014) by Brian Witte, PhD, BRIT Research Associate There is a stereotype of the scientist as a lone genius, laboring in obscurity until their “Eureka!” moment changes the world. If Hollywood is to be believed, this Eureka moment is usually followed by the destruction of Tokyo and/or New York by a giant robot/genetic mutant/superstorm. In reality, we have a tragic lack of giant robots, and nothing that we’ve done in the herbarium has (yet) threatened a major metropolitan area. We also rely heavily on collaboration, rather than solitary toil. In fact, I would venture to say that collaboration is the fundamental characteristic of science. NOT what we do…exactly. Nowhere is this more on display than in the herbarium at BRIT. Over the past month,...
Read More >

The Sweep of Time

Article originally submitted for The Leaflet (June 2014) by Brian Witte, PhD, BRIT Research Associate Most of us live in the moment. Paycheck-to-paycheck, living for the weekend, summer vacation, twitter updates. Updates now are measured in seconds. America, too, is a young nation. Few places west of the Appalachians boast buildings over 150 years old, and most of us live in suburbs built in the decades following World War II. So much around us is new…even our landscapes are new, transformed by mechanized farming, car culture, and introduced species. That’s not entirely news, and it’s not entirely new, either. Look, for example, at this sheet I recently encountered while tidying up a database of digitized herbarium specimens. Click to enlarge and read labels. This was one of the last colle...
Read More >

A 54-Year Celebration

The Journal of the Botanical Research Institute of Texas ( JBRIT ) is celebrating its 54th year of continuous publication. It all started when Lloyd H. Shinners —a member of the Southern Methodist University (SMU) faculty and a prolific botanical researcher and writer who wanted to edit his work and the work of others—founded and published the first two issues of Sida, Contributions to Botany on November 23, 1962. He named the journal for a genus ( Sida ) of yellow-flowered plants of the mallow or cotton family (Malvaceae), distributed throughout the world and especially common in Texas. Shinners served as editor and publisher until his death in 1971, after which William F. Mahler, professor of botany at SMU, inherited the journal and continued to edit and privately publish it. Barney Lips...
Read More >