Acorn Mysteries

Before September was over, you couldn't walk under a red oak without crunching acorns underfoot.

A friend recently urged me to write a post on acorns.  "Why are there so many?" she wanted to know.  Indeed, after closely observing a number of oak trees and seeing none of the expected acorns last year, I was very pleased to see some of the same trees bearing heavily this year (I was ready to go nuts last winter!)  First, in September, a couple of red oaks of my acquaintance began covering the ground with stout, barrel-shaped acorns.  Other neighborhood oaks announced themselves to be red oaks by doing likewise.  Soon afterwards, white oaks with graceful long, tapering acorns began to weigh in with heavy crops of their own--so many acorns that, in a few walks, I filled my pockets with enough sound acorns to experiment on using them for food.  (Acorns were the staple food of a variety of North American tribes in the Pacific Northwest.)  The single scarlet oak I know nearby produced an admirable crop, as well, peaking in late October.  Black oaks, meanwhile, dropped acorns one tree at a time, with no pattern apparent to me.

This may even turn out to be a "mast year": one with many times the typical yield in tree nuts that are food for animals.  The random nature of mast years makes them an enduring mystery of their own.

Red Oak (Quercus rubra).

White Oak (Quercus alba) dropped its slender acorns so thickly I sometimes

avoided walking under the trees rather than murder them underfoot wholesale.

 White and black oak acorns compared.  White have warty caps, red, black, etc are scaly.

 Black oak (Quercus velutina) acorns may be as large as red, but are rounder, with deeper cups.

So why, then, was I left with the (ungrateful) question: why aren't there MORE acorns?  It turns out the most common of our oak species was almost a no-show.

Here's the census: 

The one scarlet oak I know finished dropping an impressive crop of nuts a couple of weeks ago.

Of the five red oaks I know, all dropped good crops.  (These I did not collect for food, because red oaks and their near relatives, black and scarlet, have two-year acorns that are very bitter.)  It's entirely possible that some red oaks didn't, but those I didn't know about.

White oaks are more common: I kept tabs on seventeen, of which about half had respectable crops.  (The two in my yard, among others, remain recalcitrant.)

Black oaks are commonest of all; I know of perhaps forty trees big enough to bear well.  (Some few of these may be the similar scarlet oak.)  I am only counting mature trees that overhang some pavement, making it easier to spot fallen acorns.  Yet only four black oaks out of the forty dropped any acorns worth mentioning!  And only one dropped a crop comparable to the better-bearers among the red and white oaks.  


Where are all the black oak acorns??

There are, of course, many factors that affect whether a particular tree will bear a good crop in any year.  A cold, wet spring could inhibit pollination.  A host of other environmental conditions including disease and insect predation could reduce yield.  And the trees themselves differ: oaks tend not to bear good crops under about age fifty, and drop off production when they become old; and some trees may simply be poor producers for genetic reasons.  Here in the city, other factors that occur to me include poor soil, or inadequate water due to pavement-covered tree roots.

All oaks, like other trees, produce flower buds in the summer or fall of one year and flower the next year.  But black oaks, like the related red and scarlet oaks, don't mature and drop their acorns until the fall of the year following the year they flower.  Thus any particular acorn on such a tree is vulnerable to the environment (late frosts, drought, extreme cold, and the like) for the better part of three years.  White oaks, by contrast, mature their acorns the same year they flower, and so have one less year of vulnerability.

I have made a point of watching bigger trees, so I think youth unlikely to be the problem.  And weather conditions should be similar in the local area.  So what are we left with?

The most useful explanation probably came from ehow.com, and the above Illinois forestry extension agent.  White oaks and their near relatives tend to produce a regional crop: all trees in the region bearing about the same in a given year.  Black oaks and their relatives produce in individual patterns in which a good crop may be followed by two to four years of small crops.

Even so, in a five-year cycle we should expect good crops from about one-fifth the trees in any given year.  One-tenth seems awfully low.  And the red oaks--with similar vulnerabilities--had a bumper crop.

So, for black oaks at least, the mystery remains!

White Oaks are Having Their Babies

A few white oaks of my acquaintance have been dropping acorns since about September 4th.  I have held off declaring White Oak Season until today: more than half of the white oaks I can think of are now launching babies.  Squirrels are "rescuing" a significant fraction of these on most trees; in fact, though these kids have a low chance of survival in the paws of a squirrel, they have an even lower chance on the street, sidewalk or lawn.  Of the hundreds or thousands of acorns produced by a tree this year, perhaps one will survive to its first birthday.

At the end I'll help you tell white oak from other oaks.

Here is the development of acorns on a single branch around the corner from my house.

July 5th

July 31st

August 10th

August 31st

September 2nd

September 7th

Will a squirrels get these?  or the lawnmower?  September 9th

 

September 15th

What's the Difference?
Red oaks have been dropping acorns for weeks.  How are white oaks different?  White oak leaves have rounded lobes, while leaves of red, black and scarlet oaks all have pointed lobes with bristle tips.  Both white and red oaks have acorns with shallow cups covering only the end of the nut, but only white oaks have rough, corrugated caps; the others are smoothly covered with scales.  White oaks also have "sweeter" acorns that mature in the same year, while red oak acorns mature in their second year and so are very bitter due to tannin.

Rounded leaves and warty cap of white oak (Quercus alba) above; red oak (Quercus rubra),

with its pointed and bristle-tipped lobes (below),is nearly done dropping acorns.

 White oak cap (left) is sometimes described as "warty" to the red oak's "scaly."

Soon I will be experimenting with acorns as "wild" food!

Mighty Oaks from Acorns Grow

Once every few days or week I log into Nature's Notebook (NN) to continue my contribution to citizen science.  NN is a long-term project of the National Phenology Network, in which ordinary citizens track seasonal changes in plants and animals nationwide, to provide data to scientists interested in, for example, the rate at which climate is changing, and how living things are adapting--or not.  Most of the half-dozen or so species I track in my neighborhood are trees, and big trees, at that.  So I am mildly frustrated to come to the following questions on the data sheet: Do you see pollen release?  Do you see fruits?  Do you see ripe fruits?  The fact is I seldom see any of these, mainly because they are too too far away up in the tree's canopy.

A thunderstorm last night finally wetted the dusty ground here and might perk up the wilted leaves for a day, but it had another helpful side effect: it knocked down a few twigs from a majestic but inaccessible red oak on the other side of my block. Finally I got a look at something I seldom see: baby acorns.  

 

Branchlet of red oak (Quercus rubra).  The developing acorns are invisibly small here, 

but you can make them out in the photo below.

 

Here are three or four acorns in the axils of the leaves, while buds (for comparison) are at the tip of the twig.

When I first saw these on a fallen twig a few weeks ago, I first thought they were deformed buds.  I had to stare at them awhile and finally decided they must be acorns. 

 

 The previous photo, cropped.  At the tip of each acorn, you can just make out 

the three curly stigmas left over from the female oak flower.

The acorns are curiously small so late in the season, until you realize they will not finish maturing until next summer.  

Red oak belongs to the group of oaks that takes two years to mature its acorns.  Called the "red oak group," these oaks have leaves with pointed, bristle-tipped lobes.  Around here, they include scarlet oaks and black oaks as well as red.  

 

Scarlet oak (Quercus coccinea).  Another tree, pin oak, has similar leaves 

("more hole than leaf"), but lives in wetlands.

 

Black oak (Quercus velutina) has a leaf with fewer and broader lobes than red oak,

and is less deeply-cut than scarlet oak.

 

The acorns of red oak are broad faces wearing berets.  

The acorns of the red oak group mature the second year, so these came from last year's flowers.

 

Acorns of white oak have thinner faces wearing  knit caps.  They mature the same year, and are sweeter and more readily edible than acorns of red oak, etc.