Friday, November 13, 2015

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, 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!

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