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Good year for black oak acorns

Several weeks ago, Tom Hopkins invited me over to see the phenomenal number of acorns dropping from the black oaks around his house. Such a ridiculously abundant crop of nuts, as Walter Koenig of the University of California, Berkeley, has described it, is a “mast crop.” For black oaks, this is a “mast year.”

The question that has long intrigued ecologists is: Why?

Hundreds of millions of years ago, trees reproduced, just as ferns and mosses do today, with spores and a few cells and a smidgen of food, locked within a tough protein coat.



The success of the tree ferns and other spore-bearing trees can be judged by the prodigious amount of coal these tree trunks have become today, but very few such trees are still alive.

The evolution of seed plants presaged their doom, because seeds make reproduction a much more reliable event.




Rather than just a few cells, the seed contains a tiny embryo, a baby plant with roots and shoots ready to grow and collect light, water and nutrients as soon as the seed germinates.

And, rather than just a smidgen of food, the parent plant betters the babe’s chances of survival by filling the seed with a generous portion on nutrients.

It’s the seed’s food-store that makes wheat (or rice or corn) the “staff of life” and our major source of nutrition.

Many other animals like to also eat seeds … which means that most seeds never get to grow.

Some plants species have responded by poisoning their seeds, adding acids (hot!), alkaloids (bitter, mind-bending, toxic), or anti-digestants.

Among the last are tannins, which bind to proteins, making food indigestible and making skin, such as that which lines the digestive tract, leathery (whence the name). It’s the tannin in tea that makes one’s mouth pucker.

Acorns are full of tannins. Before eating acorn people must first leach out the tannin. Other animals have evolved biochemistries to neutralize the compounds.

Deer and squirrels, birds and weevils (whose presence is betrayed by a tell-tale hole in the seed coat) take such a heavy toll on the acorn crop, it’s a wonder any seed survives to grow into an oak.

ally, not many do. And herein, some believe, lies the clue to what has driven the evolution of masting.

Walt Koenig has found that blue oaks are in-synch throughout California. Virtually all trees mast, or don’t, at the same time. Other trees are synchronized over large parts of the continent.

The size of an animal population is closely tied to the size of its food base. If, year after year, trees put out small crops of nuts, the population of seed-eaters stays small.

If, in just one year, the trees suddenly put out a large crop of nuts, the small population of seed-eaters won’t be able to eat them all and some seeds will escape predation to germinate and grow into oaks. Such “predator saturation” is a favored explanation for what has selected for masting behavior, not just in oaks, but in many species of trees.

But what’s the signal that cues the trees to mast? A warm spring can result in more successful pollination, but a mast crop is not a simple response to the weather and, usually, something else must be involved. Just what that “something else” is remains to be discovered.

As Koenig puts it, “No fool-proof predictors [of future masting] have been discovered so far.”

Alan Stahler trained as a biologist and is an amateur astronomer. He teaches enrichment classes for children and adults at Sierra Friends Center. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).


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