How Oaks and Beeches Control Their Predators

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During a mast year, squirrels find more acorns than they can eat. Ill.: http://www.sccpre.cat, montage: laidbackgardener.blog

Squirrels, jays and other nut-eating animals like nothing better than fattening up on acorns, chestnuts and beechnuts in preparation for winter. They’re rich in oils, protein and carbohydrates, as well as calcium, phosphorus, potassium and the vitamin niacin. Not only do nut-munchers stuff themselves on the nuts, they bury countless others for future eating… then either forget to dig them back up or finally don’t use them, as they typically hide far more than they’d need when the harvest is good. Thus, they actually plant future beech, chestnut and oak forests, bending to the will of the trees.

Population Control

And oaks, chestnuts and beeches are even craftier than you might think. If they produced equal numbers of nuts every year, the squirrel and jay population would remain high and they’d consume most of the fallen nuts. Instead, they have “mast* years”: years in which they produce huge quantities of nuts, far more than their predators could possibly eat, followed by several to many years of slim pickings. This keeps the squirrel and jay population relatively low. 

*Mast: name for the fruit of beeches, oaks, chestnuts and other woodland trees.

How? When a mast year occurs and thus food is plentiful, there aren’t that many nut predators around, as they’ve gone through several low-nut winters in a row. The winter and summer following a mast year, since food is plentiful, many more predators survive the winter and the forests fill with squirrels and jays. But the following winter, after a skimpy harvest when there’s little to eat, fewer squirrels and jays survive. And several years in a row of low nut production really keep the population in check. Mast years occur every 3 to 15 years, depending on factors yet unknown.

And the off years, when few nuts are produced, allow the nut trees to store up more energy for the next mass year.

Coordination Is the Key

Of course, this wouldn’t work if each oak, chestnut and beech in the forest were on its own schedule and thus there were always trees having a mast year each fall. Instead, the trees somehow collaborate locally. All the nut trees in a given region will have their mast year all at once, followed by several coordinated years of low production. 

It’s possible trees communicate with each other to coordinate a mast year. Ill.: thekidshouldseethis.com

How do the trees coordinate their mast years? That remains a mystery. Perhaps there is a climatic signal of some kind that indicates that a certain year would be a good one to hold a mast year. Others theorize that the trees actually communicate with each other, not in words, but by sending signals from tree to tree via their roots. (“Hey, guys! We’ve been taking it easy for the last six seasons, storing up our energy. Why don’t we make this year a mast year!”)

Whatever the reason, mast years have been recognized by foresters (and nut harvesters) for generations, but it’s only recently that the reason behind them – over- then under-producing to control predators – has been studied. 

Ain’t nature wonderful!

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2 thoughts on “How Oaks and Beeches Control Their Predators

  1. Many species do this. Some species of Yucca sort of do it with their bloom, but it is probably not as effective, or as intentional. (It is probably caused by weather.) They still rely on the same yucca moth for pollination. (each species of Yucca has its own species of moth.) Fewer moths mean less pollination. Each fruit still contains larvae that eat almost all of the seed. It makes no sense to me, but I figure that the Yuccas know what they are doing.

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