On Evolution and Acorns
Why oak trees periodically produce bumper crops
Covid-19 days are, as mentioned in my previous article, a time of outdoors, social distancing — and discourse. After dealing with the mathematics of tennis matches, we helped the family shovel acorns that covered the ground, around a giant oak tree. And doing this, we discussed a little science problem: why do oak trees sometimes produce such an immense quantity of acorns — ten times as many as they normally do? And why does this happen every sixth year (here in northern Europe)?
The reason that is usually given for the periodic profusion of acorns is weather. A cool, wet spring, or a dry summer, or winds, can affect pollination and acorn maturation. But these factors cannot account for the enormous increase in yield, and for the fact that this seems to occur in regular periodicity, every three to six years (depending on region), independent of weather. And rainfall and temperature fluctuations are usually much too small to explain the massive increase in acorn crop size.
The latest studies suggest it is most likely due to evolution. Acorns are nuts that are eaten by squirrels, mice and birds. The advantage to the oak trees is that these animals disperse the acorns. Squirrels in particular will bury large numbers of acorns in the ground, as storage for winter, and in the process provide for ideal oak proliferation — by forgetting some of the nuts. They practically plant new trees in remote locations. Birds also do the same. I have described how I bonded with an acorn-burying raven crow here.
But why are there “mast years” during which inordinate amounts of acorns are produced? I asked the lads to speculate what the reason could be. I had just told them about evolution — Enders said he had heard the word before, but didn’t know exactly what it meant. It was all new, and they didn’t come close to a plausible explanation. I predict that you, dear reader, will fail to do so as well.
In normal years squirrels and other frugivores will bury acorns and later dig them up and eat them. Only, a few will be overlooked or forgotten. That defeats the purpose of wide proliferation. So periodically the oak will stage a mast year, producing so many acorns that the birds and squirrels cannot possibly eat them all. But, driven by their instincts, they will bury as many as possible. And there will be a bumper crop of oak saplings.
Now comes a decisive question: why don’t the oaks do this every year? Well, on the one hand it costs a lot of energy — in the intervening years the trees need to recuperate. But there is another, more important reason.
In mast years, due to food abundance, the population of acorn-eating “predators” increases rapidly. If this happened every year their population would explode, and most of the abundant acorns would be eaten. So from an evolutionary point of view it is beneficial to interspace bounty years with years of want. As food becomes scarce, when the oaks produce little to no fruit, the wildlife population decreases. Essentially the trees are starving the predators to keep their population in check. Classical evolutionary strategy.
I should end here, but I cannot resist sharing a little story with you. There is a beautiful bird called the Acorn Woodpecker. It lives in small colonies and is best known for hoarding acorns. It drills holes in dead trees and stores acorns in these holes, to be eaten during winter. You can find storage trees with tens of thousands of holes, drilled by generations of woodpeckers.
In 2009, in Bear Creek, Central California, a problem arose with a microwave antenna. Workers sent to fix the malfunction saw that the antenna cover was bulging. When they removed it, this is what happened:
Out poured over 300 pounds of acorns, and a technician filmed it happen! The video published at the time blamed squirrels for filling the antenna, but in the meantime the blame has switched to acorn woodpeckers, which are more likely to store nuts in unusual elevated caverns. Many a street or traffic light has been switched off by these industrious little birds.