Why Did the Squirrels Go Nuts?

The year 2019 was another masting year for oak trees in the New England region. This picture of acorns was taken in the spring of 2020. The combination of a large acorn crop and smaller squirrel population made for an overabundance of acorns. Photos courtesy of the author.

It was the fall of 2018, and I was shooting a round of golf with my brother-in-law. He’s a letter carrier for the U.S. Postal Service and therefore does a lot of driving. He mentioned to me that he thought it was going to be a very bad winter. He based his conclusion on the fact that he was seeing squirrels everywhere on his postal route, and they seemed to be extra busy collecting food.

I think anyone who has driven a car and played “dodge the squirrel” could vouch for his observation about the squirrels’ heightened activity that fall. Perhaps they did know it was going to be a bad winter and were trying to commit suicide. Why else would they zig when you zig and zag when you zag? At that time, we did not know it was going to be the year of “squirrelmageddon.”

While my brother-in-law’s observation was accurate, the conclusion he drew might have been a little misguided. The events that caused squirrels to be so active that year began a couple of years earlier, and all started with the mighty oak tree. You see, trees may not be as slow witted as we once thought. Trees, and plants in general, all have one major disadvantage when it comes to competing with animals – they cannot pack their bags and move when trouble comes their way! If someone tries to eat them, they cannot run or hide. If the weather turns cold and snowy, they cannot fly to Florida. To overcome this basic lack of mobility, they have evolved some interesting strategies for survival.

So, how has the mighty oak managed to make squirrels look like they’ve been munching on coffee beans from the dumpster at Starbucks? Oaks have developed an interesting method of producing acorns called “masting.” The word “mast” is a noun of Anglo-Saxon origin that refers to the accumulation of nuts on the forest floor. When oak trees are masting, it means all the oak trees within a region are producing huge quantities of acorns. It seems 2015, 2016 and 2017 were all mast years. What advantage does masting offer? If all oaks produce huge quantities of acorns, then squirrels, deer, mice, turkeys and others that feed on acorns cannot possibly eat them all, and therefore some acorns will survive to become oak trees. By definition, a single tree cannot mast, it is a group activity and all are asked to participate.

This picture was taken in the late summer of 2019. The abundance of oak seedlings on the forest floor clearly illustrates the effective strategy of masting.

So why not produce large amounts of acorns every year? If oaks had large acorn crops every year, then the acorn-eaters’ populations would increase to match the acorn crop. By randomly producing large crops of acorns, predators are kept out of sync with acorn production. Since the previous two years were very good for acorns, they were also very good years for squirrels. With a large acorn crop, winter mortality is diminished and baby-squirrel survival in the spring is higher. The end result is a larger squirrel population.

In 2018, there were very few acorns but many squirrels and other animals that were depending on a large acorn crop. That is why they were so active. After the salad days of 2015, 2016 and 2017, there were lots of squirrels with virtually no food source to help them overwinter.

Besides driving squirrels nuts, there are other important ecological implications of masting. For example, mice and deer eat acorns. Mice and deer are also important carriers of Lyme disease. If there are more deer and mice, then we might expect an uptick (pun intended) in Lyme-disease cases. Mice also eat the eggs of gypsy moths, an important defoliator of Eastern forest. More mice, fewer gypsy moths – fewer mice, more gypsy moths. Mice also prey on bird nests, especially ground nests. Songbird populations can fluctuate wildly depending on the mouse population.

Scientists have been trying to figure out exactly what causes oak trees to all produce huge amounts of acorns at the same time, but to no avail. Weather has always been a suspect, but favorable growing conditions do not translate into “good” acorn years. Instead, certain other environmental conditions may trigger the oaks to flower profusely in the spring and thereby set a large crop of acorns. Whether it is a good growing season does not seem to matter. If you think back to last year, it was a very hot and dry season with very poor growing conditions, yet the acorn crop was gigantic. In California, the oak trees seem to be showing some correlation between masting and the weather phenomenon known as El Niño. In this case, the temperature when the trees are in flower seems to play an important role.

It’s easy to overlook just how special trees are and what they are up against, since we are surrounded by and work with them every day. We may curse the rain of acorns as they fall and dent our cars and fill our gutters to the brim. But remember, those trees are in a life-and-death struggle to survive, and they have managed to come up with some very interesting and complicated strategies to do so.

As the oak tree has taught us, it’s better to leave ’em guessing sometimes.

Edward Roy is an arborist with Urban Tree Service/A Tree Health Co., Inc., an accredited, 28-year TCIA member company based in Rochester, New Hampshire.

TCI will pay $100 for published “From the Field” articles. Submissions become the property of TCI and are subject to editing for grammar, style and length. Entries must include the name of a company and a contact person. Send to: Tree Care Industry Magazine, 670 N. Commercial Street, Suite 201, Manchester, NH, 03101, or editor@tcia.org.

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