Why Do Oak Trees Produce More Acorns Some Years but Not Others?

Many animals rely on acorns and other nuts that fall to the ground to survive the winter. liz west [CC BY 2.0]/Flickr

If you have lots of oak trees in your area, you know there can be good years for acorns and bad years. What's strange is that when a bumper crop occurs in one part of the U.S., the opposite can happen in other areas.

For example, 2015 was a bumper crop year in Georgia, but not far away in North Carolina and Tennessee, the fall crop of acorns, walnuts and other food that bears gorge on before heading to their dens to hibernate was noticeably lower than normal, according to biologists.

What's up with feast or famine nut-producing trees? Why would someone in one area see an unusually heavy mast — that's the fruit and seed volume of forest trees — while someone else in the same general region might not?

When it comes to the boom-and-bust cycles of nut production, blame Mother Nature and not the trees, said Kim Coder, a professor of tree biology and health care in the Warnell School of Forestry and Natural Resources at the University of Georgia. "These cycles are weather-driven. They are also very localized."

The primary weather factors influencing nut production are spring frosts, summer droughts and fall rains, Coder said. And the most important of these are spring frosts.

Trees have what Coder calls "inside timers" that tell them to do different things at different times, such as when to flower and hold fruit. Just how many acorns any single tree produces depends on many things. "Some trees produce some acorns every year," Coder said, "while others almost never produce acorns every year." Still others, he said, will always have a good acorn crop if the weather cooperates.

Harvest time

Pile of acorns on the ground
The weather, not the tree, is the main reason behind a big acorn harvest. Naoto Shinkai/Shutterstock

A common term for those good years is "a big mast year." However, Coder urges people who observe nature to be careful in using the term. While it means a bountiful harvest of tree seeds and fruit, he stressed that it's important that people understand two things about what causes a big mast. One is that the weather, not the tree, is the driving force behind the harvest. The second is that the same harvest is not necessarily happening in equal proportions across a large blanket of the landscape.

Here's Coder's take on how nature doesn't play fairly from one year to the next. It begins with the oak tree's flowers:

"Most people have never seen the female flowers that produce acorns," he said. "What they tend to see are the male catkins. The female flowers on oak and walnut trees are itty-bitty."

The internal timers tell the trees to open their buds in the spring after the danger of frost has passed. Once the buds flower, the blooms are open for only a week, during which time they are pollinated by the wind. However, a late frost when the flowers are open stops the flowering process. If that happens, the results show up in the fall with greatly limited nut production regardless of what happens with the weather in the summer and autumn.

On the one hand, even if there's a good spring fruit set, summer droughts can cause acorn fungal problems that can limit production. On the other hand, significant rain during the fall can get the trees ready for a great flowering next spring, Coder said. This is an example of how nut trees are one year behind in the climate process that affects how much mast they produce, he said.

Localized nut production

Acorns still on a tree
Each area's nut production will be a little different depending on its particular conditions. Cimermane/Shutterstock

Micro-climates also affect nut production. "In many localized areas, conditions have been great for nut production this year," Coder said. What's happening in North Georgia is an example of what he means. "From Atlanta up the Chattahoochee River Valley to Rabun County in far North Georgia some hillsides have been covered with nuts," Coder said. "Others have not been. It's very localized."

Each valley can be a little different, he said. "The places that were protected from spring frosts are getting lots of acorns and walnuts now. The ridges and passes are where we are seeing the most variability. The bears are coming down into the valleys to search for food because back in the spring the trees that are higher up didn't set fruit."

The same localized conditions apply to your neighborhood and to the neighborhoods nearby, Coder said, but probably wouldn't apply to a place 100 miles from where you live.

Variations in the amount of mast in the fall has repercussions far beyond whether there are so many acorns that cover the sidewalk that you feel like you're slipping on marbles when you walk your dog. In a year when there's a large volume of mast or the year after, depending upon animal reproduction cycles, there can be surges in the populations of rodents such as mice, small mammals such as squirrels and chipmunks, larger mammals such as deer and bears as well as game birds such as turkeys. The impact of these population increases can even affect humans. An increase in the number of mice and deer, for example, can lead to an increase in ticks, which can cause a spike in Lyme disease.

On the other hand, when acorn production is low the news isn't all bad. "That's a bust for pests," said Coder.