Why Are There So Many Acorns in 2023? Trees Are ‘Masting’ and Scientists Aren’t Sure Why

This fall, a black walnut tree has become my nemesis.

The tree had largely escaped my notice all summer, even though I pass under it routinely on my daily runs around Washington, DC. But then, in October, I nearly got hit on the head by one of the tree’s lime-green, fist-sized pods that shot down just inches in front of my face, before landing with a loud thud. 

In the weeks since, I have approached the tree with new trepidation, shielding my head from an unpredictable onslaught from above, while also trying not to sprain an ankle by slipping on the pile of seeds around the tree.

If you live in the U.S., then my experience likely sounds familiar. Across large swaths of the United States, many nut-producing trees — including oak, beech and walnut — are collectively going gangbusters on seed production this year, unleashing a hailstorm of acorns, hickory nuts and other (surprisingly hard!) seeds. During boom times like these, a single oak tree can shed more than a thousand acorns, despite producing barely a trickle in previous years.

Scientists call this masting, which means the synchronized and highly variable production of seeds. Scores of different tree species mast, but how and why thousands of individual plants across thousands of square miles apparently coordinate their reproduction in this way has perplexed scientists and nature observers for centuries.

“It’s a population-level phenomenon, which makes it really interesting because evolution is usually thought to act on the individual,” said Walt Koenig, a behavioral ecologist at Cornell University who studies masting in oak trees.

The strategy has a few clear benefits, including overwhelming predators who eat seeds, which might explain why it evolved. But how trees actually pull off this feat largely remains a mystery. 

Weather conditions likely play a role in triggering masting, though different species exhibit different masting cycles and put out seeds based on differing conditions. Still, the association between weather and masting has sparked folklore, including the theory that a big acorn haul means a harsh winter.

Why so many trees are masting this year is also not clear, but Koenig advises against looking to the trees to forecast the seasons. 

“As I always say, if acorns predicted the weather, I’d be a rich man today,” he said. 

Why Do Trees Mast?

A tree’s goal, evolutionarily speaking, is to transform sunlight, water, earth and air into vessels — seeds and nuts — that will propagate their genes into the next generation. Squirrels, birds and other seed-eating creatures can thwart this goal by gobbling up those vessels. 

Trees can’t physically fight back, but they have other ways of outsmarting their predators.

By releasing seeds in boom-or-bust cycles, trees can overwhelm predators through sheer numbers, pumping out more seeds than squirrels and other critters could possibly eat to ensure some eventually sprout. “This helps trees escape seed consumption,” said Michał Bogdziewicz, an ecologist at Adam Mickiewicz University in Poland. 

The intervening years “starve trees’ enemies,” said Bogdziewicz, by producing very few seeds, or none at all. The down years buy time for the tree to build up the resources it needs for another round of masting. 

“Then, once massive reproduction happens there are few consumers around, and these few are easily satiated. Plants cannot run away from seed predators, so they basically kill them first.”

This idea makes theoretical sense to most biologists, and many studies support it, especially in more temperate climates.

Masting may also help trees with mating, especially species that just depend on wind to carry pollen. “Pollination efficiency can be pretty darn low when you’re relying on the wind,” said Koenig. Species that flower all at once saturate the air with pollen, “and everyone does better,” he said. Months later, the resulting seeds all get shed at once.

What Weather Has To Do with It

Masting is likely a form of environmental prediction for some species.

Trees might be placing bets, so to speak, based on certain environmental cues that are associated with conditions that will be most favorable to their offspring. What sorts of environmental conditions portend future success often varies from species to species and region to region.

White spruce trees, for instance, tend to mast after hot and dry summers, which are good predictors of fires the subsequent year, said Bogdziewicz. “Post-fire areas are a great, competition-free place to start a new generation for this species.” 

Several other species follow similar fire-chasing patterns.

The environmental cues for some species can get quite complicated. In valley oaks, a cold spring, followed by dry conditions the subsequent winter and spring, often sparks masting in this Californian tree. Other oak species seem to like warmer springs.

Piecing together the bigger picture for all masting species has been tough, said Koenig. “Even though there are all these correlations with weather, not too many people have figured out what it is about the weather that the trees care about.” 

That’s in part because understanding how trees pull off this mass coordination has been a tough nut to crack. 

One idea is that trees are all simply “listening” for the same sort of environmental cues. When a given area experiences the right conditions, mass coordination follows because all the individual trees are experiencing roughly the same conditions.

Another intriguing idea is that the trees are talking to each other.

“We know that trees can signal to each other when there are predators around,” said Emily Moran, an ecologist at the University of California Merced. “So that’s a possibility, though there’s not much evidence that that’s going on when it comes to reproduction.”

Can Climate Change Affect Masting

Since masting seems intimately tied to weather, scientists are racing to understand how climate change could upend traditional masting cycles.

“It’s a really important question,” said Bogdziewicz, one whose answer will vary a lot from species to species.

 In regions where springs are getting warmer, species that rely on cold springs to mast could be in trouble. Or, species that mast after warm springs or summers could be induced to do it more frequently, ultimately diminishing the overall seed output.

Worryingly, that seems to be happening for the European beech trees that Bogdziewicz studies. 

“Increasingly hot summers are triggering reproduction too frequently, which is leading to a masting breakdown,” he said. The trees are putting out seeds too frequently, upending the normal pattern that kept predation rates lower by “starving out” seed eaters. In turn, the overall reproductive success of these beech trees is declining, he said.

“We need to understand how widespread impacts like these will be,” Bogdziewicz said. 

“What species are sensitive? What are the consequences of masting breakdown for the regeneration potential of forests,” he said. “I am worried that these problems are not just local.”