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Stroll the sidewalks of the eastern United States this fall and you might think you’re walking on ball bearings, as a bumper crop of acorns rolls under your feet. Pine and spruce drip with cones, and rafts of maple seeds twirl toward the ground. This is a mast year, in which tree species reproduce prolifically and in sync, creating a bounty that will reverberate through the ecosystem for years.

The phenomenon mystifies and awes ecologists. “It seems like a superorganism that covers the entire continent that takes a big breath every couple of years,” says Michał Bogdziewicz, a forest ecologist at Adam Mickiewicz University in Poland.

Now, they are exploring a once-ignored idea about its trigger: that masting occurs in years when seeds are likely to have favorable weather for sprouting the next spring or even in the next year or two. It’s not that trees have crystal balls. Instead, researchers suggest trees are alert to large-scale, long-term climate patterns, which can cause, for example, wet weather one month and dry weather months or a year later.

The basic idea dates back decades, but until recently, scientists have had little luck correlating major climate oscillations with masting. Now, inspired by the potential of a large database that will be published early next year, a group of researchers has explored this idea, called the environmental prediction hypothesis, in several papers appearing in the 1 December issue of the Philosophical Transactions of the Royal Society. If climate swings are as important as this hypothesis suggests, “we could have the basis for a large-scale ‘pacemaker’ of global forest ecosystem dynamics,” says plant ecologist Andrew Hacket-Pain of the University of Liverpool, lead developer of the database. “This would be a tremendously exciting result, and we now have the data in place to test it.”

By concentrating resources, masting has powerful impacts on species, including humans. In some years, beech and spruce mast together throughout Europe. The seeds—up to 500 per square meter from beeches—dump enough organic matter to effectively double the nitrogen in the ground, fueling fungal and microbial growth. Rodent densities soar, followed within 1 year by rising numbers of predators like foxes and owls.

In upstate New York, 2018 was a big mast year. The bounty of acorns caused white-footed mice to surge in 2019—and Lyme disease–transmitting ticks to proliferate in 2020. “That sequence is something seen over and over” across the globe, says Richard Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies. In Switzerland, for example, a study published in 2020 found an 82% rise in infected ticks 2 years after a big masting year.

Researchers have tried to explain masting for decades. They have noted, for example, that by overwhelming mice, squirrels, and other seed predators, masting helps ensure that at least some seeds survive. Other scientists have suggested fluctuations in nutrient availability and even sunspot activity might trigger it. But Davide Ascoli, a forest management ecologist at the University of Turin, wondered whether large-scale climate patterns might be at work, as earlier biologists had hinted.

So he, Hacket-Pain, and colleagues compiled 17,000 records, some going back centuries, of nut production in beeches and cone production in Norway spruce, creating a database called MASTREE. Comparing the data with climate records, they found masting events in beeches coincided with climate patterns produced by the North Atlantic Oscillation (NAO), in which high and low air pressure flip-flops between the eastern United States and Europe. The seesaw affects the jet stream and weather, with the “positive” NAO phase favoring both masting and seedling growth in the eastern United States and Central and Northern Europe, as he, Giorgio Vacchiano from the University of Milan, and colleagues reported in 2017 in Nature Communications.

Tracking years of plenty

European beeches produce piles of nuts during high mast years (orange), synchronized across Northern and Central Europe. That’s also often when the positive phase of the North Atlantic Oscillation (NAO) brings warm, wet winters and dry springs to the region. The opposite climate pattern often leads to masting failures (teal), suggesting the decadal trend in masting (wavy line) is led by this climate oscillation.

Chart of nut producing during high mast years
(GRAPHIC) K. FRANKLIN/SCIENCE; (DATA) D. ASCOLI ET AL., PHIL. TRANS. R. SOC. B, 376, 20200380 (2021)

Trees have good reason to track major climate oscillations, Ascoli argues. For example, in the NAO+ phase, warm wet winters favor seed production and dry springs favor seedling growth. In North America, conditions fostered by the El Niño-Southern Oscillation (ENSO) can promote both masting in white spruce and forest fires, which open up space for seedlings to grow. For the special issue, the team tracked down about 40 studies that support the environment prediction hypothesis to some degree. Their synthesis suggests the NAO and ENSO help synchronize masting in temperate and tropical regions, says Akiko Satake, a mathematical biologist at Kyushu University, who was not part of the work.

Satake saw a link between climate patterns and reproduction in her own study in Southeast Asia. Common timber trees in the family Dipterocarpaceae in Malaysia appear to respond to the weather created by ENSO, with several species responding simultaneously in what’s called general flowering. Satake and colleagues compared weather data with 14 years of data on flowering and seedling growth for five species. El Niño events brought cool, dry weather in January and February, which prompted general flowering in spring, followed by wet fall weather that encouraged seedling survival in those species, the researchers reported last year in BioTROPICA. The ENSO is “driving flowering cues through space and time,” Satake says.

The researchers make a good evolutionary argument, adds Gabriela Marie Garcia, a graduate student at Tufts University. Because the climate modes align conditions for seed germination and flowering, natural selection could favor individuals that flower in sync with those modes.

However, the environmental prediction hypothesis doesn’t explain all masting, cautions Mario Pesendorfer, an ecologist at the University of Natural Resources and Life Sciences, Vienna. Another idea—that a burst of nuts uses up so many resources that it takes a tree more than 1 year to restore them—can also help explain why masting is erratic, says Tufts ecologist Elizabeth Crone, whose work backs that idea. And David Kelly, an ecologist at the University of Canterbury who studies masting in alpine grasses, says environmental prediction is “an appealing idea that doesn’t work.”

He says masting in many species doesn’t match the big climatic patterns. He agrees selection could, in principle, favor plants that made accurate predictions, but he points out that even the best supercomputer can’t forecast weather months ahead. “There’s nothing for selection to work on if predictions are just not possible,” he says.

Instead, he has another hypothesis related to weather. He works in New Zealand, where southern beech trees turn hillsides red with blossoms during masting events and produce a bounty of nuts. The next spring, introduced mice multiply; later, so do the introduced stoats that prey on them. Once the beech nuts are gone, the mice dwindle, and conservationists have to poison the stoats so they don’t kill native birds.

Kelly thinks summer temperatures orchestrate this ecological dance. He suggests they temporarily modify a plant’s DNA—a process called epigenetics—such that the plant “remembers” how hot the weather was the previous summer. For some reason, masting occurs when the second summer is much warmer than the previous summer, he says. He’s transferring alpine grasses to different elevations and therefore temperatures to test his idea. Already, he has shown that temperature can predict when beech masting will occur and stoat control will be needed.

More data on the global waves of tree reproduction could help winnow the ideas. At Duke University, researchers have developed the Mast Inference and Prediction database, which compiles annual masting data from research plots, most of them in the United States. And Hacket-Pain and Ascoli’s team is building a new database, MASTREE+, with 65,000 records on 715 species across 63 countries. To be published in Global Change Biology, it’s “the most comprehensive plant reproduction database that exists,” says Jalene LaMontagne, a population ecologist at DePaul University. Researchers are using it to see how species vary in the intensity and geographic extent of their masting and whether climate change is breaking down masting patterns. 

Meanwhile, don’t slip on the acorns.

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