Mast Years

In many parts of the Midwestern and Eastern United States (and perhaps more of the country), Autumn 2023 has contained a bit of an oddity.

Yard owners and hikers have noticed a spate of squirrels on the prowl and a seemingly unending deluge of nuts on the ground. A member of my family in New Jersey has taken to cleaning walnuts off his driveway each day with a hockey stick. Acorns litter forest floors. Walking across green spaces can be a hazardous activity.

Why is nature going so nutty this year?

There's no need to raid the bird feeders in 2023 - photo by waferboard

The answer is a phenomenon dubbed a mast year, and it has nothing to do with ships.

Botanists call the fruit of the forest mast. Any fruit produced by trees or shrubs is considered mast, though scientists break it down into two classifications: hard and soft. Hard mast contains all the nuts we associate with trees, such as acorns, walnuts, beechnuts, and hickory nuts; things we consider wild berries comprise soft mast, such as blueberries, blackberries, and raspberries.

Why call this natural cornucopia mast? The name derives from the Old English term mæstPeople of the era used a specific word for the food from the woods because they used to unleash their domesticated swine onto the free food each fall. They called this practice pannage. In Old English, the accompanying verb – mæsten – meant “to fatten or feed.” 

The term survived as scientific jargon.

Knocking down acorns to feed pigs via pannage - illuminated manuscript by Queen Mary Master

As it turns out, trees don’t drop nuts at the same rate each year.

Trees irregularly produce bumper crops, where the mast from each individual spikes. This irregular autumn crop can occur between 2 and 12 years apart depending on the species. During a mast year, animal activity increases, humans sprain more ankles in the grass, and lawnmowers receive more dings. Why do mast years occur and why do trees not just make mast in equal tonnage yearly?

The short answer is “We don’t know.”

Of course, a few plausible hypotheses exist. The first deals with “predators.”

The point of an oak dropping an acorn is not to feed adorable rodents but to ensure the survival of the tree’s genes. As I’m currently teaching my daughter, acorns are baby trees. Each time a squirrel, a chipmunk, or a mouse munches on a nut from a tree, that seed will obviously never become a new tree. Scientists theorize that trees might produce mast years for the same reason that prime-period cicadas emerge from the ground at strange intervals. If cicadas arrived above ground each year – or even in predictable two-, three-, or four-year patterns, for example – the animals that feast on them would know exactly when to wait for easy meals. This predictability would severely limit the number of cicadas that survive to reproduce. Appearing at 13- or 17-year intervals makes it very hard for birds to time, allowing as many cicadas to survive as possible. Trees might take the same tactic. Popping off the same number of nuts each year would allow forest scavengers to easily time the mast. Vary the crops, however, and the advantage moves to the trees. Drop a low number of fruit and the animals might not show up in high numbers next year; drop a huge number of fruit and, even if the predator population is high, a certain number of nuts will likely live to sprout into new trees.

Acorns from a Kermes Oak during a mast year - photo by Hans Hillewaert

Hypothesis two deals with resource matching. The act of reproducing is energy- and nutrition-intensive. Trees require water, carbon, nitrogen, and phosphorous in abundant quantities in order to send nuts into the world. Studies have shown that trees run low on these resources after a mast year and build them up between mast years. Could the timing of mass seeding events be dependent on the ability to store enough of these resources, a process that takes multiple years?

The final line of inquiry relates to the weather. External parameters seem to affect seasonal behavior by trees. Cues such as temperatures in the spring, droughts, and frosts can have drastic impacts on how and when trees produce fruit. Does a warmer winter the year before trigger the ability to mast seed? 

Is the ultimate answer a mix of some or all of the three hypotheses? The predation angle seems logical, while the resource theory could be the method used to bamboozle the squirrels. Perhaps weather is a wildcard that might swing things one way or the other?

The arboreal jury remains out.

Tree ring sample from the 1700s - photo by Stefan Kühn

Despite the uncertain causes of the phenomenon, the ecological effects on ecosystems are a bit clearer.

Mast years have big effects on the populations of certain animals, including squirrels, mice, rats, and stoats. Even if a big seeding year helps to ensure the survival of a tree, the animals still benefit from the extra food. Their populations rise after a mast year. Conversely, populations of nut eaters decrease in intervening years.

Studies have indicated these population fluctuations have positive and negative effects on ecosystems. For example, white-footed mice increase in population when a mast year occurs, which increases the presence of Lyme disease in that ecosystem. However, the mice also consume more gypsy moths, a destroyer of forests. Ecosystems are complexly tangled webs, which is a huge reason why we should aim to conserve the species of the planet. When one goes away, the consequences can ripple in many ways.

Dendrochronological investigations display that mast years are more energy-intensive than non-mast years. Trees grow less in mast years than they do in normal situations, indicating their energy went into making a bumper crop instead of getting bigger. The energy usage does seem to benefit the trees, however. A survey of white spruce found that areas ravaged by wildfires in mast years tend to repopulate with more trees than areas where fires occur in off years.

Oddly, scientists have determined that the variability of mast years has increased over the past century. The exact cause of this increase is unknown, but links to certain global climate changes and oscillations appear to play a part. How this metamorphosis will affect the world’s forests is unclear.

If you encounter some overzealous squirrels this autumn, don’t fear some sort of animal uprising; they’re simply lining up at the all-you-can-eat buffet. A walk in the forest might require more vigilance from falling acorns, though.

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