Why Do Trees Drop So Many Seeds One Year, and Then Hardly Any the Next? (2024)

Why Do Trees Drop So Many Seeds One Year, and Then Hardly Any the Next? (1)

A mighty oak covers the ground in piles of acorns. Squirrels gather them up, growing fat on the rich bounty and storing more of the seeds away for the winter.

If you live in the temperate Northern Hemisphere, this may be one of your most familiar natural scenes of autumn, because various species of both oak trees and squirrels are common and visible throughout that range. It represents a simple predator-prey model that ecologists have studied since the early days of their science. But those early scientists—and, long before them, Indigenous people—noticed something unusual about this scene. Some years, a glut of acorns fell, creating a squirrel’s paradise under every tree. Other years, almost none did.

Plants dropping most of their seeds together in one year, then taking years almost or completely off from seed production, is called “seed masting.” Oak trees are one example, but thousands of species of trees and other long-lived plants use this boom-and-bust strategy. The most common explanation has involved those hungry squirrels, birds and countless other species that eat acorns. Drop enough seeds at once, the theory says, and some will survive the predators’ feast. Ecologists call this the “predator satiation hypothesis,” and it has been a widely accepted explanation for seed masting for decades.

Why Do Trees Drop So Many Seeds One Year, and Then Hardly Any the Next? (2)

But predator satiation is far from the only theory. Another idea suggests masting helps insects like bees most efficiently pollinate a plant. If all of the trees of one species flower and set seed at once, that theory goes, bees or other pollinators have better odds of bringing pollen directly from one tree to another. But what if something less obvious and visible than either of these theories helped explain this phenomenon? What if the force driving it was something much smaller than a squirrel, or even a bee?

Researchers in Canada published a paper this past February in Current Biology proposing a new hypothesis for the evolution of seed masting: disease. While acorns are being gobbled up from above by hungry squirrels, they are also being attacked from below, and within, by fungi, bacteria and other pathogens. Scientists have understood for a long time that these agents can kill large numbers of seeds, but their role in determining the timing of seed release has been largely ignored. But some scientists wondered whether masting trees could drop fewer seeds in some years to break cycles of disease, rather than just to overwhelm predators in high years.

“Look at what farmers do,” says Jonathan Davies, a botanist and forest conservation scientist at the University of British Columbia, and one of the authors of the recent paper. “They often let the fields lie fallow, and that clears the pests and pathogens. You remove the crop for two, three or four years. It clears pathogens and pests from that field, and you can plant again.”

The idea that disease could play an important role was born, as many ideas are, not in a formal lab but in a casual conversation. Davies was talking to plant community ecologist Janneke Hille Ris Lambers of ETH Zurich about how variable the seed production was on the trees she studied in Washington state. The concept of pathogens as a driver came up, and Davies assumed that someone would have looked at that possibility before. But when he searched for references in journal databases, he was surprised to find an empty results screen.

“There was literally nothing in the literature about it,” says Davies.

Collecting data to support a theory like this would take decades, because of the time scales that govern tree reproduction. But before the pathogen escape hypothesis could be tested in the field, a solid foundation would have to be built, to make sure it worked even in theory. To start that process, the paper’s other author, math professor Ailene MacPherson of Simon Fraser University, came in and did what mathematical biologists do: She built a model.

The basic units of ecological theory are models, simplified representations of natural relationships that are expressed using math. Ecological models can be extremely complex, accounting for multiple species, environmental conditions and other variables. Since they were starting from a clean slate in terms of past research on the subject, MacPherson chose to use mathematical models that were as simple as possible.

“The idea was not to build the most robust models ever,” says MacPherson about the paper’s math, which she sees as a starting point and hopefully a launchpad for other researchers. “Our models are very much focused on illustrating that there might be a reason to study this.”

The closest thing to a pathogen model for seed masting in the literature was a 1992 study that looked at parasites. The paper used a version of a standard predator-prey model, like the squirrel and acorn, with basically two moving parts: seed and parasite. To adapt it for the new hypothesis, MacPherson considered two different ways that pathogens can spread: direct and environmental. Direct transmission spreads from one host to another. Environmental transmission can involve another step, either an intermediate host or another sort of reservoir where a pathogen can live between infections. A classic example is the bacterium that causes plague, which can be carried by rodents and then transmitted to humans through fleas.

Whichever method the pathogen uses, direct or environmental, there are two kinds of hosts to consider in a model: the already infected, and the susceptible, or not yet infected. According to MacPherson’s models, seed masting creates many susceptible seeds at once. In slow seeding years, the number of susceptible seeds can be so low that it could starve the next epidemic of hosts, cutting it off before it begins.

Why Do Trees Drop So Many Seeds One Year, and Then Hardly Any the Next? (3)

Now that the first steps of the theory are in place, Davies and MacPherson hope that other researchers can take the next steps, using more complex models and testing the theory against data in the field. One scientist who might incorporate some aspects of the theory into her work is the ecologist whose conversation with Davies sparked the idea in the first place.

Hille Ris Lambers has been studying trees and their population dynamics in Mount Rainier National Park since 2007. That data set has only recently gotten long enough, 16 years and counting, to start looking at masting patterns and, potentially, their relationship with disease. She finds the recent paper a promising start.

“I thought it was really nicely written and convincing to me that, yes, this is something that we’ve ignored as a potential long-term driver of some of these dynamics,” she says.

Rather than unseating predator satiation or pollinator efficiency as a leading theory, pathogen escape may just add to a mixture of drivers that all work together to push plant species toward masting.

“The reality is, there’s probably no one explanation,” says Davies. “This is probably going to be part of the explanation when we put this puzzle together.”

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Why Do Trees Drop So Many Seeds One Year, and Then Hardly Any the Next? (2024)

FAQs

Why Do Trees Drop So Many Seeds One Year, and Then Hardly Any the Next? ›

The most common explanation has involved those hungry squirrels, birds and countless other species that eat acorns. Drop enough seeds at once, the theory says, and some will survive the predators' feast.

Why is my tree dropping so many seeds? ›

One possibility is that, high costs and multiple risks notwithstanding, trees need to produce seed to pass along their genes and perpetuate their kind. This is the biological imperative. It is so important that heavy seed production may be triggered in some trees as a reaction to significant, threatening stress.

Why are trees producing so many seeds this year? ›

In recent years, some maple trees dropped seeds in overwhelming numbers potentially due to stress from the drought. "There's a tendency to just flower like crazy and produce seeds like crazy. Plants just want to live, so it's just to keep that next generation going," he said.

Do dying trees produce more seeds? ›

It was once believed that prolific tree seed/fruit production is connected to tree stress. The theory was that heavy seed production occurred on stressed or dying trees as a last hurrah in support of the species. However, research has failed to provide consistent support for this speculative conjecture.

Do trees produce more seeds when stressed? ›

These stressed trees will also often have stunted shoots and may produce more seeds than typically seen for a tree. Conifers will often produce an abundance of cones the second year of a drought. Trees that are stressed by drought are also more susceptible to insect borers and canker diseases.

How to stop trees from dropping seeds? ›

Debris reduction pruning is conducted from January through March. It involves pruning your trees to minimize the blooms and prevent subsequent seeds or seed pods from growing. This means that your trees will not produce blooms (or not as many), and therefore few or no seeds or seed pods will grow.

Why is my maple tree dropping so many seeds? ›

After periods during which maple trees experience stress, for example, drought or a harsh winter, they can grow a greater number of seed pods, or samaras. This spring, I began to worry about my several large maple trees throughout our yard.

Why are so many trees dying this year? ›

California's forests continue to see mass die-offs of trees as a result of too little water, insect infestation and disease. But last year's wet winter provided at least some relief, new federal data shows. Roughly 28.8 million trees died statewide in 2023, according to the U.S. Forest Service's annual aerial survey.

Is the U.S. planting more trees? ›

WASHINGTON, July 25, 2022 — Today, Agriculture Secretary Tom Vilsack announced a strategy for how the Biden-Harris Administration, through the U.S. Department of Agriculture, will address a reforestation backlog of four million acres on national forests and plant more than one billion trees over the next decade.

How do you stop plants from producing seeds? ›

Pinch off flower buds: If you notice flower buds starting to form, pinch them off as soon as possible. This will help prevent the plant from shifting its energy to producing flowers and seeds.

What speeds up tree growth? ›

Make Sure You're Watering Enough: Making sure that you are providing your tree with the water it needs is essential for growth. Supplemental water at least 1-2 times a week is important, especially at seasons of minimum rainfall. You also want to make sure that you are not overwatering your tree.

Will fertilizer make trees grow faster? ›

Actually, by pushing too much fertilizer, you are forcing your tree to focus on top growth, not a strong, healthy, supportive root system. By mulching trees and giving them supplemental water when needed, you will be giving them a big advantage, creating healthier trees that grow faster.

What triggers a tree to begin growing again? ›

There are two ways trees know when to wake up for spring. First, they respond to noticeably warmer days after a stretch of cold temperatures in winter. At the same time, they react to a change in light duration, when shorter nights and longer days of sun exposure, spur new growth and development.

Why is my tree dropping so many seeds this year? ›

The most common explanation has involved those hungry squirrels, birds and countless other species that eat acorns. Drop enough seeds at once, the theory says, and some will survive the predators' feast.

Does pruning a tree stress it? ›

“At other times of the year, such as leaf out, leaf drop or during flowering, pruning can have a very negative impact on plant health.” Plants expend a lot of energy during those stages, and pruning can place undue stress on trees and shrubs because it causes so much of that vital energy to be lost, he said.

Do female trees drop seeds? ›

Female flowers and female trees produce fruit and seeds. Male flowers and trees produce pollen. Making the correct choice of tree gender can be important. Anyone who has ever smelled putrid ginkgo fruit, washed mulberries off their car or sneezed at tree pollen should understand.

Why does my plant have so many seeds? ›

Plants produce so many seeds so that enough of them will grow into mature plants to ensure survival of the species.

How do you stop new sprouts at the bottom of a tree? ›

You can remove the sprouts with a pruning saw or sharp pruning shears. Cut as close to the base as possible without damaging the trunk or main branches when controlling tree sprouting with pruning.

Do male or female trees drop seeds? ›

Female flowers and female trees produce fruit and seeds. Male flowers and trees produce pollen.

What tree drops seed balls? ›

The dried spiky balls from sweetgum trees that fall to the ground develop holes that reveal seeds attracting dozens of birds, butterflies, and animals.

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