When it comes to seed survival, trees have friends and enemies in a process known as masting

Unprecedented challenges such as wildfires, diseases, droughts, and deforestation are confronting forests worldwide. The survival of these expansive habitats hinges on seeds and their capacity to regrow.

In this endeavor, a process known as “masting” plays a pivotal role. Masting is the unpredictable boom-and-bust cycle of seed production.

For the first time, an international team of scientists has examined the intricate balance between seed defense, seed dispersal, and the process of masting at a global scale. The research, based on millions of tree-year observations, is published in the scientific journal Nature Plants.

Seeds, fruits, and nuts, which are rich in carbohydrates, fats, and proteins, are among the highest quality plant foods in nature. Study lead author Tong Qiu, an assistant professor in Penn State’s College of Agricultural Sciences, explained this dynamic.

Studying seed survival and masting

Qui said that when there is a large seed crop, consumers such as birds, squirrels, and insects thrive. This is due to the plentiful food supply. However, during years of low seed production following a masting, these animal populations may face hardship. This could lead to a population decline.

“At the same time, some seeds that aren’t eaten during the mast year might germinate and grow into new trees, and this can lead to an increase in tree numbers, affecting the forest’s overall growth and the habitat for other creatures,” said Qiu.

A comprehensive understanding of masting patterns is thus vital for forest managers to direct their conservation efforts effectively. They sometimes choose to manually plant seeds. Other times they implement measures to safeguard struggling animal populations during lean seed production years.

The researchers were intrigued by the impact of erratic seed crops. While keeping seed predators at bay, how does it affect the seed dispersers and seed survival that are necessary for successful germination.

They theorized that if unpredictable seed production, which foils a tree’s “enemies,” also negatively impacted their disperser allies and seed survival, then tree species heavily reliant on animal dispersers might have to abandon this defensive strategy.

Three critical elements of the study

“When trees have big swings in seed production, take a long time between high seed years and all produce lots of seeds at the same time, predators can be overwhelmed,” says Qiu.

“Our research revealed that masting relies on three critical aspects that affect both trees’ friends and foes, seed dispersers and seed predators.”

These three critical elements include volatility, which indicates the degree of fluctuation in seed production year over year. Periodicity, which reflects the time interval between years of high seed production. Finally, synchronicity, which denotes the common trend where many trees produce large seed crops in the same years.

Mast Inference and Forecasting network, or MASTIF

Despite the clarifying insights provided by these elements, the explanation for masting isn’t entirely straightforward. Senior author James Clark, a professor of Environmental Science at Duke University, helped to explain. He developed the Mast Inference and Forecasting network, or MASTIF.

Professor Clark suggests that tree species attracting seed predators may also depend on mammals and birds for seed dispersal.

He posits that these “friends” are so beneficial that many tree species invest extra resources into their fruits and nuts. They adorn them with colorful displays to attract these crucial mutualist dispersers.

Masting as a defense strategy

The researchers examined whether the unpredictability of seed production that deters a tree’s enemies also negatively affects their disperser allies. If masting effectively defends against enemies, then, the team confirmed, tree species heavily dependent on animal disperser species might need to relinquish this defensive strategy.

“An analysis of seed production in hundreds of tree species across five continents shows this mixed benefit of masting – the tree species that depend most on animal dispersers are the ones that avoid masting,” said Clark. 

“In the temperate forests of North America and Eurasia, oaks and firs are prolific mast species. Pines and spruces also mast, but to a lesser degree. Hickories and walnuts still less. Chestnut and the fleshy fruits of black gum, holly, hack- and sugarberry, persimmon, juniper, yew and pawpaw, hardly at all – they are reliable resources.”

Delving further into the masting behavior, Clark explained how certain tree species like firs, pines, and spruces are subjected to the feeding frenzy of birds and rodents. This happens both in the canopy and when they reach the forest floor. 

Conifer trees defend their seeds via woody, resin-coated cones, many of which are armed with spines. However, once these seeds make their way to the forest floor, they are quickly consumed by foraging rodents. With few mutualist dispersers to aid them, these tree species make ideal candidates for masting as a defense strategy.

Nutrient availability and climate gradients

The study’s findings go beyond the interaction between tree species and their animal allies and enemies. The researchers discovered that nutrient availability and climate gradients also play a significant role in masting. 

The experts found that tree species requiring substantial nutrients generally exhibit low year-to-year variations in seed production.

Simultaneously, species prevalent in nutrient-rich, warm, and wet areas tend to show shorter intervals between years of high seed production. By contrast, however, masting is more common in regions that are cold and dry.

“Interestingly, this happens in areas where weather conditions mean there is less need for animals to spread seeds, unlike in the wet and warm tropics where such help from animals is more common,” Qiu said.

“This fascinating interplay reminds us that our diverse forests are a result of countless factors working together in harmony, adapting to their unique circumstances to thrive.”

Trees with rich and colorful fruits avoid wild fluctuations

At the opposite extreme, rich, colorful fruits avoid wild fluctuations. The trees that produce them depend on their animal dispersers, Clark said. There is still plenty of year-to-year variation, because a large, expensive fruit is sensitive to moisture stress.

“A good two-week drought in mid-summer will see many trees abandoning much of their fruit crop — early abortion,” said Professor Clark.

“This includes not only fleshy fruits like persimmon, hackberry (including nettle tree in Europe), and black gum. Acorns and hickory nuts also have high moisture content. They too will abort many partially developed seeds to trim the resource demand. Still, a string of years with suitable climate conditions can see reliable crops in many of these species, one after another.”

This study was conducted with contributions from 70 institutions. The research was primarily funded by the National Science Foundation, the Belmont Forum, NASA, and France’s Programme d’Investissement d’Avenir (Make Our Planet Great Again) initiative.

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