Discovery Files

For larger, older trees, it’s all downhill from here

As trees grow, does their seed production, too?

As trees age and grow, it seems logical to assume their ability to produce seeds, nuts or fruits will continue to grow, too, but a U.S. National Science Foundation-funded, Duke University-led study of nearly 600 species worldwide nips that idea in the bud.

"By testing the assumptions of forest models, the surprising results from this research could have major implications for understanding forest dynamics through time, and improve forest models," said Betsy Von Holle, a program director in NSF's Division of Environmental Biology.

In about 80% of the species examined, trees' fecundity, or physical potential to reproduce, peaked or plateaued as they reached an intermediate size. After that, fecundity declined.

The other 20% of species do not necessarily have a secret elixir of youth to ward off this deterioration, the researchers say. Those trees, too, likely experience a decline in fecundity past a certain age and size. There is not enough data yet on older, larger trees to know for sure.

"Tree fruits and nuts comprise 3% of the human diet and are also important for many birds and small mammals, while tree seeds are vital for forest regeneration," said Tong Qiu, who led the study. "To manage and conserve these resources effectively, we need to know if declines in fecundity are likely to occur, and at what size or age they might set in."

Answering those questions has, until now, forced ecologists to go out on a limb.

"On one hand, it's extremely implausible that fecundity in trees indefinitely increases with age and size, given what we know about senescence, or age-related deterioration, in humans and all other multi-celled organisms," said James Clark, the study’s senior author. "On the other hand, strictly speaking, there's been no conclusive evidence to disprove it."

Because many fruit tree crops are replaced every two or three decades as yields begin to decline, and because of the difficulty of monitoring seed production in non-cultivated trees, most studies on tree fecundity have relied on datasets that skew toward younger trees that are still small or medium-sized, Clark said. Lacking sufficient data on seed production in a species' later stages of development, scientists have had to approximate these numbers based on averages from earlier stages.

The new study avoids this pitfall by synthesizing data on seed production and maturation status for 585,670 individual trees from 597 species monitored through the Masting Inference and Forecasting network of long-term research sites, or MASTIF. Clark has helped develop MASTIF in recent years in collaboration with dozens of institutional partners around the world.

Clark, Qiu and their colleagues published their study in Proceedings of the National Academy of Sciences.