DURHAM, N.C. – 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 new Duke University-led study of nearly 600 species worldwide nips that assumption in the bud.

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 don’t necessarily have a secret elixir of youth to ward off this deterioration, the researchers say. They, too, likely experience a decline in fecundity past a certain age and size. There just isn’t enough data yet on older, larger trees of their species 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, a postdoctoral researcher at Duke’s Nicholas School of the Environment, 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 S. Clark, Nicholas Distinguished Professor of Environmental Science at Duke. “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 explained. 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.

An aging tree
An aging tree. Credit: Jim Clark, Duke University

The problem with that, Clark said, is that trees don’t necessarily produce a regular number of seeds each year at any size or age. Though production tends to be heavier overall in a tree’s earlier stages, there can be huge variations from year to year and tree to tree – from zero seeds one year to millions the next. So, using past species averages to project future production can lead to overestimation of a tree’s actual potential.

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 (MASTIF) network of long-term research sites. Clark ‘s lab has helped develop MASTIF in recent years in collaboration with dozens of institutional partners around the world.

The network’s global database contains detailed records, often stretching back many decades, on the annual seed production of trees growing at more than 500 different sites in North America, South America, Asia, Europe and Africa. New or recent observations –typically obtained through seed traps or estimates of total seed production based on cone counts – can easily be entered into the database to add to our knowledge base.

Having access to such a vast repository of raw, unfiltered data enabled Qiu, Clark and their colleagues to develop a model calibrated to avoid overestimation and calculate long-term fecundity more accurately.

“For most of the species we study, including humans, one of the most fundamental variables we measure is birth rates,” Clark said. “For animals, it’s often easy –you count the number of eggs in a nest or pups in a litter. But when you get to trees, it’s trickier. You can’t directly observe how many seeds are being produced, and, as this study shows, approximation doesn’t work either. You need another way to do that. Our model can help meet that need.”

Clark, Qiu and their colleagues published their peer-reviewed study the week of Aug. 16 in the Proceedings of the National Academy of Sciences.

Fifty-nine researchers from 13 countries or territories – Chile, Italy, Canada, Poland, France, Spain, Switzerland, Japan, Slovenia, Germany, Panama, Puerto Rico and the United States – co-authored the study with Clark and Qiu.

Primary funding came from the National Science Foundation (NSF), NASA and the French Ministère de ’Enseignement Supérieur de la Recherche et de l’Innovation’s “Make Our Planet Great Again” initiative. Data sources included the National Ecological Observation Network (NEON) funded by the NSF.

In addition to Clark’s primary faculty appointment at Duke’s Nicholas School of the Environment, he holds an appointment at the Université Grenoble Alpes through the Institute National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement.

CITATION: “Is There Tree Senescence? The Fecundity Evidence,” T. Qiu, M.C. Aavena Acuna, R. Andrus, D. Ascoli, Y. Bergeron, R. Berretti, M. Bogdziewicz, T. Boivin, R. Bonal, T. Caignard, R. Calama, J.J. Camarero, C. Clark, B. Courbaud, S. Delzon, S. Donoso Calderon, W. Farfan-Rios, C.A. Gehring, G.S. Gilbert, C.H. Greenberg, Q. Guo, J. Hille Ris Lambers, K. Hoshizaki, I. Ibanez, V. Journé, C.L. Kilner, R. Kobe, W.D. Koenig, G. Kunstler, J.M. LaMontagne, M. Ledwon, J.A. Lutz, R. Motta, J.A. Myers, T.A. Nagel, C.L. Nuñez, I.S. Pearse, Ł. Piechnik, J. Poulson, R. Poulton-Kamakura, M.D. Redmond, C.D. Reid, K.C. Rodman, C.L. Scher, H. Schmidt Van Marle, B. Seget, S. Sharma, M. Silman, J.J. Swenson, M. Swift, M. Uriarte, G. Vacchiano, T.T. Veblen, A.V. Whipple, T.G. Whitham, A.P. Wion, J. Wright, K. Zhu, J.K. Zimmerman, M. Zywiec, and J.S. Clark; Proceedings of the National Academy of Sciences, Aug. 16, 2021. DOI: 10.1073/pnas.2106130118.