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Too Many Acorns?

Ann-Sophie Qvarnström
Wikimedia Commons

Ask any squirrel — acorns are delicious! Over our long cold winters, these portable packets of oak fruit provide nourishment to mammals and birds. On average, a single oak tree can produce over 10,000 of these nuggets each year.

But that’s an average. Oaks bear super-abundant fruit crops only every three to five years, what botanists call “mast years.” These masting events are preceded and followed by years of very small crops — sometimes, by complete crop failure. What’s the reason for this irregular seed production?

One theory is “predator satiation,” an idea grounded in the fact that making acorns is a huge energy investment. The tree’s payoff comes when animals transport their offspring to places far from their own crowns, where the acorns will have a better chance of thriving, free from competition with their parents. And the animals get a food source that is rich in carbohydrates and nutrients, and that can be stored for months at a time.

When oak trees produce lots of fruits during a mast year, they make more food than acorn-consuming animals can possibly eat, so there are plenty of acorn leftovers that can germinate and grow into the next generation. And during low years, some animals will starve, which effectively keeps those populations in check. This pattern ensures that, in the long term, some acorns make it into the next generation.

But still, oak masting puzzles remain: How do oak trees across populations coordinate the timing of their masting events? How do local weather patterns affect acorn variability? And how is climate change affecting these patterns? Happily, long-term studies are now probing these questions to better understand the complex dance between trees and their associates.

Dr. Nalini Nadkarni is an emeritus professor of both The Evergreen State College and the University of Utah, one of the world’s leading ecologists and a popular science communicator. Dr. Nadkarni’s research and public engagement work is supported by the National Geographic Society and the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. @nalininadkarni
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