"It's a zero year. There's zero production. I've never seen anything like this before," said Rod Simmons, a field botanist in Virginia.

At least 20 different species of oak trees in a large swath of North America failed to produce acorns this year and botanists are watching closely to determine if it is part of a natural cycle for the trees, which can live more than 200 years, The Washington Post reported Sunday.

The cause also may be weather-related or environmental and related to global warning, though botanists in other parts of North America report a normal acorn harvest, the Post reported.

"If you're a squirrel, it's a big worry. But it's no problem for the oak tree. They live a long time. They'll produce acorns again when they're ready to," said Alan Whittemire, a botanist at the U.S. Arboretum in Washington.