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Acorn cycle tough to crack

For two autumns in a row, acorns fell like rain from the tall oak trees in George Robertson's yard in Rockport, carpeting his lawn so thickly that raking seemed almost futile.

Not this year.

"Last year there must have been thousands of acorns and squirrels all over the place," Robertson said. "And this year there are hardly any."

After two years of record acorn crops, people across the Northeast are scratching their heads at the sudden absence of the telltale underfoot crunch of fall.

Enormous swaths of oaks in the Northeast are taking a year off from producing acorns, and scientists aren't quite sure why. The nut shortage appears to be a periodic blip in the life cycle of oak trees, and one that is already beginning to have dire consequences for animals such as chipmunks and mice that feed on acorns. It is also bringing new annoyances for gardeners, as squirrels and chipmunks dig into gardens to get at bulbs or other protein sources.

New England is home to at least 10 different kinds of oak trees, and the acorn, the fruit of the oak, has long played a role in local culture. Though toxic for humans to eat, acorns were long believed to bring luck. For many New Englanders, including Robertson, folk wisdom dictates that a big autumn acorn crop is a harbinger of a tough winter.

Scientists know that oak trees produce one bumper crop of acorns every two to seven years and then a small crop the following year. But this year's nut drought appears to be more severe, and it seems more extreme following two back-to-back years of stupendous acorn crops.

At the Acorn Alpaca Ranch in Millis, the namesake nut has been scarcer this year. Last year, "the animals were tripping on them they were so thick," said Louise Hebeler, who owns the farm with her husband, Bob. "We had to rake the pastures near the trees where the acorns fell." This year, she says, the acorns haven't vanished, but there are far fewer.

Here and there, some homeowners are reporting yet another year of large acorn crops, but those appear to be isolated cases.

Scientists suspect that the oak trees might be tired, literally. Drought and unusual weather from 2000 to 2002 resulted in oak trees dropping large numbers of acorns for the last two years, possibly because stressed trees drop more seeds to ensure propagation. But the weather has returned closer to normal in the last year and the trees may not have the energy or the need to bear lots of acorns.

"It takes a huge amount of energy to create that fruit," said Ben Staples, a certified arborist with Bartlett Tree Experts in Beverly Farms.

Though for homeowners it may sometimes seem that the acorn's main role is to make small dings in the car roof, acorn production is the first link in a long chain of events in the natural world. Black bears feed on acorns. So do deer, chipmunks, and mice. The enormous overrun of squirrels in suburbia that started early this summer was probably due in part to the enormous acorn crops of 2002 and 2003.

This year's acorn drought could mean that there isn't enough food to go around. Researchers expect that many small animals will die from lack of food, and deer and bears may venture closer to houses looking for a meal. Already, scientists in New York are noticing a die-off of chipmunks and white-footed mice because of the lack of acorns there.

"We know it's happening for the smaller rodents so far," said Richard Ostfeld, senior scientist at the Institute of Ecosystem Studies in Millbrook, N.Y., who studies acorn production and its effects on the environment.

The next link in the chain: Once the mice go, the gypsy moths can move in. Mice also feed on gypsy moth pupae, and a die-off in the mouse population may lead to a proliferation of gypsy moths.

"And when gypsy moths defoliate trees, their preference is for oaks," Ostfeld said. "And we know that if gypsy moths are eating all your leaves, you are less likely to seed. It's a feedback loop."

Oak trees still remain a mystery to researchers, in part because they exhibit strange synchronized behavior. Perhaps because of a mix of genetics and climate, trees are able to turn acorn production off and on in nearly perfect rhythm, with no scientific predictability except that it appears to happen every two to seven years. Some scientists suggest that the trees may be "smart," turning off production when they sense a threat in their surroundings, such as an abundance of rodents eating their acorns and preventing new trees from taking root.

"The tree isn't doing this intentionally, but it makes sense in a natural selection kind of way," said John O'Keefe, coordinator of the Fisher Museum at Harvard Forest in Petersham.

Science aside, some homeowners are gleeful that they don't have to pick up buckets of acorns from driveways and yards. Some get so exasperated with the nuts that arborists say they get calls from people asking for removal of oak trees simply because so many acorns fall on cars.

"Some people hate them," Staples said of the region's huge oak trees. "But I say, 'When is the last time you made oxygen?' "

Beth Daley can be reached by e-mail at bdaley@globe.com.

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