Acorn Synchrony

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Patterns of Acorn Production by California Oaks

Walt Koenig and Jean Knops
Hastings Reservation and Museum of Vertebrate Zoology
University of California Berkeley

The acorns of 9 species of tree oaks provide a vast resource of food for wildlife throughout much of California. Major acorn consumers include mule deer, wild boar, scrub-jays, acorn woodpeckers, and (historically) both Native Californians and grizzly bears.

Since 1980 we have studied acorn production by 6 species of California oaks. Surveys are done visually by counting as many acorns as possible in 30 secs (15 secs for each of 2 observers) in individually-marked trees each fall. Several hundred trees are surveyed each year.

Acorn production also varies strikingly from tree to tree. Some trees in our study have produced good to excellent crops of acorns almost every year, whereas a few individuals have never produced an acorn since 1980!

We're particularly interested in the geographic synchrony in acorn production, and have conducted a statewide survey at 14 localities since 1994. Results thus far suggest that synchrony in several species is widespread, in the case of blue oaks encompassing the entire range of the species!

Acorn production varies dramatically from year to year, a phenomenon known as "mast-fruiting" or "masting". In valley and blue oaks, 2 common species requiring a single year to mature acorns, the size of the fall acorn crop is highly correlated with warm, dry conditions during flowering and fertilization the prior spring. This is not true for other species of oaks, especially those requiring 2 years to mature acorns.

Our work on acorn production has been sponsored by the University of California's Integrated Hardwood Range Management Program and the California Department of Fish and Game