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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
Further Reading....
Koenig, W. and J. Knops. 1997. Geographical ecology of acorn production by California oaks. Oaks 'n' Folks, Univ. of California Integrated Hardwood Range Management Program 12(2): 6-7.
Koenig, W. and J. Knops. 1995. Why do oaks produce boom-and-bust seed
crops? California Agriculture 49:7-12.
Koenig, W. D., J.M.H. Knops, W. J. Carmen, M. T. Stanback, and R. L. Mumme. 1996. Acorn production by oaks in central coastal California: influence of weather at three levels. Canadian Journal of Forestry Research 26: 1677-1683.
Koenig, W.D, J.M.H. Knops, W.J. Carmen, M.T. Stanback and R. L. Mumme. 1994. Estimating acorn crops using visual surveys. Canadian Journal of Forest Research. 24:2105-2112.
Koenig, W. D., R. L. Mumme, W. J. Carmen, and M. T. Stanback. 1994. Acorn production by oaks in central coastal California: variation within and among years. Ecology 75:99-109.