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Geographical Ecology of Acorn Production by California Oaks

Koenig, Walt and Jean Knops, Oaks ‘n’ Folks – Volume 12, Issue 2 – September, 1997

Intermittent seed production, also known as mast seeding, is a widespread phenomenon among forest trees. One of the defining features of mast seeding is that it is a population phenomenon. In other words, a single tree does not mast. Rather, masting implies that a population of trees all produce (or fail to produce) seeds synchronously. This leads to the obvious question: how geographically synchronous is seed production in masting species?

California oaks offer a good opportunity to examine this question both because of their wide distribution and their diversity. Most considerations of masting focus on only a single species, but it is entirely plausible that sympatric species of oaks mast synchronously as well. In fact, such interspecific synchrony is predicted if masting is an adaptation to counter predation by generalist seed predators that do not discriminate among different species growing together.

Currently, the most extensive data addressing this question come from three sites in central coastal California that we and our colleagues have surveyed every year since 1989. These sites, which include Jasper Ridge in San Mateo County, Hastings Reservation in Monterey County, and Pozo in San Luis Obispo County, are approximately 93 miles apart, yielding a total range of nearly 180 miles from north to south. At each site, we visually surveyed the acorn crops of valley, blue, and coast live oaks. Sample sizes vary between species and sites, but in all cases involve the same individuals each year.

The results are clear: for all three species, acorn production, while not identical across sites, is statistically synchronous across years. This is particularly striking for coast live oaks, which demonstrate no statistical decrease in the degree of synchrony with distance up to the 179 miles separating Jasper Ridge from Pozo. In other words, two coast live oaks growing literally next to each other produce annual acorn crops that are just as synchronized as two coast live oaks growing 180 miles apart. This suggests a high degree of geographic synchrony in acorn production of California oaks at least approaching the statewide level.

What about interspecific comparisons? Interestingly, there also is a reasonably high degree of synchrony when comparisons are made between, rather than within, these three species of oaks. This suggests that widespread geographic synchrony in acorn production by California oaks is interspecific as well as intra-specific.

There are two caveats to these analyses. First, the geographic range is fairly limited compared to the 750+ mile range of oaks in California, and second, all three of these species are similar in that they require a single year to mature acorns. Although we currently have limited data to extend these results, we are now in our fourth year of a larger, statewide survey of acorn production involving a total of 14 sites and six species, including four that require one year to mature acorns (valley, blue, Engelman, and coast live oaks) and two species that require two years to mature acorns (California black and canyon live oaks). The results, although preliminary,support the conclusion that geographic synchrony is widespread, both intra-and interspecifically, among species of oaks requiring one year to mature acorns. However, there appears to be considerably less synchrony in the species that require two years to mature acorns, and there is apparently no synchrony between the two types of species.

This suggests several important messages for patterns of acorn production in California. First, widespread geographic synchrony in several species implies an environmental cue synchronizing acorn production. This is consistent with our work at Hastings Reservation demonstrating that acorn production of several species correlates strongly with environmental cues including weather during early spring and rainfall in prior years. Second, because acorn production is synchronized both intra- and interspecifically overlarge areas, populations of animals dependent on acorns for food, including Native Americans before European influence, are or were likely to have been affected over comparably large areas when the crop failed or was exceptionally good. However, because of the asynchrony between acorn production of oaks requiring different number of years to mature acorns, total crop failures affecting all species of oaks in a region are likely to be rare and at least some acorns should be produced in most regions in most years. Of course,this would only work to the advantage of a species as long as individuals were willing to eat any species of acorns that were available. In fact,most vertebrate acorn predators are generalists willing to do exactly this,in contrast to some boreal seed-eating birds that often specialize on the seeds of a particular species, or even subpopulation, of conifers.

Although much remains to be done, many of the mysteries of acorn production by California oaks are finally starting to be revealed. Understanding these patterns promises to yield insights into both the population biology of the many species of wildlife dependent on acorns throughout California and beyond, as well as the factors that have shaped the evolution of the oaks themselves.

Walt Koenig Research Zoologist, Hastings Reservation, U.C. Berkeley 

Jean Knops Research Director, Cedar Creek Natural History Area, University of Minnesota

prepared and edited by Richard B. Standiford and Pamela Tinnin