Acorn Production by Oaks in Central Coastal California: Variation within and among Years

We measured acorn production by individual oaks of five different species at Hastings Reservation in central coastal California between 1980 and 1991. Variation in acorn production was considerable both within and among years and was generally uncorrelated between species. Compared to expected values, variance within years in the size of acorn crops was small, while variance among years was high. Crop failures occurred fairly frequently and large crops in successive years were observed, but not more than expected by chance. Individual trees masted at species—specific intervals, but these patterns did not result in regular masting cycles at the production level. We compared these patterns to predictions of four hypotheses for the evolution of seed production patterns. Observations did not support the hypotheses that production patterns track resource availability (the ”resource matching” hypothesis) or that they have evolved to attract seed dispersers (the “seed dispersal” hypothesis). However, they are generally consistent with two additional hypotheses, that masting in these wind—pollinated species evolved because of a proportional increase in fertilization and seed set during mast years (the “wind pollination” hypothesis) and that masting has evolved to maximize the probability of avoiding predation via predator satiation (the “predator satiation” hypothesis).