Acorn production by oaks in central coastal California: Influence of weather at three levels

Mean annual acorn crop size of oaks in central coastal California correlated with weather for four of five species. For the deciduous Quercus lobata and Quercus douglasii, larger crops were produced in years when April temperatures were warmest, while for the evergreen Quercus agrifolia and Quercus chrysolepis, crops were larger when there was more rain one and two years prior to the year when acorns were produced. No correlations were found between weather and mean annual acorn production by Quercus kelloggii. Within years, individual Q. douglasii that flowered during periods of greater solar radiation produced more acorns, and during 1 of 5 years individual Q. lobata and Q. agrifolia produced larger crops when they flowered during warm, dry periods. These results indicate that conditions favorable for pollination and fertilization strongly enhance mean annual acorn production in Q. lobata and Q. douglasii and may have an effect on differences in acorn crop size among individual Q. lobata, Q. douglasii, and Q. agrifolia. Mean annual acorn crops of these three species were significantly concordant among three sites in central coastal California 300 km apart. Weather conditions were also significantly correlated, especially annual rainfall and mean April temperatures. Thus, the environmental factors that have the strongest influence on mean annual acorn production within a site are also those that arc the most geographically concordant. However, we found no relationship between environmental factors and differences in mean annual acorn production by these same three species at the three sites.