Timing of Flowering and Seed Production in Three California Oaks

We examined the importance of pollen limitation to variation in acorn production of three species of California oaks (Quercus lobata, Q. douglasii, and Q. agrifolia) by testing for relationships between phenology and acorn production. Within years, trees flowering closer to the mean flowering date of the population produced significantly more acorns in two of the species. Assuming that more pollen is available when more conspecifics are blooming, this result is consistent with pollen limitation affecting individual variation in acorn productivity. Across years, relatively warm, dry conditions during the peak of flowering correlated with larger acorn crops the following fall in all three species. Assuming that such conditions favor either increased pollen movement or increased fertilization, this is consistent with the hypothesis that pollen limitation plays a significant role in the highly variable seed crops characteristic of masting in these species. Overall, the proportion of total variance in the acorn production of individual trees explained by these two indices of pollen availability ranged from 28 to 38 percent. These results support the hypothesis that pollen limitation plays an important role in causing variance in seed production in these wind-pollinated oaks.