Highly variable and synchronous seed production, or ‘masting’, is hypothesized to be driven, in part, by resource limitation. The resources that are most relevant for limiting seed production differ across studies, however, and studies that include multiple masting species studied within the same ecosystem are rare. We studied two closely related, sympatric, masting oaks (Quercus lobata and Quercus douglasii) with contrasting water use strategies to test: (1) whether resources had effects on flowering, fruit set, or acorn crop; and (2) whether these two species differed in which resources were most limiting for reproduction. Quercus douglasii reaches lower water potentials during summer than Q. lobata, and concurrently experiences a sharp decline in photosynthetic rates. Therefore, we predicted that Q. douglasii would deplete carbon reserves during acorn development. In contrast, Q. lobata avoids low water potentials and maintains photosynthetic rates during the dry summer months. Therefore, we predicted that nitrogen would limit acorn production in Q. lobata. We measured non-structural carbohydrates (NSC) in trunks and branches, and nitrogen (N) in branches and leaves in June and September over a two-year period. We counted female flowers and tracked their fates on subsampled branches of each tree. We estimated acorn production with visual surveys.
Results/Conclusions
In the more water-limited Q. douglasii, trees with greater acorn crops accumulated less branch NSC. This suggests that acorn development and carbon reserves are competing sinks for carbon. There was no effect of N on acorn production, and no effects of any measured resources on flowers. In the less water-limited Q. lobata, trees that retained more leaf N during flower initiation produced more flowers in the following spring, and trees with larger acorn crops also experienced more N depletion in branches during acorn maturation. There were no effects of resources on fruit set and no effects of NSC on flowers or acorns. Overall, we found some support for resource-limited seed production in both masting species, however there were different resources associated with reproduction. Q. lobata in this population do not reach the same levels of pre-dawn water potentials as Q. douglasii, which suggests that they have greater access to water, even during the driest months. Furthermore, the two species do not differ in soil fertility at this site. We interpret this as evidence that species functional traits, such as those associated with water relations, are potentially important drivers of differences in limiting resources among masting species.