Individual resource limitation combined with population-wide pollen availability drives masting in the valley oak (Quercus lobata)

1. Masting, the synchronized production of variable seed crops, is widespread among woody plants, but there is no consensus about the underlying proximate mechanisms. To understand this population-level behaviour, it is necessary to dissect the behaviour of individual trees as well as the interactions that synchronize them. 2. Here, we test a model of masting in which variability in seed set is driven by resource limitation within trees and synchrony is driven by pollen limitation due to phenological asynchrony in some years. 3. We used a 35-year seed set data set and a 12-year phenological data set to analyse seed production of 84 valley oaks (Quercus lobata) in central coastal California. Individual trees varied tremendously in their seed production patterns; trees with high levels of seed production were less variable over time, but showed stronger negative autocorrelation between years, suggesting that they are more resource-limited than unproductive trees. In years of more asynchronous flowering, Q. lobata produced fewer seeds, consistent with the importance of phenological synchrony. 4. We parametrized a model with these results to investigate how individual resource limitation and population-wide pollen limitation - a consequence of asynchronous flowering during cold spring temperatures-interact to shape annual variation in seed production. The model illustrates that this proximate abiotic driver can synchronize the behaviour of individuals, resulting in population-wide seed production patterns that closely resemble the field data. 5. Synthesis. Our findings support the hypothesis that an interaction between two proximate mechanisms, individual resource limitation and environmental variation affecting population-wide pollen availability, drives masting in this population of Quercus lobata. This combination of internal and external proximate drivers may underlie masting behaviour in many wind-pollinated plants.