Stabilizing selection for within-season flowering phenology confirms pollen limitation in a wind-pollinated tree

1. We examined the fitness consequences of pollen limitation in the valley oak (Quercus lobata Nee, Fagaceae), a wind-pollinated, predominantly outcrossing tree endemic to California, by examining the relationship between within-year phenology and acorn production over an 8 year period. 2. We predicted that pollen limitation should result in trees flowering relatively early and late in the season, when fewer trees are in flower, to experience reduced reproductive success compared to trees flowering during the middle of the season. 3. Despite considerable variation in acorn production from year to year, analyses confirmed this prediction and indicated that there was significant overall stabilizing selection on budburst date consistent with the pollen limitation hypothesis. 4. Analyses including herbivory and pre-budburst temperatures at individual trees revealed significant correlations with acorn production, but we still found significant stabilizing selection on budburst date even when considering these variables. 5. Synthesis. Pollen limitation, as indicated by stabilizing selection on within-year flowering phenology, significantly affects acorn production by individual trees in Q. lobata and has the potential to influence other key life-history features including masting behaviour and spatial synchrony.