Seed production that varies greatly from year to year, known as "masting" or "mast-fruiting" behavior, is a population-level phenomenon known to exhibit geographic synchrony extending, at least in some cases, hundreds of kilometers. The two main nonexclusive hypotheses for the driver of such geographically extensive synchrony are (1) environmental factors (the Moran effect), and (2) the mutual dependence of trees on outcrossed pollen (pollen coupling). We tested 10 predictions relevant to these two hypotheses using 18 years of acorn production data on two species of California oaks. Data were obtained across the entire ranges of the two species at 12 sites (10 for each species) separated by up to 745 km. In general, our results provided strong support for the importance of the Moran effect as a driver of spatial synchrony in and between these two species. Particularly compelling was evidence of close concordance between spatial synchrony in acorn production and key environmental factors extending over the range of both species and significant spatial cross-synchrony between the two species, despite considerable differences in their geographical ecology. Because oaks are monoecious, female flowers are not necessarily related to pollen production, and thus, our tests do not address the role of pollen coupling in bisexual species where pollen and flower production are necessarily correlated. For the oak species considered here, however, the Moran effect is a key driver of large-scale spatial synchrony in acorn production.