Mast production has strong effects on frugivore feeding tactics, resulting in altered behavioral patterns. Although such effects are more obvious in cool-temperate forests with less diverse nuciferous tree species, empirical research supporting such hypotheses, especially regarding entire assemblages of mammals in wide geographical distributions, has been limited. Therefore, we focused on the considerable fluctuation in the yearly production of beechnut (Fagus crenata), the dominant nuciferous tree species in cool-temperate forests, and compared the spatiotemporal variations in mammal assemblages at different forest patches with diverse landscapes throughout three study sites (in total, 2,000 km²) in northeastern Japan between 2015 (masting year) and 2016 (lean year). In each study site, we set eight 1-km² quadrats and provided four camera traps for each quadrat from August to November. A total of 13,201 trap-nights yielded five carnivore, two rodent, and one primate species. From this evaluation, we determined that the diurnal rhythms of several species were sensitive to beechnut production: (1) the mammal species diversity decreased during the nocturnal period in the lean year; and (2) the activity of Ursus thibetanus and Sciurus lis increased during the diurnal period in the lean year. However, these tendencies differed between study sites. Based on these findings, we discussed the ecological rationale for the shift of diel rhythms and distribution based on feeding tactics and landscape structure, respectively.