Evolutionary advantages of mast seeding in Fagus crenata

1 Two non-mutually exclusive hypotheses have been proposed to explain the evolutionary advantages of mast seeding (the intermittent production of large crops of flowers or seeds by a population of perennial plants). Mast seeding could have evolved as a result of increased pollination efficiency in mast-flowering years and/or as an anti-predator adaptation that increases the survival of seeds by alternately starving seed predators in non-mast years and satiating them in mast years. 2 We investigated annual seed crops to test the relative contributions of pollination efficiency and pre-dispersal predator satiation to mast seeding in Fagus crenata, a tall tree species dominating cool-temperate forests in Japan. Thirteen-year (1990-2002) time series data were collected for five beech forests in south-western Hokkaido. 3 The negative relationship observed between the pollination failure rate and the total seed crop in the current year supports the pollination efficiency hypothesis. The predator satiation hypothesis was also supported by the fact that the predation rate showed a good fit to the ratio of successive total seed crops, suggesting that a numerical response (starving the predator in low seed years) operated in F. crenata. 4 Key-factor analysis revealed that pre-dispersal seed predation had a larger effect on seed production per flower than did pollination efficiency. 5 We used a simulation model to examine how the magnitude of fluctuation in the total seed crop would influence the pollination failure rate, the predation rate and the viable seed rate. The mean levels of fluctuation of total seed crops of F. crenata were just large enough to provide maximum benefits from predator satiation at some sites. 6 Mast seeding in F. crenata thus appears to be determined by selective pressures from its seed predators.