Fruiting phenology affects the direction of vertical seed dispersal by mammals and birds across mountain ranges

Vertical seed dispersal towards higher or lower altitudes is an important process for plants' adaptation to climate change. Although many plants depend on animals for seed dispersal, studies on vertical seed dispersal by animals, determined by complex animal behaviours, are scarce. Previous studies hypothesised that animals inhabiting temperate regions disperse seeds uphill in spring/summer and downhill in autumn/winter due to their seasonal movement following the altitudinal gradients in food phenology. However, this hypothesis has only been tested in seed dispersal by mammals on one mountain range. Vertical seed dispersal by birds might differ from that by mammals, and frugivorous megafauna extinction and mountain topography may affect seed dispersal patterns. Here we assessed the vertical seed dispersal of summer and summer-to-autumn fruiting cherries by mammals and birds across three Japanese mountain ranges, two of them with presence of a megafauna, the Asian black bear. We found strong uphill seed dispersal of summer fruiting cherry species and weak downhill seed dispersal of summer-to-autumn fruiting cherry species, irrespective of the frugivore community and mountain topography. These indicate that the fruiting phenology affects the direction of vertical seed dispersal by mammals and birds across mountains. Mammals and birds dispersed seeds over a similar vertical profile, although birds are likely to be low-quantity seed dispersers. The absence of bears, which dispersed the majority of the seeds, was not compensated by the remaining mammal species. The results suggest that the fruiting phenology and megafauna presence affect whether animal-dispersed temperate plants can migrate efficiently under climate change.