Pollination linkage between canopy flowering, bumble bee abundance and seed production of understorey plants in a cool temperate forest

In forest ecosystems, canopy trees provide a large amount of floral resources for pollinators but flower production fluctuates highly from year to year. Temporal variation in floral resources may influence the population dynamics of pollinators. We investigated the relationship between the population dynamics of bumble bees (Bombus spp.) and canopy floral resources over 5 years in a cool temperate forest. We predicted that the yearly fluctuation of canopy resources affects colony development and queen production, resulting in the variation of the pollination success of understorey plants the following spring. Flower production of canopy trees showed clear biyearly fluctuations, synchronously among most species. Worker production of Bombus ardens and Bombus hypocrita were positively related to floral resources during colony establishment by overwintered queens, while that of Bombus diversus was positively related to floral resources during the worker-active period. An abundance of overwintered queens was positively related to both worker abundance and floral resources in the previous summer in B. ardens and to only previous-year worker abundance in B. hypocrita but was dependent on neither factor in B. diversus. These differences in causal effects on queen production among species might be a reflection of interspecific differences in the degree of dependence on canopy resources for queen production due to species-specific phenologies and/or foraging preferences of other species of flowers. The extent of pollen limitation in the spring-ephemeral Corydalis ambigua, which is exclusively pollinated by overwintered queens, was negatively correlated with the abundance of B. hypocrita queens. Thus, population dynamics of bumble bees has a crucial effect on seed production of understorey plants. Synthesis. The dynamics of flower production in overstory trees can indirectly affect the pollination efficiency of understorey plants in the next season via the population dynamics of pollinators. Our results suggest that a cascade effect occurs through pollination networks in a cool temperate forest ecosystem.