Site-specific variation in flowering phenology of a spring ephemeral plant and its implications for phenological mismatch with pollinators under climate change.

BACKGROUND AND AIMS: Disruption of plant-pollinator interactions occurs when interacting species exhibit different phenological responses to climate change. Flowering of Corydalis ambigua, a spring ephemeral herb growing in deciduous forests, is highly dependent on snowmelt time, while its main pollinators, bumblebee queens, emerge from overwintering after a certain degree of soil warming. In springs with early snowmelt, flowering often occurs earlier than bumblebee emergence, resulting in greater pollen limitation and reduced seed production, suggesting that phenological mismatch affects plant fitness. This study investigates how flowering traits vary among populations with different snowmelt times.

METHODS: To reveal site-specific differences in flowering phenology, phenological traits and the effects of flowering timing on seed set were compared in four populations with different snowmelt times and pollinator compositions over 3-5 years. Furthermore, a common garden experiment was conducted to quantify the thermal requirement for flowering onset of individual populations.

KEY RESULTS: Flowering onset was strongly related to snowmelt time in each population, which showed greater annual fluctuations (2020-2024) in early-snowmelt than late-snowmelt populations. Bumblebee abundance, and thus pollinator availability, varied greatly between years. Seed set varied significantly among years and populations. Common garden plants showed significant variation in flowering phenology and thermal requirements for flowering with population of origin and transplant location, indicating genetics-environment interaction effects on flowering phenology. Long-term monitoring in an early-snowmelt population revealed that phenological mismatch significantly affects seed production, and advancing snowmelt time increases the risk of mismatch.

CONCLUSIONS: The thermal requirements for flowering onset vary among populations with different snowmelt times. As pollinator availability has a significant impact on seed production, regulating flowering phenology could minimize the phenological mismatch between flowering onset and bumblebee emergence, thereby maintaining local populations. A comprehensive assessment of population dynamics linked to variation in seed production is needed to clarify this.