Warming-driven shifts in floral traits generate flower-pollinator size mismatch and decrease reproductive output.

Plant-pollinator interactions are essential for maintaining pollination services but may be sensitive to disruption from climate change. Higher temperatures can reduce flower size, nectar availability, and other floral traits, and shorten pollinator tongue and increase body size. This could, in turn, reduce flower-pollinator size matching, leading to difficulties accessing nectar and effectively transferring pollen between flowers. However, the effects of climate change on plant-pollinator trait matching are rarely experimentally tested. We conducted a 2-year simulated warming experiment and a follow-up trait manipulation experiment in the field. Our aim was to link the effects of simulated warming on flower size and morphology, pollination efficiency, and ultimately, reproductive success of the bumblebee-pollinated Impatiens oxyanthera (Balsaminaceae). Warming significantly decreased the lengths of corolla tubes and nectar spurs in I. oxyanthera but did not affect the size of bumblebee visitors, leading to a size mismatch between the flowers and their pollinators. This mismatch resulted in a shorter visit duration of bumblebees on a single flower, reducing pollen deposition on the stigmas and female reproductive success. Experimental reductions in nectar spur length, mirroring the decreases observed under warming, impaired pollen deposition and reduced seed production, independent of bumblebee size. These findings suggest an additional mechanism through which climate change may result in size mismatch between interacting partners, consequently disrupting pollination services beyond climate-driven distribution range and phenological shifts.