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 two-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 depositio,n 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.