Professor University of California Davis, United States
Abstract: Heat waves are extreme weather events anticipated to increase in frequency and severity in the coming decades and may impact particularly sensitive ecological processes such as animal-mediated pollination. Pollination is vulnerable during heat waves because elevated heat stress can simultaneously impact floral development and alter pollinator availability and foraging behavior. Previous work studying extreme heat on pollination assess how flowers, reproductive organs, and gametes are affected, but do not include the active role of pollinators which may exacerbate or mediate extreme heat effects on successful pollination. Thus, our understanding of how the dynamics of pollination changes under heat wave conditions is incomplete, and studies evaluating the relative impacts of extreme heat on pollination through individual plant and pollinator responses is urgently needed.
Our research objective was to partition the effects of heat waves on the dynamics of pollination and post-pollination through direct effects on floral development and elevated air temperature on pollinator foraging.
Our experiment employed a fully crossed design of experimental heat treatments and baseline control temperatures to 1) flowers during development and 2) flowers and bumble bees during a foraging period. Treatments were applied using growth chambers and experimental foraging arenas containing flowering Brassica napus plants connected to active bumble bee colonies.
We demonstrated that temperatures emulating moderate heat waves significantly reduce pollen deposition through reduced bumble bee visitation regardless of flower heat stress. In contrast, when flowers develop during heat stress, pollen production in anthers is reduced so dramatically that it limits pollen availability and subsequent pollen deposition despite adequate visitation by bumble bees. Pollen limitation at the pollen dispersal stage appears to create the largest bottleneck in successful fertilization and seed production, but flowers developed during heat treatments also show additional limitation through reduced pollen tube survival. These mechanisms combine to meaningfully reduce seed set and suggest that pollination and floral reproduction will be highly vulnerable when heat waves occur during flowering periods, with potentially profound consequences for crop yields in pollinator dependent crops and seed set in wild plant species. Additionally, because our experiments focused on independent replication to elucidate mechanisms and lacked temporal context, we may be underestimating effects of prolonged heat waves on pollination and successful floral reproduction.
