Rising temperatures and variable precipitation will continue to accentuate aridity globally. Direct effects of drought and warming on plants are well known but surprisingly little information exists concerning how these environmental changes affect floral resources and, in turn, how such changes affect visitation by pollinators. Here we experimentally examine how warming and drought affect plant-pollinator interactions under field conditions. Using bee-pollinated squash (Cucurbita pepo), we manipulated warming and drought in a fully-crossed design (with two levels of each factor) that employed passive, open-top warming chambers and differing levels of irrigation. For replicate plants in the four experimental groups, we bagged flowers to ensure that plants received pollen only from plants within the same experimental group. Interspersed among bee-pollinated plants were hand-pollinated plants that experienced the same range of altered conditions. We measured the daily production of floral advertisements (i.e. flower size, nectar production, and pollen production) and monitored bee visitation and pollination by both specialist (Peponapis pruinosa) and generalist (Apis mellifera) pollinators. At the end of the season, we measured fruit and seed set for all plants. To test how drought and warming affect the pollination mutualism, we compared response variables between hand-pollinated and bee-pollinated plants.
Both warming and drought affected reproductive performance in Cucurbita pepo, with irrigation levels having the strongest effects. Compared to hand-pollinated plants, bee-pollinated plants produced fruit that were smaller and contained fewer seeds when grown under drought conditions. Plants grown under warmed, drought conditions produced the most pollen, but were also the only experimental group to experience pollen limitation. These findings suggest that squash grown in warmed, drought conditions were pollen-limited due to insufficient pollination by bees, leading to decreased fruit volume and seed set. These results, in turn, imply that floral resource quality affects decision-making by bees in ways that can affect plant reproductive performance.