Floral traits and their connection with pollinators and climate.

Background and aims: Floral characteristics vary significantly among plant species, and multiple underlying factors govern this diversity. Although it is widely known that spatial variation in pollinator groups can exert selection on floral traits, the relative contribution of pollinators and climate to the variation of floral traits across large geographical areas remains a little-studied area. Besides furthering our conceptual understanding of these processes, gaining insight into the topic is also of conservation relevance: understanding how climate might drive variation in floral traits can serve to protect plant-pollinator interactions in globally change conditions.

Methods: We used Rhododendron as a model system and collected floral traits (corolla length, nectar volume and concentrations), floral visitors and climatic data on 21 Rhododendron species across two continents (North America, Appalachians and Asia, Himalaya). Based on this, we quantified the influence of climate and pollinators on floral traits using phylogeny-informed analyses.

Key results: Our results indicate that there is substantial variation in pollinators and morphological traits across Rhododendron species and continents. We came across four pollinator groups: birds, bees, butterflies and flies. Asian species were commonly visited by birds, bees and flies, whereas bees and butterflies were the most common visitors of North American species. The visitor identity explained nectar trait variation, with flowers visited by birds presenting higher volumes of dilute nectar and those visited by insects producing concentrated nectar. Nectar concentration and corolla length exhibited a strong phylogenetic signal across the analysed set of species. We also found that nectar trait variation in the Himalayas could also be explained by climate, which presented significant interactions with pollinator identity.

Conclusions: Our results indicate that both pollinators and climate contribute and interact to drive nectar trait variation, suggesting that both can affect pollination interactions and floral (and plant) evolution individually and in interaction with each other.