Floral nectar is a vital resource for pollinators, playing an important role in ecosystem structure and functioning. Ongoing climate warming could have a negative effect on nectar secretion where a strong temperature rise is forecasted, e.g. the Mediterranean. Decreased nectar secretion, coupled with strong shifts in plant flowering phenology can disrupt plant–pollinator interactions, both expected to largely affect the entire ecosystem. The present study builds on previous work by some of us which provided evidence that in Mediterranean communities, only early-flowering species are prone to shifts in phenology due to climate change, and that pollinator phenology is largely unaffected.
Under fully controlled conditions, we tested how increased temperature influences nectar secretion in six Mediterranean plant species (Rosmarinus officinalis, Asphodelus ramosus, Lavandula stoechas, Echium plantagineum, Ballota acetabulosa, and Teucrium divaricatum) covering a flowering period from winter to summer. We compared the changes in nectar secretion within a range of temperatures, including temperature expected by the end of the century according to IPCC. The experiments were conducted using potted plants (19-23 per species) in a climate chamber and outdoors as a control. Per day and plant, we measured nectar volume and concentration per flower, and counted the number of open flowers.
Results/Conclusions
We found a significant unimodal effect of temperature on nectar secretion, with a negative effect of very high temperatures in all species. Optimal temperatures for nectar secretion were close to and slightly higher than the current long-term mean temperatures for the late- and early-flowering species, respectively. Increasing temperatures will, however, set the average temperature beyond the optimum range by the end of the century, especially in the case of summer-flowering species. As a consequence, we expect a future decrease in nectar secretion of the summer-flowering species, which will reduce the amount of nectar resources available for pollinators. Nectar secretion of spring flowering plants being more prone to shifts in phenology, will either be not affected (plants shifting their flowering phenology) or, in case of slight temperature increase, may benefit from rising temperatures (plants not shifting their flowering phenology). Because such a shift may not equally apply to pollinators, as shown in earlier studies, this may lead to a decoupling resulting in pollination limitation and asymmetric exploitation of nectar (maybe except social bees, such as honeybees). The nectar secretion patterns of summer-flowering plants will be most affected, which could in turn affect plant–pollinator interactions and overall community composition.