Climate change reduces nectar secretion in two common Mediterranean plants

Global warming can lead to considerable impacts on natural plant communities, potentially inducing changes in plant physiology and the quantity and quality of floral rewards, especially nectar. Changes in nectar production can in turn strongly affect plant-pollinator interaction networks-pollinators may potentially benefit under moderate warming conditions, but suffer as resources reduce in availability as elevated temperatures become more extreme. Here, we studied the effect of elevated temperatures on nectar secretion of two Mediterranean Lamiaceae species-Ballota acetabulosa and Teucrium divaricatum. We measured nectar production (viz. volume per flower, sugar concentration per flower and sugar content per flower and per plant), number of open and empty flowers per plant, as well as biomass per flower under a range of temperatures selected ad hoc in a fully controlled climate chamber and under natural conditions outdoors. The average temperature in the climate chamber was increased every 3 days in 3 °C increments from 17.5 to 38.5 °C. Both study species showed a unimodal response of nectar production (volume per flower, sugar content per flower and per plant) to temperature. Optimal temperature for sugar content per flower was 25-26 °C for B. acetabulosa and 29-33 °C for T. divaricatum. According to our results, moderate climate warming predicted for the next few decades could benefit nectar secretion in T. divaricatum as long as the plants are not water stressed, but have a moderate negative effect on B. acetabulosa. Nevertheless, strong warming as predicted by climate change models for the end of the 21st century is expected to reduce nectar secretion in both species and can thus significantly reduce available resources for both wild bees and honeybees in Mediterranean systems.