Warming and elevated CO₂ induces changes in the reproductive dynamics of a tropical plant species

Tropical plant species are vulnerable to climate change and global warming. Since flowering is a critical factor for plant reproduction and seed-set, warming and elevated atmospheric carbon dioxide concentrations (eCO₂) are crucial climate change factors that can affect plant reproductive dynamics and flowering related events in the tropics. Using a combined free-air CO₂ enrichment and a free-air temperature-controlled enhancement system, we investigate how warming (+2 °C above ambient, eT) and elevated [CO₂] (~600 ppm, eCO₂) affect the phenological pattern, plant-insect interactions, and outcrossing rates in the tropical legume forage species Stylosanthes capitata Vogel (Fabaceae). In comparison to the control, a significantly greater number of flowers (NF) per plot (+62%) were observed in eT. Furthermore, in warmed plots flowers began opening approximately 1 h earlier (~09:05), with a canopy temperature of ~23 °C, than the control (~09:59) and eCO₂ (~09:55) treatments. Flower closure occurred about 3 h later in eT (~11:57) and control (~13:13), with a canopy temperature of ~27 °C. These changes in flower phenology increased the availability of floral resources and attractiveness for pollinators such as Apis mellifera L. and visitors such as Paratrigona lineata L., with significant interactions between eT treatments and insect visitation per hour/day, especially between 09:00-10:40. In comparison to the control, the additive effects of combined eCO₂ + eT enhanced the NF by 137%, while the number of A. mellifera floral visits per plot/week increased by 83% during the period of greatest flower production. Although we found no significant effect of treatments on mating system parameters, the overall mean multilocus outcrossing rate (tm = 0.53 ± 0.03) did confirm that S. capitata has a mixed mating system. The effects of elevated [CO₂] and warming on plant-pollinator relationships observed here may have important implications for seed production of tropical forage species in future climate scenarios.