Warming and soil water availability affect plant-flower visitor interactions for Stylosanthes capitata, a tropical...

The reproductive success of a zoophilous plant species depends on biological interaction with pollinators, which involves both the provision and exploitation of flower resources. Currently, there is little information about how future climate change scenarios will impact interactions between plants and their flower visitors in the tropics. This study analyzes the effects of warming and two soil water conditions on interactions between the tropical forage legume species Stylosanthes capitata and its floral visitors during the flowering period. We used a temperature-free air-controlled enhancement (T-FACE) facility to simulate future warming scenarios by increasing canopy temperature. The tested treatments were: irrigated and ambient canopy temperature (Control); non-irrigated and ambient canopy temperature (wS); irrigated and elevated canopy temperature (eT, +2 °C above ambient canopy temperature); and non-irrigated and elevated canopy temperature (wSeT). The effects of treatments on the time of flower opening and closing, sugar concentration in the nectar, and plant-flower visitor interactions were assessed. In the warmed treatments, S. capitata flower opening occurred ~45 min earlier compared to non-warmed treatments, and flowers remained opened for only ~3 h. Further, the sugar concentration in the nectar from eT was 39% higher than in the Control. The effects of warming on floral biology and flower resource production in S. capitata had an impact on the plant-floral visitor relationships with the bees Apis mellifera and Paratrigona lineata, the most abundant potential pollinating floral visitors, and the butterfly visitor Hemiargus hanno. Additionally, around noon, the interactive and additive effects of the combined wS and eT treatments decreased insect visiting frequency. These results suggest that warming and soil water deficiency could affect flower-visitor interactions and thus the reproductive success of S. capitata in tropical belts.