Contrasting responses of stomatal conductance and photosynthetic capacity to warming and elevated CO<inf>2</inf> in the tropical tree species Alchornea glandulosa under heatwave conditions

Factorial experiments of combined warming and elevated CO2 are rarely performed but essential for our understanding of plant physiological responses to climate change. Studies of tropical species are particularly lacking, hence we grew juvenile trees of Alchornea glandulosa under conditions of elevated temperature (+1.5 °C, eT) and elevated CO2 (+400ppm, eC) in a factorial open top chamber experiment. We addressed three questions: i) To what extent does stomatal conductance (gs) reduce with eT and eC treatments?; ii) Is there an interactive effect of eT and eC on gs?; iii) Does reduced gs as a result of eT and/or eC cause an increase in leaf temperature?; iv) Do the photosynthetic temperature optima (Topt) and temperature response of photosynthetic capacities (Vcmax, Jmax) shift with higher growth temperatures? The experiment was performed during an anomalously hot period, including a heatwave during the acclimation period. Our key findings are that: 1) the eT treatment reduced gs more than the eC treatment, 2) reduced gs caused an increase in leaf temperatures, and 3) net photosynthesis and photosynthetic capacities showed very high temperature tolerances with no evidence for acclimation to the eT treatment. Our results suggest that A. glandulosa may be able to cope with increases in air temperatures, however reductions in gs may cause higher leaf temperatures beyond those induced by an air temperature rise over the coming century.