Stomata Are Driving the Direction of CO₂-Induced Water-Use Efficiency Gain in Selected Tropical Trees in Fiji

Understanding plant physiological response to a rising atmospheric CO₂ concentration (c_a) is key in predicting Earth system plant-climate feedbacks; however, the effects of long-term rising c_a on plant gas-exchange characteristics in the tropics are largely unknown. Studying this long-term trend using herbarium records is challenging due to specimen trait variation. We assessed the impact of a c_a rise of ~95 ppm (1927-2015) on the intrinsic water-use efficiency (iWUE) and maximum stomatal conductance (g_smax) of five tropical tree species in Fiji using the isotopic composition and stomatal traits of herbarium leaves. Empirical results were compared with simulated values using models that uniquely incorporated the variation in the empirical g_smax responses and species-specific parameterisation. The magnitude of the empirical iWUE and g_smax response was species-specific, ranging from strong to negligible. Stomatal density was more influential than the pore size in determining the g_smax response to c_a. While our simulation results indicated that photosynthesis is the main factor contributing to the iWUE gain, stomata were driving the iWUE trend across the tree species. Generally, a stronger increase in the iWUE was accompanied by a stronger decline in stomatal response. This study demonstrates that the incorporation of variation in the g_smax in simulations is necessary for assessing an individual species' iWUE response to changing c_a.