Michelle Murray

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.

Stomatal conductance (g_s) in terrestrial vegetation regulates the uptake of atmospheric carbon dioxide for photosynthesis and water loss through transpiration, closely linking the biosphere and atmosphere and influencing climate. Yet, the range and pattern of g_s in plants from natural ecosystems across broad geographic, climatic, and taxonomic ranges remains poorly quantified. Furthermore, attempts to characterize g_s on such scales have predominantly relied upon meta-analyses compiling data from many different studies. This approach may be inherently problematic as it combines data collected using unstandardized protocols, sometimes over decadal time spans, and from different habitat groups. Using a standardized protocol, we measured leaf-level g_s using porometry in 218 C₃ woody angiosperm species in natural ecosystems representing seven bioclimatic zones. The resulting dataset of 4273 g_s measurements, which we call STraits (Stomatal Traits), was used to determine patterns in maximum g_s (g_smax) across bioclimatic zones and whether there was similarity in the mean g_smax of C3 woody angiosperms across ecosystem types. We also tested for differential g_smax in two broadly defined habitat groups – open-canopy and understory-subcanopy – within and across bioclimatic zones. We found strong convergence in mean g_smax of C3 woody angiosperms in the understory-subcanopy habitats across six bioclimatic zones, but not in open-canopy habitats. Mean g_smax in open-canopy habitats (266 ± 100 mmol m^-2 s^-1) was significantly higher than in understory-subcanopy habitats (233 ± 86 mmol m^-2 s^-1). There was also a central tendency in the overall dataset to operate toward a g_smax of ∼250 mmol m^-2 s^-1. We suggest that the observed convergence in mean g_smax of C3 woody angiosperms in the understory-subcanopy is due to a buffering of g_smax against macroclimate effects which will lead to differential response of C3 woody angiosperm vegetation in these two habitats to future global change. Therefore, it will be important for future studies of g_smax to categorize vegetation according to habitat group.