Emergence of the physiological effects of elevated CO 2 on land-atmosphere exchange of carbon and water.

Elevated atmospheric CO 2 (eCO 2 ) influences the carbon assimilation rate and stomatal conductance of plants, thereby affecting the global cycles of carbon and water. Yet, the detection of these physiological effects of eCO 2 in observational data remains challenging, because natural variations and confounding factors (e.g., warming) can overshadow the eCO 2 effects in observational data of real-world ecosystems. In this study, we aim at developing a method to detect the emergence of the physiological CO 2 effects on various variables related to carbon and water fluxes. We mimic the observational setting in ecosystems using a comprehensive process-based land surface model QUINCY to simulate the leaf-level effects of increasing atmospheric CO 2 concentrations and their century-long propagation through the terrestrial carbon and water cycles across different climate regimes and biomes. We then develop a statistical method based on the signal-to-noise ratio to detect the emergence of the eCO 2 effects. The eCO 2 effect on gross primary productivity (GPP) emerges at relatively low CO 2 increase (∆[CO 2 ] ~ 20 ppm) where the leaf area index is relatively high. Compared to GPP, the eCO 2 effect causing reduced transpiration water flux (normalized to leaf area) emerges only at relatively high CO 2 increase (∆[CO 2 ] >> 40 ppm), due to the high sensitivity to climate variability and thus lower signal-to-noise ratio. In general, the response to eCO 2 is detectable earlier for variables related to the carbon cycle than the water cycle, when plant productivity is not limited by climatic constraints, and stronger in forest-dominated rather than in grass-dominated ecosystems. Our results provide a step toward when and where we expect to detect physiological CO 2 effects in in-situ flux measurements, how to detect them and encourage future efforts to improve the understanding and quantification of these effects in observations of terrestrial carbon and water dynamics.