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\title{Emergence of the physiological effects of elevated CO2 on
land-atmosphere exchange of carbon and water}



\author[1]{Chunhui Zhan}%
\author[1]{René Orth}%
\author[1]{Mirco Migliavacca}%
\author[2]{Sönke Zaehle}%
\author[1]{Markus Reichstein}%
\author[2]{Jan Engel}%
\author[3]{Anja Rammig}%
\author[1]{Alexander J Winkler}%
\affil[1]{Department for Biogeochemical Integration, Max Planck Institute for Biogeochemistry, Jena, Germany}%
\affil[2]{Department of Biogeochemical Signals, Max Planck Institute for Biogeochemistry, Jena, Germany}%
\affil[3]{Land Surface-Atmosphere Interactions, Technical University of Munich, TUM School of Life Sciences Weihenstephan, 85354 Freising, GermanyLand Surface-Atmosphere Interactions}%


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  \date{May 28, 2024}


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\begin{abstract}
Elevated atmospheric CO2 (eCO2) influences the carbon assimilation rate
and stomatal conductance of plants, and thereby can affect the global
cycles of carbon and water. However, the extent to which these
physiological effects of eCO2 influence the land-atmosphere exchange of
carbon and water is uncertain. In this study, we aim at developing a
method to detect the emergence of the physiological CO2 effects on
various variables related to carbon and water fluxes. We use a
comprehensive process-based land surface model QUINCY (QUantifying
Interactions between terrestrial Nutrient CYcles and the climate system)
to simulate the leaf-level effects of increasing atmospheric CO2
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 eCO2 effects. The
signal in gross primary production (GPP) emerges at relatively low eCO2
(\selectlanguage{english}\selectlanguage{greek}Δ{[}CO2{]} \textasciitilde{} 20 ppm) where the leaf area index (LAI) is
relatively high. Compared to GPP, the eCO2 effect causing reduced 28
transpiration water flux (normalized to leaf area) emerges only at
relatively high CO2 increase (\selectlanguage{english}\selectlanguage{greek}Δ{[}CO2{]} \textgreater{}\textgreater{} 40
ppm), due to the high sensitivity to climate variability and thus lower
signal-to-noise ratio. In general, the response to eCO2 is detectable
earlier for variables of 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 towards when and where we expect to detect
physiological CO2 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.%
\end{abstract}%



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