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\begin{document}

\title{Estimating the CO2 fertilization effect on extratropical forest
productivity from Flux-tower observations}



\author[1,2]{Chunhui Zhan}%
\author[1,3]{Rene Orth}%
\author[1]{Hui Yang}%
\author[4]{Markus Reichstein}%
\author[1]{Soenke Zaehle}%
\author[5]{Martin G De Kauwe}%
\author[2]{Anja Rammig}%
\author[1]{Alexander J. Winkler}%
\affil[1]{Max Planck Institute for Biogeochemistry}%
\affil[2]{Technical University Munich}%
\affil[3]{University of Freiburg}%
\affil[4]{Max Planck Institute fur Biogeochemistry}%
\affil[5]{University of Bristol}%


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  \date{May 25, 2024}


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\selectlanguage{english}
\begin{abstract}
The land sink of anthropogenic carbon emissions, a crucial component of
mitigating climate change, is primarily attributed to the CO2
\selectlanguage{english}fertilization effect on global gross primary productivity (GPP).
However, direct observational evidence of this effect remains scarce,
hampered by challenges in disentangling the CO2 \selectlanguage{english}fertilization effect
from other long-term drivers, particularly climatic changes. Here, we
introduce a novel statistical approach to separate the CO2 \selectlanguage{english}fertilization
effect on GPP and daily maximum net ecosystem production (NEPmax) using
eddy covariance records across 38 extratropical forest sites. We find
the median stimulation rate of GPP and NEPmax to be 16.4 \selectlanguage{ngerman}± 4\% and 17.2
± 4\% per 100 ppm increase in atmospheric CO\selectlanguage{english}2 across these sites,
respectively. To validate the robustness of our findings, we test our
statistical method using factorial simulations of an ensemble of
process-based land surface models. We acknowledge that additional
factors, including nitrogen deposition and land management, may impact
plant productivity, potentially confounding the attribution to the CO2
\selectlanguage{english}fertilization effect. Assuming these site-specific effects offset to
some extent across sites as random factors, the estimated median value
still reflects the strength of the CO2 \selectlanguage{english}fertilization effect. However,
disentanglement of these long-term effects, often inseparable by
timescale, requires further causal research. Our study provides direct
evidence that the photosynthetic stimulation is maintained under
long-term CO2 \selectlanguage{english}fertilization across multiple eddy covariance sites. Such
observation-based quantification is key to constraining the
long-standing uncertainties in the land carbon cycle under rising CO2
\selectlanguage{english}concentrations.%
\end{abstract}%



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\textbf{Hosted file}

\verb`979646_0_supp_11594875_s469tn.docx`  available at \url{https://authorea.com/users/528109/articles/687595-estimating-the-co2-fertilization-effect-on-extratropical-forest-productivity-from-flux-tower-observations}

\selectlanguage{english}
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