article published yes Simone Fatichi author cf8e546b-a9b0-11f0-a43b-aa89ed1b56d6 Sebastian Leuzinger author Athanasios Paschalis author J. Adam Langley author Alicia Donnellan Barraclough author Mark J. Hovenden author Increasing concentrations of atmospheric carbon dioxide are expected to affect carbon assimilation and evapotranspiration (ET), ultimately driving changes in plant growth, hydrology, and the global carbon balance. Direct leaf biochemical effects have been widely investigated, whereas indirect effects, although documented, elude explicit quantification in experiments. Here, we used a mechanistic model to investigate the relative contributions of direct (through carbon assimilation) and indirect (via soil moisture savings due to stomatal closure, and changes in leaf area index) effects of elevated CO2 across a variety of ecosystems. We specifically determined which ecosystems and climatic conditions maximize the indirect effects of elevated CO2. The simulations suggest that the indirect effects of elevated CO2 on net primary productivity are large and variable, ranging from less than 10% to more than 100% of the size of direct effects. For ET, indirect effects were, on average, 65% of the size of direct effects. Indirect effects tended to be considerably larger in water-limited ecosystems. As a consequence, the total CO2 effect had a significant, inverse relationship with the wetness index and was directly related to vapor pressure deficit. These results have major implications for our understanding of the CO2 response of ecosystems and for global projections of CO2 fertilization, because, although direct effects are typically understood and easily reproducible in models, simulations of indirect effects are far more challenging and difficult to constrain. Our findings also provide an explanation for the discrepancies between experiments in the total CO2 effect on net primary productivity. National Academy of Sciences2016 eng carbon dioxidemodelingFACEsoil moistureevapotranspiration 0027-8424 1091-6490 2779107410.1073/pnas.1605036113 1134512757-12762 yes Fatichi S, Leuzinger S, Paschalis A, Langley JA, Donnellan Barraclough A, Hovenden MJ. Partitioning direct and indirect effects reveals the response of water-limited ecosystems to elevated CO. <i>Proceedings of the National Academy of Sciences</i>. 2016;113(45):12757-12762. doi:<a href="https://doi.org/10.1073/pnas.1605036113">10.1073/pnas.1605036113</a> Fatichi S, Leuzinger S, Paschalis A, Langley JA, Donnellan Barraclough A, Hovenden MJ. 2016. Partitioning direct and indirect effects reveals the response of water-limited ecosystems to elevated CO. Proceedings of the National Academy of Sciences. 113(45), 12757–12762. Fatichi, Simone, Sebastian Leuzinger, Athanasios Paschalis, J. Adam Langley, Alicia Donnellan Barraclough, and Mark J. Hovenden. “Partitioning Direct and Indirect Effects Reveals the Response of Water-Limited Ecosystems to Elevated CO.” <i>Proceedings of the National Academy of Sciences</i>. National Academy of Sciences, 2016. <a href="https://doi.org/10.1073/pnas.1605036113">https://doi.org/10.1073/pnas.1605036113</a>. Fatichi, S., Leuzinger, S., Paschalis, A., Langley, J. A., Donnellan Barraclough, A., &#38; Hovenden, M. J. (2016). Partitioning direct and indirect effects reveals the response of water-limited ecosystems to elevated CO. <i>Proceedings of the National Academy of Sciences</i>. National Academy of Sciences. <a href="https://doi.org/10.1073/pnas.1605036113">https://doi.org/10.1073/pnas.1605036113</a> S. Fatichi, S. Leuzinger, A. Paschalis, J. A. Langley, A. Donnellan Barraclough, and M. J. Hovenden, “Partitioning direct and indirect effects reveals the response of water-limited ecosystems to elevated CO,” <i>Proceedings of the National Academy of Sciences</i>, vol. 113, no. 45. National Academy of Sciences, pp. 12757–12762, 2016. Fatichi, Simone, et al. “Partitioning Direct and Indirect Effects Reveals the Response of Water-Limited Ecosystems to Elevated CO.” <i>Proceedings of the National Academy of Sciences</i>, vol. 113, no. 45, National Academy of Sciences, 2016, pp. 12757–62, doi:<a href="https://doi.org/10.1073/pnas.1605036113">10.1073/pnas.1605036113</a>. S. Fatichi, S. Leuzinger, A. Paschalis, J.A. Langley, A. Donnellan Barraclough, M.J. Hovenden, Proceedings of the National Academy of Sciences 113 (2016) 12757–12762. 225362026-07-27T12:30:24Z2026-08-06T08:08:46Z