Low and contrasting impacts of vegetation CO₂ fertilization on global terrestrial runoff over 1982–2010: accounting for aboveground and belowground vegetation–CO₂ effects

• Main Authors: Y. Yang, T. R. McVicar, D. Yang, Y. Zhang, S. Piao, S. Peng, H. E. Beck
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-06-01
• Series: Hydrology and Earth System Sciences
• Online Access: https://hess.copernicus.org/articles/25/3411/2021/hess-25-3411-2021.pdf
• ISSN: 1027-5606; 1607-7938

<p>Elevation in atmospheric carbon dioxide concentration (eCO<span class="inline-formula">₂</span>) affects vegetation water use, with consequent impacts on terrestrial runoff (<span class="inline-formula"><i>Q</i></span>). However, the sign and magnitude of the eCO<span class="inline-formula">₂</span> effect on <span class="inline-formula"><i>Q</i></span> are still contentious. This is partly due to eCO<span class="inline-formula">₂</span>-induced changes in vegetation water use having opposing responses at the leaf scale (i.e., water-saving effect caused by partially stomatal closure) and the canopy scale (i.e., water-consuming induced by foliage cover increase), leading to highly debated conclusions among existing studies. In addition, none of the existing studies explicitly account for eCO<span class="inline-formula">₂</span>-induced changes to plant rooting depth that is overwhelmingly found in experimental observations. Here we develop an analytical ecohydrological framework that includes the effects of eCO<span class="inline-formula">₂</span> on plant leaf, canopy density, and rooting characteristics to attribute changes in <span class="inline-formula"><i>Q</i></span> and to detect the eCO<span class="inline-formula">₂</span> signal on <span class="inline-formula"><i>Q</i></span> via vegetation feedbacks over 1982–2010. Globally, we detect a very small decrease of <span class="inline-formula"><i>Q</i></span> induced by eCO<span class="inline-formula">₂</span> during 1982–2010 (<span class="inline-formula">−</span>1.7 %). Locally, we find a small positive trend (<span class="inline-formula"><i>p</i></span> <span class="inline-formula"><i>&lt;</i></span> 0.01) in the <span class="inline-formula"><i>Q</i></span>–eCO<span class="inline-formula">₂</span> response along a resource availability (<span class="inline-formula"><i>β</i></span>) gradient. Specifically, the <span class="inline-formula"><i>Q</i></span>–eCO<span class="inline-formula">₂</span> response is found to be negative (i.e., eCO<span class="inline-formula">₂</span> reduces <span class="inline-formula"><i>Q</i></span>) in low-<span class="inline-formula"><i>β</i></span> regions (typically dry and/or cold) and gradually changes to a small positive response (i.e., eCO<span class="inline-formula">₂</span> increases <span class="inline-formula"><i>Q</i></span>) in high-<span class="inline-formula"><i>β</i></span> areas (typically warm and humid). Our findings suggest a minor role of eCO<span class="inline-formula">₂</span> on changes in global <span class="inline-formula"><i>Q</i></span> over 1982–2010, yet we highlight that a negative <span class="inline-formula"><i>Q</i></span>–eCO<span class="inline-formula">₂</span> response in semiarid and arid regions may further reduce the limited water resource there.</p>