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

Abstract

Elevation in atmospheric carbon dioxide concentration (eCO2) affects vegetation water use, with consequent impacts on terrestrial runoff (Q). However, the sign and magnitude of the eCO2 effect on Q are still contentious. This is partly due to eCO2-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 eCO2-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 eCO2 on plant leaf, canopy density, and rooting characteristics to attribute changes in Q and to detect the eCO2 signal on Q via vegetation feedbacks over 1982–2010. Globally, we detect a very small decrease of Q induced by eCO2 during 1982–2010 (−1.7 %). Locally, we find a small positive trend (p < 0.01) in the Q–eCO2 response along a resource availability (β) gradient. Specifically, the Q–eCO2 response is found to be negative (i.e., eCO2 reduces Q) in low-β regions (typically dry and/or cold) and gradually changes to a small positive response (i.e., eCO2 increases Q) in high-β areas (typically warm and humid). Our findings suggest a minor role of eCO2 on changes in global Q over 1982–2010, yet we highlight that a negative Q–eCO2 response in semiarid and arid regions may further reduce the limited water resource there.