Low and contrasting impacts of vegetation CO 2 fertilization on global terrestrial runoff over 1982–2010: accounting for aboveground and belowground vegetation–CO 2 effects

摘要

Elevation in atmospheric carbon dioxide concentration (eCO 2 ) affects vegetation water use, with consequent impacts on terrestrial runoff ( Q ). However, the sign and magnitude of the eCO 2 effect on Q are still contentious. This is partly due to eCO 2 -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 2 -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 2 on plant leaf, canopy density, and rooting characteristics to attribute changes in Q and to detect the eCO 2 signal on Q via vegetation feedbacks over 1982–2010. Globally, we detect a very small decrease of Q induced by eCO 2 during 1982–2010 ( ? 1.7?%). Locally, we find a small positive trend ( p ? ?0.01) in the Q –eCO 2 response along a resource availability ( β ) gradient. Specifically, the Q –eCO 2 response is found to be negative (i.e., eCO 2 reduces Q ) in low- β regions (typically dry and/or cold) and gradually changes to a small positive response (i.e., eCO 2 increases Q ) in high- β areas (typically warm and humid). Our findings suggest a minor role of eCO 2 on changes in global Q over 1982–2010, yet we highlight that a negative Q –eCO 2 response in semiarid and arid regions may further reduce the limited water resource there.