Global tree intrinsic water use efficiency is enhanced by increased atmospheric CO 2 and modulated by climate and plant functional types

Significance Changes in tree physiology driven by environmental change can alter the balance of forest ecosystem carbon and water fluxes. We performed a meta-analysis of published tree ring literature, comprising 36 different species across 84 sites globally, to show stimulated leaf photosynthesis, not reduced stomatal conductance, is primarily responsible for recently increasing tree intrinsic water use efficiency (iWUE), which integrates the balance between carbon and water fluxes. Furthermore, we show trends in tree iWUE are similar in magnitude to the increase in atmospheric CO 2 over the 20th century and that climate interacts with CO 2 to modulate tree iWUE. These findings will help to guide efforts of refining the role of forests in process-based models under future environmental change.