Despite the wide agreement that increased plant biomass accumulation under elevated CO2 concentrations (e[CO2]) might play a key role in climate change, the effect of e[CO2] on plant biomass levels remains a major uncertainty in climate models. In the review associated with this dataset, we discuss the evidence for increased biomass levels under e[CO2] across multiple levels of ecological organization, scaling from physiological responses to changes in population-, community-, ecosystem-, and global-scale dynamics. We find that evidence for a sustained biomass response to e[CO2] varies across ecological scales, leading to diverging conclusions about the responses of individuals, populations, communities, and ecosystems. We identify key research gaps in our understanding of the effect of e[CO2] on plant biomass and highlight the need to integrate knowledge across scales of ecological organization so that large-scale modeling can represent the finer-scale mechanisms needed to constrain our understanding of future terrestrial C storage.