Whether rising atmospheric carbon dioxide (CO 2 ) concentrations will cause forests to grow faster and store more carbon is an open question. Using free air CO 2 release in combination with a canopy crane, we found an immediate and sustained enhancement of carbon flux through 35-meter-tall temperate forest trees when exposed to elevated CO 2 . However, there was no overall stimulation in stem growth and leaf litter production after 4 years. Photosynthetic capacity was not reduced, leaf chemistry changes were minor, and tree species differed in their responses. Although growing vigorously, these trees did not accrete more biomass carbon in stems in response to elevated CO 2 , thus challenging projections of growth responses derived from tests with smaller trees.