AbstractClimate warming is currently advancing spring leaf‐out of temperate and boreal trees, enhancing net primary productivity (NPP) of forests. However, it remains unclear whether this trend will continue, preventing for accurate projections of ecosystem functioning and climate feedbacks. Several ecophysiological mechanisms have been proposed to regulate the timing of leaf emergence in response to changing environmental cues, but the relative importance of those mechanisms remains unclear. Here, we use 727,401 direct phenological observations of common European forest trees to examine the dominant controls on leaf‐out. Using the emerging mechanisms, we forecast future trajectories of spring arrival and evaluate the consequences for forest carbon dynamics. By representing hypothesized relationships with autumn temperature, winter chilling, and the timing of spring onset, we accurately predicted reductions in the advance of leaf‐out. There was a strong consensus between our empirical model and existing process‐based models, revealing that the advance in leaf‐out will not exceed 2 weeks over the rest of the century. We further estimate that, under a ‘business‐as‐usual’ climate scenario, earlier spring arrival will enhance NPP of temperate and boreal forests by ~0.2 Gt per year at the end of the century. In contrast, previous estimates based on a simple degree‐day model range around 0.8 Gt. As such, the expected NPP is drastically reduced in our updated model relative to previous estimates—by a total of ~25 Gt over the rest of the century. These findings reveal important environmental constraints on the productivity of broad‐leaved deciduous trees and highlight that shifting spring phenology is unlikely to slow the rate of warming by offsetting anthropogenic carbon emissions.