Photosynthesis and tree growth respond differently to different climate cues, according to a new study, suggesting that current forest carbon sequestration models may be overestimating forests’ ability to store atmospheric carbon. The findings illustrate the importance of considering processes other than photosynthesis for estimating how much carbon trees can sequester. Through photosynthesis, forests capture and sequester atmospheric carbon as woody biomass and soil carbon. Currently, this process offsets roughly 25% of annual anthropogenic carbon emissions. With elevated atmospheric carbon dioxide (CO2) boosting photosynthesis through a phenomenon known as carbon fertilization, using forests to sequester carbon is often viewed as an attractive natural solution to mitigating climate change. It's been assumed that photosynthesis and plant growth are generally limited by the amount of atmospheric carbon – more carbon, more growth, more storage. However, a growing body of research has indicated that this might not be the case, suggesting that forest carbon storage is sensitive to other factors, including temperature, water, and nutrient availability. This means forest carbon sequestration represents a source of major uncertainty for projections of global forests’ carbon storage potential. To better understand forest carbon uptake and its relationship to woody growth, Antoine Cabon and colleagues used estimates of the amount of carbon taken up by plants during photosynthesis from 78 forests worldwide and compared them to tree-ring growth data from the Tree-Ring Data Bank. Cabon et al. found a strong decoupling between photosynthesis (productivity) and plant growth, with substantial variation based on tree species, ecosystem traits, and climate conditions, indicating that the relationship between the two isn’t as linear as has been assumed. The findings highlight limits to tree growth, particularly in cold and dry areas, which may continue to constrain the carbon storage potential of forests under ongoing climate change. “The results reported by Cabon et al. have implications for using natural ecosystems to sequester carbon and for the success of natural climate solutions, such as planting trees, in combating climate change,” write Julia Green and Trevor Keenan in a related Perspective.