Although tropical rainforests in the Amazon are green year-round, their ability to photosynthesize is seasonal. During the dry season, scientists have noticed a peculiar rise in the amount of carbon dioxide (CO2) they suck up. The cause of this dry season “greening up” has been debated; many scientists have pinned it on extra sunlight or on drought, which they say could make plants more efficient photosynthesizers. Now, a team of researchers reports that fresh, new-growth leaves may best explain the phenomenon. At four sites in the Amazon, the team used cameras to monitor leaf changes throughout the canopy and CO2-measuring sensors to measure shifts in photosynthesis. They found that during the dry season, old leaves are shed and rapidly give way to a generation of young leaves that are particularly efficient at absorbing CO2, they report today in Science. The finding could transform models that study the interactions of climate and tropical forests, which typically assume a constant canopy greenness throughout the year. The authors say that these models need to incorporate the more realistic seasonal patterns in leaf growth in order understand how tropical forests will respond to a warming world.