When Scott Saleska first began studying the Amazon rainforest, he knew the critical role that forests play in the health of the planet. As trees in the forest photosynthesize, they produce life-giving oxygen and pull in carbon dioxide—including some of what humans pump into the atmosphere as we burn fossil fuels. But the trees’ rate of photosynthesis isn’t constant. Before launching their nearly two-decade-long study, Saleska and his colleagues assumed—based on research in temperate forests and on ecological models—that photosynthesis would peak during the wet season, when the rainforest appears to be at its lushest. When the scientists looked closely, though, they found just the opposite: The rainforest, at least in in north-central Brazil’s Tapajós National Forest, is most productive when conditions are driest. To understand why, and what these findings might mean for the health of the forest in a changing climate, the researchers have studied the ecosystem at every possible scale—from leaf to landscape.

Despite long periods without rainfall the forest’s five-month dry season is a period of rebirth in the rainforest canopy. Having dropped their old leaves at the end of the wet season (July), trees begin to grow new foliage. By November, leaves less than six months old outnumber older leaves. This shift in leaf “quality,” as scientists describe it, leads to a 27 percent increase in photosynthesis in the dry season compared to the wet season.