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Trees in tropical forests grow more slowly in years when the nights are warmer than average or dry-season days are unusually hot, according to a 21-year study. This suggests such forests will grow less as the world warms due to climate change 鈥� potentially taking up less carbon dioxide from the air and exacerbating warming.

鈥淔or the first time, we have a window on what a whole tropical forest is doing,鈥� says at the University of Missouri-St Louis. 鈥淚t is very scary.鈥�

Tropical forests contain an enormous amount of carbon, because the trees take in CO鈧� from聽the air and use it to grow. Droughts, which are becoming more severe due to climate change, may harm the forests and release some of the stored carbon.

For over two decades, Clark and聽her husband , also at the University of Missouri-St Louis, lived at La Selva Biological Station in Costa Rica. From 1997 to聽2018, they took detailed measurements of the surrounding tropical forest, tracking how much new wood was produced and the amount of leaves and other material that fell as litter.

The pair, with Steven Oberbauer at Florida International University, discovered that wood production fell in years with warmer nights 鈥� something previous studies had already suggested. The team also found that the trees produced less wood in years when temperatures in the dry season exceeded 28掳C more often than usual.

While this hadn鈥檛 previously been shown, Deborah Clark says聽it聽was expected because photosynthesis dramatically slows above this temperature.

The study adds to growing evidence of climate impacts on tropical forests, says Iain Hartley at the University of Exeter in the UK. 鈥淚t looks like, as you warm the聽climate, you stress the plants and you reduce productivity. If聽you link that to drought as well, these ecosystems that have been helping us in the fight against climate change are going to have less ability to do that in the future.鈥�

He and his colleagues have studied another natural store of carbon, the soil. There is evidence that warmer temperatures will cause soils to release more greenhouse gas, as the heat activates microorganisms that decompose organic matter.

Hartley鈥檚 team compared more than 9000 soil samples and found that, for the same temperature rise, coarse-grained soils lost more than three times as much carbon as fine-grained soils.

Soils in the tropics are mostly very fine-grained, says Hartley, so they ought to be relatively resilient to warming. Soils in temperate and polar regions are coarser and more likely to release their carbon.

To find out even more about how tropical forests will respond, Hartley is collaborating on a new聽project called AmazonFACE. This involves pumping controlled volumes of CO鈧� into patches of the聽Amazon rainforest to see how聽plants fare in simulated conditions of the year 2050.