One of our planet’s best buffers against the impacts of climate change are trees. Forests pull massive amounts of carbon dioxide from the atmosphere and store the carbon in their woody tissues. But a severe drought can slow a forest’s growth for up to 4 years afterward. And during this rebound period, they’re less effective as carbon sinks, according to findings published in Science this week.

Models trying to capture the impacts of more frequent, intense climate extremes typically assume that vegetation recovers both quickly and completely from extreme drought. But is this actually the case? "If forests are not as good at taking up carbon dioxide, this means climate change would speed up," Princeton’s William Anderegg says in a statement.

He and colleagues examined the recovery of tree stem growth after severe drought events at 1,338 forest sites across the planet using records dating back to the 1940s from the International Tree Ring Data Bank. Tree ring growth helps researchers to keep track of all sorts of changes in the tree’s environmental conditions, such as rising temperatures. The broader the area between two rings, Science explains, the more productive the tree, the more carbon it stores. 

After a severe drought, trees took between one and four years to fully recover – they call this “legacy effects.” This was the case across many different species spanning a wide geographic range, though drier ecosystems were harder hit. Growth was about 9% slower than expected during the first year and about 5% slower in the following year. 

Over the span of a century, carbon storage capacity in semi-arid ecosystems would drop by about 1.6 metric gigatons. That’s the equivalent of one fourth of emissions in the U.S. in a year.

Then, the team compared these observations to simulated results from various climate-vegetation models for the same areas. In stark contrast, the models showed minimal legacy effects after droughts. The discrepancy between observed and simulated data suggests that some of our tools might not be accurately predicting the impacts of droughts on the world’s carbon budget. "In most of our current models of ecosystems and climate, drought effects on forests switch on and off like a light. When drought conditions go away, the models assume a forest's recovery is complete and close to immediate," Anderegg adds. "That's not how the real world works."

Exactly how drought causes this long-lasting harm is unclear. It may have to do with the loss of foliage and carbohydrate reserves, damage to tissues vital for water transport, or simply succumbing to pests and diseases because of drought-induced stresses. 

Images: Stressed forests in southwestern U.S. Leander Anderegg.