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The Long Lasting Legacy of Droughts in Forests

New research shows forests are slow to recover from droughts, and climate models that overlook recovery time are likely overestimating how much carbon forests can absorb after droughts.
Stressed forests in the southwestern United States. (Photo: Leander Anderegg)

Stressed forests in the southwestern United States. (Photo: Leander Anderegg)

As California grinds through its fourth year of severe drought, a strong El Niño is expected to bring at least some relief to the state. Unfortunately, it might not make much of a difference: New research finds that forests can take years to recover from the damage droughts inflict.

In the study, published today in Science, researchers looked at 1,338 forest sites where droughts occurred within the last half century, and combed through tree growth measurements in the International Tree Ring Data Bank to find out how long it took each site to recover.

The team found that most trees took two to four years to bounce back to pre-drought growth levels. These "legacy effects" were most pronounced in dry forests, like those of the southwestern United States. But the team also found that this recovery time is almost always overlooked in current climate models.

"The future of forests, and whether they continue to take up carbon, is really uncertain."

"In almost all of our mathematical models of ecosystems and forests, drought is generally treated like a light switch," says William Anderegg, an assistant professor at the University of Utah, and lead author on the study: "When drought stress is present, the switch is flipped and trees grow more slowly and die more often, but as soon as the drought disappears, the switch is flipped back the other way." In other words, drought recovery is seen as immediate and complete, partly because models are, by definition, simplified versions of the real world, but also because scientists are just beginning to understand these damage processes.

"These legacy effects have to be driven by damage ... and the primary way we think that this damage occurs is to the piping systems that move water through trees," Anderegg says. During drought, parched soil and dry air tug the water inside trees' hydraulic piping in opposite directions. Over time, Anderegg thinks, the tension weakens the pipes, allowing in air bubbles damaging to the plant's cells.

Now that researchers are illuminating the mechanisms of drought damage, Anderegg believes these legacy effects could be incorporated into climate models within a year or two—an important step if climate models are to truly capture the effect of drought on the carbon cycle.

The study found these legacy effects could cause some forests to take up 1.6 metric gigatons less carbon over the next century—a quarter of the carbon emissions the U.S. churns out each year—and Anderegg cautions, "that number is almost certainly very low."

"The big picture concern here, especially for carbon, is that the future of forests, and whether they continue to take up carbon, is really uncertain," he says. Forests soak up roughly 25 percent of human carbon emissions, but some models suggest that by the end of the century the world's forests could flip from a critical carbon sink to a major source.

The frequency and severity of droughts are only expected to increase, Anderegg says, and once climate models take recovery time into account, we may find that future forests will harbor even less carbon than current models predict.

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