NAU researchers contribute to finding of drought’s lasting impact in forests


In forests around the world, drought leaves a legacy that endures even after the rains return.

Three Northern Arizona University researchers contributed to a study published this week in Science that showed surviving trees took an average of two to four years to recover and resume normal growth rates after droughts ended.

The finding runs counter to climate models that assume instant recovery, said George Koch, a professor in NAU’s Center for Ecosystem Science and Society. Koch focused on project design for the research, while Christopher Schwalm, assistant research professor at NAU, applied his expertise in land atmosphere modeling. Kiona Ogle, who recently moved to NAU from Arizona State University, also contributed to the design and analysis of the study.

“What we’ve found is that recovery takes a while,” Koch said. “For most forested ecosystems, growth following a dry year is slower than expected.” Now Schwalm will help other modeling groups build this nuanced reality into better models, Koch said.

Koch, meanwhile, will conduct research to “understand more of the mechanistic basis of this legacy effect of drought.” He and collaborators will use data they’ve collected over dozens of areas in the Four Corners region.

“We can define drought and study its impacts to trees in more detail when looking at specific regional sites,” Koch said. “This may help us understand why some trees survive while others die. And it will improve predictions of the impacts of future droughts on forest carbon sequestration.”

The implications of the global study relate to climate change and the anticipated increase in drought frequency and duration.

Forest trees play a big role in buffering the impact of human-induced climate change by removing massive amounts of carbon dioxide emissions from the atmosphere and incorporating the carbon into woody tissues. The finding that drought stress sets back tree growth for years suggests that Earth’s forests are capable of storing less carbon than climate models have calculated.

Lead author William R.L. Anderegg at the University of Utah led a team that carefully measured the recovery of tree stem growth after severe droughts since 1948 at more than 1,300 forest sites around the earth using records from the International Tree Ring Data Bank. Tree rings provide a convenient history of wood growth and track carbon uptake of the ecosystem in which the tree grew.

The researchers found that a few forests showed positive effects, that is, observed growth was higher than predicted after drought, most prominently in parts of California and the Mediterranean region. But in the majority of the world’s forests, trees struggled for years after experiencing drought.

On average, trunk growth took 2 to 4 years to return to normal. Growth was about 9 percent slower than expected during the first year of recovery, and remained 5 percent slower in the second year. Long-lasting effects of drought were most prevalent in dry ecosystems, and among pines and tree species with low hydraulic safety margins.