CHAMPAIGN, Ill. — Some Arctic regions regain their "greenness" within a decade of a sudden permafrost collapse, while others can take a century or more to recover, researchers report in a new study. The difference is directly related to each site's gross primary productivity, a measure of its photosynthetic capacity, the researchers discovered. This finding will allow scientists to accurately predict how long it will take a specific site to recover after a permafrost collapse.
The new findings appear in the journal Nature Climate Change.
The study focused on a phenomenon known as retrogressive thaw slumps, "the sudden, landslide-like features that sometimes occur when permafrost thaws," said Mark Lara , a professor of plant biology at the University of Illinois Urbana-Champaign who led the study with postdoctoral researcher Zhuoxuan (Summer) Xia, and Liu Lin , a professor of earth and environmental sciences at The Chinese University of Hong Kong.
Thaw slumps occur when ground ice melts, suddenly destabilizing parts of the terrain. Individual slumps can affect many acres of territory, and the soil surface can shift or drop by hundreds of feet. Slumps uproot, erase and displace much of the plant life that was there before and cause soil-carbon losses, Lara said.
"Abrupt thaw features like thaw slumps currently affect an estimated 5% of the global permafrost area," amounting to about 905,000 square kilometers (nearly 350,00 square miles) of territory, he said. Understanding how plants reestablish themselves on areas degraded by thaw slumps can help scientists make better predictions about how these events may affect carbon cycling and contribute to — or perhaps even help mitigate — the negative effects of climate change.
For the new study, Xia assessed vegetation greenness in eight slump-affected permafrost regions, including two in Alaska, three in northern and northwestern Canada, one in Siberia and two in the Qinghai-Tibet plateau.
"Our data includes sites distributed across the Arctic and also in the high-mountain region," Xia said.
She relied on decades of satellite data, using red and near-infrared signatures as a proxy for plant productivity. The satellite data allowed her to track the occurrence and consequences of thaw slumps. Uncrewed aerial surveys offered more detail about which plants were recolonizing slumped sites and how quickly they were returning.
The researchers had to rely on limited high-resolution data for slumps that occurred more than a decade ago, Lara said.
"The satellite record only goes back so many years," he said. "But we could look at the type of vegetation that recovered from slumps that occurred in the 1950s, the '60s, the '80s, and then we could associate those types of vegetation with this chronological sequence, which allowed us to see, regionally, how vegetation recovered following disturbance."
By analyzing and comparing the recovery chronology at the eight permafrost sites, Xia discovered that thaw slumps in higher Arctic latitudes and high-elevation regions took much longer to recover their surface greenness than those in the lower latitudes.
"In low-Arctic regions, it can recover within a decade," she said. "In high-Arctic regions, however, it can take as long as decades to 100 years."
The researchers found a direct relationship between a region's gross primary productivity and its recovery time, making it possible to predict how long it would take any site to fully recover its photosynthetic capacity after a thaw slump.
To test the predictive accuracy of their assessments, the team evaluated the slump-related recovery of four additional sites in unrelated permafrost regions. They found that, for highly productive low-Arctic sites, their predictions were accurate to within a year or two.
"The predictions were less certain at sites with lower productivity, where photosynthetic recovery took much longer," Lara said.
One important part of the equation, verified by ground surveys conducted by scientists around the world, is that even if a thaw-slump site recovers its greenness, it is unlikely to quickly go back to its original plant composition and diversity, Lara said. Some plants, like willows, are much better at colonizing disturbed highly productive sites, and so if they are present in the vicinity of a slump they may come in relatively quickly and dominate the site before other plants return.
There is an upside to the rapid increase in woody plants, he said. First, they can stabilize a site and reduce further soil-carbon losses that might occur after a slump. Second, woody plants can potentially take up more carbon from the atmosphere and enhance soil carbon storage more than other plants would.
If confirmed in follow-up studies, "this could potentially alter the carbon budget for hundreds to thousands of thaw-slump sites," Lara said.
"Plants are not going to save us," he said. "The plants can't uptake the amount of carbon that we're releasing from permafrost as it warms and thaws. But plants might be able to offset some of that loss."
Future studies should look more closely at the carbon-uptake capacity of specific Arctic plants, like willows, that may play a beneficial role in a radically changing permafrost landscape, he said.
Lara also is a professor of geography and geographic information science at the U. of I.
The U.S. National Science Foundation, U.S. National Aeronautics and Space Administration, National Key Research and Development Program of China, Hong Kong Research Grants Council and Guangdong Science and Technology Department supported this research.
