The resilience of the Yellowstone forests tested by an unprecedented fire

In August 2016, the areas of Yellowstone National Park burned down in 1988 burned again. Shortly after, in October 2016, ecologist Monica Turner and her team of graduate students visited the park to begin assessing the landscape.

"We saw these areas where everything was burned and we had not seen it before," says Turner, professor of integrative biology at the University of Wisconsin-Madison, who has studied the Yellowstone reaction closely. on fire since 1988. "It was surprising."

In a study published this week in the Proceedings of the National Academy of SciencesTurner and his team describe what happens when Yellowstone, adapted for recurring fires every 100 to 300 years, burns instead twice in less than 30 years. Researchers say that Yellowstone, as we know it, faces an uncertain future, and one of the big questions they hope to answer is whether forests can regenerate.

With funding from Rapid Response Research of the National Science Foundation, Turner and his team returned to Yellowstone in the summer of 2017 to study areas that were burned again. This is the Maple Fire, which burned 28-year-old lodgepole pine trees that regenerated after the 1988 North Fork fire, and the Berry fire, which contained 28-year-old lodgepole pine trees that regenerated. after the fire Huck, in 1988 and after 16 years. old trees that regenerated after the fire of Glade in 2000.

In each zone, they were compared to the burned areas in 1988 or 2000, but were not burned in 2016.

In some areas, the fire has burned so much that only the stumps of young trees remain. Logs once scattered on the forest floor burned, leaving negatives on themselves – ghost shadows – where they had fallen.

"Everything was gone," says Turner. "It was amazing."

Generally, most fire-killed trees remain upright for years. Surface fires leave dead needles on the trees. Crown fires burn needles but leave trunks upright. However, Turner's team on four of the 18 sampled fire plots saw the fire so severe that she had to come up with a new name to describe it: more peak fires. In these, 99% of the previous tree stems burned.

In 2011, the Turner Group's modeling work challenged pre-existing ideas that young forests are running out of fuel, in the form of fallen trees and logs, to withstand severe fires. The 2016 fires confirmed their forecasts.

"The idea was that if the fires recurred more frequently, we would see a self-limitation, young forests will not be able to burn again," says Kristin Braziunas, co-author of the study, graduate student. . "We definitely found that this was not the case – even at the age of 16, there was enough fuel for these forests to burn to the highest possible level of gravity."

The team also found a six-fold decrease in the number of lodgepole pine tree seedlings that recovered during the first year following the 2016 fires. In some patches of reconstituted forest, the Regeneration rates were significantly lower. Dense and young forests have been converted into more sparse forests.

Lodgepole pines are known for their serotinous cones, which are adapted to open fire and release their seeds, restoring the forest with a thicket of new trees once the fire is over. off. Historically, fire intervals of 100 to 300 years allowed trees to mature and build their seed bank.

But young trees have not yet accumulated their savings, so a quick re-burn is like tapping into a bank account before funds are replenished.

The researchers also found that burned forests lost significant carbon storage capacity. Nearly two out of every three logs on the forest floor were consumed during the 2016 fires. These pieces of dead wood were carbon sinks, storing the carbon that the tree took alive. Once burned, they release carbon into the atmosphere.

Turner explains that once an old forest burns, it takes about 90 years for the forest to recover its lost carbon.

"Carbon storage and recovery is important to us because forests play a very important role in the global carbon cycle," said Braziunas, who, before joining Turner's research group, worked for more than seven years. as a municipal firefighter in Oberlin, Ohio.

Braziunas adapted a model previously created by Turner's collaborator, Rupert Seidl, to estimate the time it would take the forest to recover the carbon lost in the atmosphere during the 2016 fires, between the loss of Trees, the consumption of felled wood and the reduced regeneration of trees. density. She found that it would take more than 150 years, assuming that the forests are no longer burning at this time.

"We were able to essentially restore what the forest looked like before the fire, how many trees there were and how big they would have been," said Braziunas. "Since we also measured nearby stands (trees) that were not burning, we could compare what happens after reburning and define the model scenarios."

The estimate, she says, as well as Turner, represents a conservative scenario in the best of cases. With global warming and increased frequency of drought, forests are likely to burn again at short intervals.

However, the forest has long been shown resilient.

"The landscapes will look different from the past," says Turner, "but that does not mean they will not be beautiful, there will be species that will benefit and species that will see their territory contract. "

"The change will happen and the change will happen faster than expected," she added. "We are learning how the system reacts, but we do not know how resilient or adaptable it will be in the future, but I'm not ready to write it. the past."