Climate change and Alaska's boreal forests

Global warming spurs changes in AK wildfire regime

More frequent and intense fires documented in Alaska’s interior region, where conifer forests are giving way to deciduous trees

Summit County Citizens Voice

25 July, 2013

FRISCO — Global warming is making some of Alaska’s interior forests more flammable, with wildfire activity higher than at any time in the past 10,0000 years, according to new research funded by the National Science Foundation.

The study documented a dramatic increase in both the frequency and severity of fires in recent decades in a 2,000-square-kilometer zone in the Yukon Flats of interior Alaska — already one of the most flammable high-latitude regions of the world.

The fires are converting the conifer-rich boreal forests of Alaska into deciduous woodlands, the study found. Whether the shift to deciduous forests — which traditionally have been thought to be more fire-resistant — will overcome the fire-inducing effects of a warming climate remains to be seen.

“The evolution of climate is a complicated response to many interacting processes,” said William Wiseman, Arctic Natural Sciences program director. 

“This study develops a theory, using historical data, concerning how boreal forests respond to climate-induced changes in fire regime through a negative feedback process that stabilizes the regional system. Although it presents initial evidence that the present day boreal forests are responding to fire in a related fashion, only time will tell whether this is indeed true,” Wiseman said.

A paper describing the study appears in the Proceedings of the National Academy of Sciences.

“We reconstructed the fire history by picking charcoal fragments out of sediments preserved over thousands of years,” said University of Illinois doctoral student Ryan Kelly, who led the study with Illinois plant biology professor Feng Sheng Hu.

 “And from what we can tell, the fire frequency at present is higher than it has been at any time in the past 10,000 years.”

The researchers looked at the charcoal and pollen content of mud collected from the bottoms of 14 deep lakes in the Yukon Flats.

“We chose this area because today it is one of the most flammable boreal ecoregions of North America,” Hu said. “So we are focusing in a specific area that is highly flammable, and we are focusing on periods of climate fluctuation during the Holocene. We’re trying to figure out what happened in the past to help us to project what may happen in the future.”

The Holocene epoch began about 11,700 years ago and continues to the present.

The team paid close attention to a particularly warm period in the Holocene. This period, called the Medieval Climate Anomaly, occurred roughly 1,000 to 500 years ago. Global temperatures and tree species in the Yukon Flats were similar during the MCA to conditions today.

“This period probably wasn’t really as warm as today, definitely not as warm as it’s bound to get in the future, but may be the most similar to today,” Kelly said. “There was lots of burning, almost as much as today, and the fires were particularly severe.”

The researchers found that the composition of tree species in the Yukon Flats gradually shifted during the MCA–from forests dominated by coniferous trees to woodlands populated by the relatively fire-resistant deciduous trees.

“The same kind of change in tree species is occurring today,” said Kelly. Much of his study area has burned in the last decade, with young deciduous trees now growing where black spruce once stood.

Current wildfire activity in the study area, however, has already surpassed the limit seen during the MCA, Kelly said. The average fire frequency in this region during the last 3,000 years was nine or 10 fire events per 1,000 years. But in the last 50 years, the number of wildfire events has doubled, to almost 20 per 1,000 years, he said.

“That’s like a fire every 50 years, whereas in the past it was closer to a fire every hundred years,” Kelly said.

The findings are notable because boreal forest covers more than 10 percent of the earth’s land surface and contains a vast amount of carbon, primarily in the soil, Kelly said.

“There is more carbon in the boreal forests than in the atmosphere,” he said. “And one of the main ways that the carbon that’s accumulated over thousands of years gets out of the soil is through burning.”

The release of this carbon from fires adds to the greenhouse gases in the atmosphere, potentially leading to additional climate warming, he said.

“The Yukon Flats region appears to be undergoing a transition that is unprecedented in the Holocene epoch,” Hu said. “And the transition may be indicative of what will happen throughout much of the North American boreal forests in the decades to come.”

“Ryan’s study area is already covered by deciduous forest because so much spruce has burned recently–it’s already different than the vast majority of boreal forests,” Hu said. “The climate today appears to be warmer than in the past 10,000 years in that region, and we know that the climate is continuing to warm up.”

As warming continues, Hu said, it’s plausible that even deciduous forests will become highly flammable.

“If it’s dry, it will burn.”

The research team also included scientists from the University of Idaho, the University of Minnesota and the University of Washington.

The burning taiga

Another positive feedback.

Arctic’s Boreal Forests Burning At ‘Unprecedented’ Rate

Climate Central,

22 July, 2013

In a sign of how swiftly and extensively climate change is reshaping the Arctic environment, a new study has found that the region’s mighty boreal forests — stands of mighty spruce, fir, and larch trees that serve as the gateway to the Arctic Circle — have been burning at an unprecedented rate during the past few decades. 

The study, published Monday in the Proceedings of the National Academy of Sciences, found that the boreal forests have not burned at today’s high rates for at least the past 10,000 years, and climate change projections show even more wildfire activity may be to come.

The study links the increase in fire activity to increased temperatures and drier conditions in the region, which is driving wholescale changes in the massive forests that encircle the northern portion of the globe.

Wildfire activity in the boreal forest biome, which is also known as taiga, plays a crucial role in the globe’s carbon budget, since these forests represent nearly 10 percent of the planet’s land surface and contain more than 30 percent of the carbon that is stored on land, in plants and soils. Globally, the boreal forest covers 6.41 million square miles, forming a ring along and just below the Arctic Circle.

Increased burning in recent years has meant that more stored carbon has been freed from these ecosystems, which acts as a feedback, leading to more global warming, and hence more wildfires. In addition, the black carbon, or soot, emitted from the fires can land on snow and ice in the Arctic, hastening melting.

Alaska has seen a significant increase in wildfire activity in recent years, which has been linked to the effects of a warming climate, including warmer, drier summers with greater thunderstorm activity. So far this year, Alaska has seen 451 wildfires (not all of them in the boreal forest), which have burned 1.3 million acres, the most of any state in the country.

The new study found that while global warming is likely to lead to even greater wildfire activity in the coming decades, vegetation changes as a result of such fires may keep a lid on the magnitude of the surge in wildfire activity, as apparently occurred during the so-called “Medieval Warm Period” between about 800 to 1400 AD.

For the study, researchers used charcoal records from 14 lakes in the Yukon Flats of interior Alaska, which is one of the most flammable parts of the boreal forest biome, to infer changes in the wildfire regime during the past 10,000 years. Scientists employ charcoal records as a “proxy” indicator of past wildfire activity, in much the same way that other climate researchers have used tree rings to study drought history.

The researchers found that recent wildfire activity exceeded the range of natural variability during the past 10,000 years, which they attributed to climatic warming during the past few decades and “the legacy effect” of the Little Ice Age, which occurred from about 1350 to 1850 AD, and brought cold and wet conditions to Alaska that encouraged the growth of trees and plants in the boreal forests. Such vegetation is now serving as fuel for wildfires.

“The ecosystems in this ecoregion appear to be undergoing a transition that is unprecedented” in the past 10,000 years, said Feng Sheng Hu, a coauthor of the study and a plant biologist at the University of Illinois. “We think this transition may occur in other boreal regions in the decades to come,” he said via email.

Most climate projections show that wildfire frequency, size, and severity are likely to increase as the northern climate becomes warmer and drier than it is today. Studies show that there may be a fivefold increase in annual area burned during the 21st century in Alaska and Western Canada, for example.

Already, the boreal forest biome has seen some of the most rapid and largest amount of warming of anywhere on earth, with a significant decline in the number of days with extremely cold temperatures, and increases in summertime overnight low temperatures and the length of the frost-free season.

Map with the boreal forest biome, also known as taiga, highlighted in green.
Credit: Wikimedia Commons.

This summer has been exceptionally warm in Alaska, with the state recording its fourth-warmest June on record. Fairbanks has seen more days with a high temperature of 85°  For higher than anytime since records began there in 1930, with an unusually high number of days with a low temperature above 60°F as well.

However, the study suggests that the increase in future burning may not be quite as significant as the projections show, because of vegetation changes set in motion by the fires themselves. According to the study, the tree species that have moved into recently-burned parts of the boreal forest have tended to be less flammable deciduous species. This vegetation change, the study found, could exert a negative feedback on future wildfire severity and frequency. From their charcoal records, the researchers found that this dynamic likely played out during the Medieval Warm Period, when climate conditions were similar to what they are today across interior Alaska.

“. . . Deciduous trees are prevalent in our study area today because of the extensive burning over the past few decades. This vegetation change will probably lead to diminished burning,” Hu told Climate Central. “However, the magnitude of climate warming within this century is projected to be greater than anything that has occurred over the past 10,000 years. So, even deciduous forests could become flammable,” he said.

Although the study did not directly address the carbon cycle, other research has shown that increased forest burning as a result of warming will be a bigger factor driving the release of stored carbon from the boreal forest than the more direct impacts that climate change will have on the carbon cycle, such as through changes in the rate at which soils can absorb atmospheric carbon. And the boreal forest is becoming more flammable at the same time as another key Arctic biome, the tundra, is as well. One massive tundra fire in Alaska in 2007 emitted 2.1 million metric tons of carbon into the atmosphere, about equal to the amount of carbon that the Arctic typically absorbs in a year, according to a 2011 study. 

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