Fire, climate change, and the reorganization of Arctic ecosystems

Alaskan nature writer Bill Sherwonit reports on Yale Environment 360 about the complex response of Arctic ecosystems to climate change in how Arctic Tundra is Being Lost As Far North Quickly Warms:

Researchers have known for years that the Arctic landscape is being transformed by rising temperatures. Now, scientists are amassing growing evidence that major events precipitated by warming — such as fires and the collapse of slopes caused by melting permafrost — are leading to the loss of tundra in the Arctic. The cold, dry, and treeless ecosystem — characterized by an extremely short growing season; underlying layers of frozen soil, or permafrost; and grasses, sedges, mosses, lichens, and berry plants — will eventually be replaced by shrub lands and even boreal forest, scientists forecast.

Much of the Arctic has experienced temperature increases of 3 to 5 degrees F in the past half-century and could see temperatures soar 10 degrees F above pre-industrial levels by 2100. University of Vermont professor Breck Bowden, a watershed specialist participating in a long-term study of the Alaskan tundra, said that such rapidly rising temperatures will mean that the “tundra as we imagine it today will largely be gone throughout the Arctic. It may take longer than 50 or even 100 years, but the inevitable direction is toward boreal forest or something like it.”

… In the course of studying caribou, Joly has also learned a great deal about the role of fire in “low,” or sub-Arctic, tundra, where for several decades at least it has been a much more significant factor than on the North Slope’s “high Arctic” landscape. About 9 percent of Alaska’s lower latitude tundra burned between 1950 and 2007, whereas only 7 percent of the North Slope caught fire during that period. That could change as the region warms and fires become more frequent farther north.

Joly’s and Jandt’s overlapping interests and study areas have prompted them to jointly analyze several interrelated factors — fires, caribou foraging, global warming, and shrub expansion — that appear to be acting “unidirectionally” to reduce lichen cover in northwest Alaska, a change likely to have substantial ripple effects.

Jandt’s fascination with tundra wildfires has now led her northeast, to the Anaktuvuk River burn, where in 2008 she and several research partners established transects to study burn severity, plant community shifts, and the effects of fire on permafrost and active soil layers. Among the team’s observations so far: virtually no lichen cover remains in the burn area; willows have begun re-sprouting, and by 2009 some were already more than a foot tall; and tundra slumping and collapse has occurred.

Many other scientists have also begun studies in the Anaktuvuk burn, which has become part of the Arctic Long Term Ecological Research (LTER) program. Based at the Toolik Field Station, in the Brooks Range’s northern foothills, the LTER study is funded by the National Science Foundation. Among the many researchers working out of Toolik is Mack, who characterized the tundra as “a very resilient plant community.” But for all its resilience, Mack wonders if North Slope tundra is headed for substantial and perhaps irrevocable change, particularly if “fire starts the ecosystem on a new trajectory,” as has happened on the Seward Peninsula.

…Other scientists are looking at a different, but related, extreme Arctic event: the development of “thermokarst failures” within the burn area and in other parts of the North Slope. …  A thermokarst is uneven terrain produced by thawing permafrost, and recent studies have revealed many more thermokarst features than anyone expected. On flat ground, that may mean bumps and hollows. But on sloping ground, it can create huge slumps in which tons of soil move downhill. Such thermokarst failures can lead to high carbon dioxide and methane emissions from newly exposed and thawing soils, thus contributing to atmospheric warming. The thermokarsts provide niches for new plants and shrubs, which add to the “greening” of the tundra and warming of the soil, which in turn favors even more shrub growth. And in moving huge amounts of sediments and nutrients, thermokarsts can have “enormous impacts” on tundra streams and lakes, Bowden says.

Anything that warms tundra and thaws permafrost — from fires to milder annual temperatures and increased rainfall, particularly in winter — can contribute to thermokarst failures. Because climate change models predict a warmer and wetter Arctic with increased summer thunderstorms and lightning, thermokarsts are likely to occur on an ever-larger scale.

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