In the 2011 Arctic Report Card,
scientists concluded that the Arctic environment has undergone a
fundamental shift in conditions, and that the Arctic of recent
years—warmer, greener, less icy—is likely to be the new normal for the
Far North. One sign of the ongoing transformation of the Arctic is the
spread of shrubs across the tundra.
A black and
white photo captured on film by the Gambit satellite in 1966 (left) shows much
more open tundra than a view of the same area in 2009 (right). Gambit image (large version) courtesy the U.S. Geological
Survey. GeoEye-1 image (large version) copyright GeoEye, Inc.,
provided through the NGA Commercial Imagery Program.
The pair of images above shows a site on the Siberian tundra near
Russia’s Yenisey River in the summers of 1966 (left) and 2009 (right).
In the 43 years that passed between the first image and the second,
shrubs colonized virtually all of the previously open tundra surrounding
a cluster of lakes.
Gerald Frost, a Ph.D. student at the University of Virginia, is
studying the site to understand how ground characteristics like soil
type and depth to permafrost interact with the changes in climate that
are driving the greening of the Arctic.
Frost is working in collaboration with his advisor, Howard Epstein at
the University of Virginia, and Donald Walker at the University of
Alaska.
At the site Frost studies, the tundra is often patterned with bald
spots—circles of bare ground where seasonal frost heave can uproot plant
seedlings. These frost circles, sometimes called “frost boils,” give
the tundra in the top center of the images its speckled look. The bare
spots create an open canvas for shrubs to colonize, presuming they can
withstand the seasonal frost heave. At this site, the colonizing shrubs
are usually alders.
Created by
seasonal frost heave, circles of bare ground create an open canvas for
colonizing shrubs on the Siberian tundra. Photo courtesy Gerald Frost.
Alders have a competitive advantage over other low-growing tundra
vegetation. They are fast growers, and so alder seedlings more quickly
develop roots that are deep enough to withstand frost heave. In
addition, alder roots harbor microorganisms that provide the plant with
biologically useful forms of nitrogen, an essential nutrient that most
plants must draw from organically rich soil. Alders at the Yenisey River
site shown int he satellite images have taken maximum advantage of the
frost circles and recent decades’ warmer summers: very little open
tundra remained at the site in the 2009 image.
The conversion of tundra to dense, tall shrubland triggers a cascade
of changes in how the ecosystem functions. Observations from Europe,
Alaska, and Siberia in recent decades have shown plant communities
became less diverse as mosses, lichens, and other shorter-growing plants
disappeared under the shade created by shrubs. The loss of lichens, in
particular, could pose a problem for caribou and reindeer, which forage
on them extensively.
The change from tundra to shrubland can also affect the thawing of
permafrost. In the winter, the shrubs trap snow, and the insulating
effect can make soil temperatures beneath shrubs up to 30 degrees
Celsius warmer than the air temperature. In the summer, though, the
shrubs provide shade, which tends to keep soil temperatures in
shrub-covered areas cooler than those in open tundra.
Expanses of
tundra in the northwest Siberian Low Arctic are being colonized by alders,
which take advantage of frost circles and recent decades’ warmer summers. Photo
courtesy Gerald Frost.
Whether the arrival of shrubs at a site will accelerate permafrost
melting or slow it will depend on whether the summer cooling or the
winter warming is stronger. The outcome of the competing influences has
global implications, as thawing permafrost can become another source of
atmospheric carbon dioxide—an important greenhouse gas.
Frost’s work in Siberia was funded by
NASA’s Land-Cover and Land-Use Change program and the National Science
Foundation’s Greening of the Arctic project.
References
Frost, G.V., Epstein, H.E., Walker, D.A., Matyshak, G., and Ermokhina, K. (2011). Patterned-ground facilitates conversion of tundra to shrubland in the northwest Siberian Low Arctic. Manuscript in progress.
Frost, G.V., Epstein, H.E., Walker, D.A., Matyshak, G., and Ermokhina, K. (2011). Patterned-ground facilitates conversion of tundra to shrubland in the northwest Siberian Low Arctic. Manuscript in progress.
Myers-Smith, I. H., Forbes, B. C., Wilmking, M., Hallinger, M.,
Lantz, T., Blok, D., Tape, K. D., et al. (2011). Shrub expansion in
tundra ecosystems: dynamics, impacts and research priorities. Environmental Research Letters, 6(4), 045509. doi:10.1088/1748-9326/6/4/045509.
Schuur, E. A. G., Vogel, J. G., Crummer, K. G., Lee, H., Sickman, J.
O., & Osterkamp, T. E. (2009). The effect of permafrost thaw on old
carbon release and net carbon exchange from tundra. Nature, 459(7246),
556-559. doi:10.1038/nature08031
Related
Arctic Tundra Growing Greener from Warmer Summers
Shrubbery on the March in Québec
2011 Arctic Report Card: Permafrost
2011 Arctic Report Card: Caribou and Reindeer
Arctic Tundra Growing Greener from Warmer Summers
Shrubbery on the March in Québec
2011 Arctic Report Card: Permafrost
2011 Arctic Report Card: Caribou and Reindeer
Science reviewers: Gerald Frost and Howard Epstein.
ANAΔΗΜΟΣΙΕΥΣΗ ΑΠΟ
http://www.climatewatch.noaa.gov
Σχόλια