The abundance of Arctic shrubs is again increasing, apparently driven by a warming climate, the authors said. It is possible we are witnessing the forerunner of another major transition in arctic vegetation.

An increase in shrubbery could have a profound effect on future warming - what climatologists call a positive feedback - because soils store about three times as much carbon as plants.

Shrubs allocate carbon to woody stems that have long turnover times compared with annual roots and ... leaves ... so shrub-dominated tundra is likely to assimilate carbon in a different way and store it for a different length of time than shrub-free tundra, said the Bioscience paper, written by Matthew Sturm of the U.S. Army Cold Regions Research and Engineering Laboratory in Fort Wainwright, Alaska, and seven others.

Since the pool is so large, even a small change could have a big effect on the atmosphere and therefore future climate, said Alan Townsend, a biology professor at the University of Colorado.

Skeptics about global warming have argued there is no polar amplification - that is, the planet's far northern latitudes are not warming faster than other areas. The biological response cited in the paper, however, argues strongly there is significant warming going on there.

Jonathan Overpeck, director of the Institute for the Study of Planet Earth at the University of Arizona in Tucson, wrote in a 1997 paper that Arctic temperatures are now at levels higher than experienced in the past 400 years, or perhaps longer. Other researchers have concluded temperatures now are rising at about 0.5 degrees Celsius per decade - five times faster than the rate of overall global warming.

The evidence for increasing shrub abundance is most comprehensive for northern Alaska, Sturm and colleagues wrote. An extensive comparison of old (1940s) and modern photographs has shown that shrubs there are increasing in size and are colonizing previously shrub-free tundra.

The shrubs also are increasing in western arctic Canada, they continued, but there the change is inferred from the recollections of long-time residents. In central Russia, a transect along the Pechore River has shown a decrease in tundra and a corresponding increase in shrubland, but for the vast tundra regions of Siberia, there are currently no data on which to make an assessment.

The Sturm group added that despite the lack of specific data for those areas, satellite remote sensing studies ... greatly strengthen the case for pan-Arctic expansion of shrubs.

Taken on its own, an increase in shrubs in the tundra might not make so strong a case, but the news comes in concert with other arctic data that make polar warming a more certain thing.

The recent Arctic Climate Impact Assessment documented a number of changes in the region, including:

- a reduction in the extent and thickness of sea ice;

- the retreat of glaciers;

- increased discharge from northward flowing rivers, and

- an arctic-wide increase in permafrost temperatures.

In addition to the issue of carbon sequestration in the tundra, the shift to shrubs could have other, potentially serious impacts. For instance, it could reduce the quantity and quality of forage plants for the caribou herds, which prefer lichens and graminoids, or grass-like plants, over shrubs.

An extensive shrub canopy would also increase summer sensible heat flux, perhaps by as much as 6 watts per meter square, the paper said, more than twice the global impact of greenhouse gases, which are estimated to be approximately 3 watts per meter square.

Darker shrubs also would cause a decrease in surface reflectivity, or albedo, meaning the surface absorbs more solar heat and reflects less back into space.

In general, the expansion of woody vegetation is thought to be a result of climate warming, Gordon Bonan, a climatologist at the National Center for Atmospheric Research in Boulder, told UPI's Climate.

I've looked in the past at how the expansion of woody vegetation into arctic tundra comes about. It can actually feed back to warm the climate further. Trees and shrubs protrude above the snow and this reduces the snow albedo, Bonan said. More sunlight is absorbed and this warms up the climate.

The question to date has been mostly hypothetical, Bonan said. In the computer simulations he has run, climate has changed the treeline between forest and tundra in the past, he said.

I can't say whether that is changing right now. From the fossil record we know that it has changed in the past. Whether that's actually occurring right now is a completely different problem, Bonan added.

The data presented by Sturm, though not conclusive, indicates the change is occurring to some extent.

Critics may be quick to point out vegetation changes have occurred in the past in the arctic. As recently as 8,000 years ago, forests grew along its coasts, and 9,000 years ago the area changed from a grassland to a tundra ecosystem, resulting in the migration or extinction of Pleistocene giant mammals, such as the mammoth.

A report issued by the National Wildlife Federation last week indicated other vegetation-related effects in the northern latitudes.

Changes in forest dynamics due to disease and insects are very likely, the report said.

In conjunction with rapid arctic warming from 1992 through 1996, it noted, a sustained outbreak of spruce bark beetles has caused over 2.3 million acres' worth of tree mortality in Alaska.

This was the largest loss to spruce bark beetles ever recorded in North America, the report said.

Climate is a weekly series by UPI examining the potential impact of global climate change, by veteran environmental reporter Dan Whipple. E-mail [email protected]

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