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Forest invades tundra

... and the new tenants could aggravate global warming

By
11:17am, June 20, 2008
Magazine issue: Vol. 174 #1, July 5, 2008
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For the Arctic, green is the new black.

People frequently say “green” to mean “environmentally friendly.” But encroaching conifer forests — really big greens — threaten to further spike the far North’s already low-grade fever.


Temperatures in the high
Arctic already are climbing “at about twice the global average,” notes F. Stuart Chapin of the University of Alaska Fairbanks.

The newest data on the advance of northern, or boreal, forests come from the eastern slopes of
Siberia’s northern Ural Mountains. Here, north of the Arctic Circle, relatively flat mats of compressed, frozen plant matter — tundra — are the norm. This ecosystem hosts a cover of reflective snow most of the year, a feature that helps maintain the region’s chilly temperatures. Throughout the past century, however, leading edges of conifer forests began creeping some 20 to 60 meters up the mountains, and in some places these forests are now overrunning tundra, scientists report in the July Global Change Biology.

Conifers here now reside where no living tree has grown in some 1,000 years, points out one of the authors, ecologist Frank Hagedorn of the Swiss Federal Institute for
Forest, Snow and Landscape Research in Birmensdorf.

Ecologists and climatologists are concerned because emerging forest data suggest that the albedo, or reflectivity, of large regions across the
Arctic will change. Most sunlight hitting snow and ice bounces back into space instead of being absorbed and converted to heat. So if a white landscape becomes open sea or boreal forest, what was once a solar reflector becomes a heat collector.

Sea-surface ice already is melting in the
Arctic, and polar ice sheets are thinning. Warming threatens to further degrade these solar reflectors. So does the advance of boreal forests, Chapin says.

“Effects of vegetative changes will be felt first and most strongly locally — in the
Arctic,” he says. However, he adds, if the Arctic’s albedo drops broadly, this could aggravate warming underway elsewhere across the planet.



Posturing

Tree rings from the Arctic Urals show that since the 15th century, many Siberian larch (Larix sibirica Ledeb.) — the primary tree species — have grown in a stunted, shrubby form, sporting multiple spindly trunks. This adaptation to harsh conditions helps the trees weather wind and snow. But the trees invest so many calories in making multistemmed clusters, Hagedorn says, that they end up puny and unable to make seeds. This infertility has thwarted the stand’s spread.


After about 1900, these larches began to switch from their creeping, multistemmed form to tall trees with a more upright posture, though sometimes with up to 20 stems, Hagedorn and his Russian and Swiss collaborators report. Over time, new trees emerged with a single, upright trunk, at the same time bulking up with more biomass than shrubby, same-age kin. Overall, 70 percent of upright larches have emerged in just the past 80 years. Since 1950, 90 percent of local upright larches have been single-stemmed.


This forest advance into former tundra coincided with a nearly 1 degree Celsius increase in summer temperature and a doubling of winter precipitation.


“That’s a good cocktail for growth,” says arctic plant ecologist Serge Payette of Laval University in
Quebec. Whether a tree grows up versus out depends on survival of its uppermost, or apical, buds. Good snow cover will protect those buds from winter damage, he says. Only if they are destroyed will the surviving lateral buds push growth horizontally, he explains.

Spruce are
North America’s more common boreal species at polar tree lines, Payette says. Some of these also assume a shrubby form, creating what he calls “pygmy forests” perhaps a meter high. But he has witnessed some of these trees assuming new, upright postures as areas warm and get wetter.

This process can create the “mirage” of tree line advance, he says. In fact, the trees may not move at all; in-place populations may simply recover from chronic stress and resume growth until they reach their normal height and mass.


Ecologist Andrea Lloyd of
Middlebury College in Vermont has been studying the health of boreal tree lines throughout the warming Arctic. As in the Urals, warmth seemed to spur American spruce to move into new terrain. “I’ve also seen spruce advancing upwards,” climbing up mountains to form dense stands, she says.

But that’s only part of the story, she finds. Even where stands are advancing, “if you look at individual trees, some are starting to decline.” They’re growing increasingly slowly. Sometimes, as growth slows, tree numbers within a stand may be increasing. “It’s a paradox,” she acknowledges.


Forest
ecologist Glenn Juday of Alaska-Fairbanks and his student Martin Wilmking have recorded similarly perplexing data from tree rings in 2,600 trees along two mountain ranges in polar Alaska. As the environment warmed, 42 percent of the trees grew more slowly and 38 grew more quickly.

Too little water seems a bigger factor affecting tree growth than temperature, although warming can foster drought, Juday acknowledges. Indeed, as the
Arctic warms, it will likely become drier, he says. “So we can expect that at least in the western North American Arctic, there are going to be sites that eventually will get too dry to grow trees.”

But their loss isn’t likely to compensate for the tundra lost to trees, at least in Arctic-warming potential. In fact, their loss could further perturb the global climate because boreal forests currently hold huge amounts of carbon that had been emitted as carbon dioxide, a greenhouse gas. Until they decompose, they darken the land and remain solar collectors. Once they rot, their carbon will enrich already high atmospheric CO2 levels.



Shrubs and microbes

The threat of tundra displacement by trees has largely escaped notice, Juday says. And indeed, boreal forest advances in
Alaska have been modest, at best. One reason: Seeds don’t normally travel far in the Arctic, and even when they land on tundra, its dense mats resist implantation.

Except when those mats have been disturbed. A dry summer and warm September last year allowed a fire to ignite 100,000 hectares (about 250,000 acres) of Alaskan tundra. The huge footprint of disturbed land is now ripe for growing seeds. Fortunately, Juday says, boreal forests are on the other side of a mountain range from this scarred landscape.


Throughout the past half-century, a far more pervasive disturbance — what ecologists have taken to calling shrubbification — has been subtly transforming the tundra landscape. It starts with the arrival of tiny shrubs, such as spreading willows perhaps only 7.5 centimeters (about 3 inches) high, explains ecologist Ken Tape, also at Alaska-Fairbanks. He compared repeat photographs of Arctic tundra scapes taken around 1950 and again a few years back.


His calculations indicated that for the sites he studied, “there’s been something like a 39 percent increase in shrub cover.” It’s consistent with data from satellite monitoring of
Alaska’s high Arctic that have shown “increases in biomass of a similar magnitude — about 25 to 30 percent,” he says.

As these willows and other shrubs start moving in, they trap snow, which begins to insulate — and warm — the soil at their feet, explains Andy Bunn, an environmental scientist at Western Washington University in Bellingham. The warming will rouse sleeping bacteria in the soil, which will then begin to feed. In the process, they’ll begin to spew much of the carbon that had been locked up in the formerly frozen soil. This fertilizes the shrubs, fostering the whole warming-growth cycle.


“There’s what people call a big Arctic carbon bomb” waiting to go off, Bunn says.
Up to 200 petagrams — that’s 200 trillion kilograms — are stored in the top meter of Arctic tundra. For comparison, the atmosphere already has 730 petagrams of carbon in it, he adds. If shrub-related warming releases much of this carbon, it could undermine much of the carbon-limiting measures people are contemplating to slow global warming, he notes.

Although trees soak up carbon, boreal trees grow so slowly they’ll likely never keep up with what the soil warming will spew, Bunn says. But forests could exacerbate the problem by darkening the still fairly light-colored shrubby landscape.
Warming has so changed the climate of a huge and growing span of tundra that it now hosts a temperature and moisture level that would support forests, Juday notes. “Today, if you planted a tree — in some cases very far up from the current tree line — it would survive in many parts of the tundra.” Just 40 years ago, he says, it wouldn’t.

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