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Boreal forests shift north

Advancing greenery could further heat the already warming climate

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7:47pm, May 15, 2008
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For the Arctic, green is the new black.

People frequently say “green” to mean “environmentally friendly.” But conifer forests — really big greens — encroaching on Arctic tundra threaten to further accelerate warming in the far North. Temperatures at these high latitudes already are climbing “at about twice the global average,” notes F. Stuart Chapin of the University of Alaska in Fairbanks. The newest data on the advance of northern, or boreal, forests come from the eastern slopes of Siberia’s 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, the leading edges of conifer forests have creeped some 20 to 60 meters up the mountains and begun overrunning tundra, scientists report in an upcoming Global Change Biology, now available online. Conifers here now reside where no living tree has grown in some 1,000 years, points out ecologist Frank Hagedorn of the Swiss Federal Institute forForest, Snow and Landscape Research in Birmensdorf. Ecologists and climatologists are concerned because the emerging forest data suggest that the albedo, or reflectivity, of large regions across the Arctic could change. Most sunlight hitting snow and ice bounces back into space. But convert a white landscape to open sea water or boreal forest, and the surface suddenly becomes a great collector of solar energy. 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. “The effects of vegetative changes will be felt first and most strongly locally — in the Arctic,” he says. If the albedo there drops broadly, this could further aggravate warming there and underway elsewhere across the planet. Tree rings from the Arctic Urals show that since the 15th century, many of the primary tree species — Siberian larch (Larix sibirica Ledeb.) — 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 into making multi-stem clusters, Hagedorn says, that they end up puny and unable to make seeds. The inability to reproduce has inhibited the stand’s spread. After about 1900, the local Siberian larch began to switch from their creeping, multi-stem form to tall trees with a more upright posture, though sometimes with up to 20 stems, Hagedorn and teams of Russian and Swiss collaborators found. 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 are no more than 80 years old. Since 1950, 90 percent of local upright larches have been single-stemmed. This forest’s movement 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 MiddleburyCollege 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,” she says, climbing up mountains to form dense stands.

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 reports. 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. Indeed, the loss could further perturb the global climate because boreal forests currently store huge amounts of carbon once emitted as carbon dioxide, a greenhouse gas. As the trees die, their carbon could be released into the air. Meanwhile, until they fall over and decompose, they’ll continue to serve as low-albedo solar collectors. 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, the dense mats normally resist implantation. However, 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 seed implantation. Fortunately, Juday says, seed-bearing boreal forests are on the other side of a mountain range from the scarred landscape. Warming has changed the climate of a huge and growing span of tundra so it now hosts a temperature and moisture level that would support forests, if the seeds ever arrived, 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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