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.
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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.
CREEPING IN A sapling sprouts near a mature tree in the Russian Arctic in January 2001. Larches and spruce there are overtaking formerly low-growing vegetation. Peter Essick/Aurora/Getty ImagesGREENING TUNDRA Satellite data map a greening Arctic tundra. Brown shows where photosynthesis decreased between 1981 and 2005, and green where it increased. This change resulted mainly from shrubs invading permafrost, beginning a chain of events that may affect global climate. Bunn, EOSINVADING LARCHES The upper photo, taken in 1962, shows mostly low vegetation and shrubs on a slope in the Siberian Urals. The lower photo of the same site in 2004 reveals larches building a true forest. Andreas RiglingHARDY UPSTARTS Although some newly invading Ural larches sport multiple, upright trunks, as seen below, others are beginning to grow with a single trunk. The latter have greater vigor and are more fertile. Global Change Biology 2008Posturing
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 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,” 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.