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Russian, Swiss and Arizona tree ring scientists have discovered why
forests in northern Siberia have not been growing faster as Siberian
summer temperatures have warmed during the past few decades.
Meteorological instruments at their tree sites have recorded not only a
temperature rise during this century, but also a slow, gradual increase
in the amount of snow. Greater snowfall is keeping the ground frozen
longer, delaying 'spring greening' for this high-latitude forest, they
report tomorrow (July 8) in Nature.
Scientists study wood density and width of annual tree rings for
information on year-to-year, or even season-to-season, changes in
temperature and precipitation. Tree ring analysis plays a major role in
reconstructing how global climate has changed over much of the past
So when European researchers last year reported that beginning in the
1960s, significant numbers of trees at timberline across the subarctic
from Alaska and Canada to Scandinavia and Siberia have not grown as much
as expected given the rise in temperature, dendroclimatologists -- tree
ring scientists who study climate -- were puzzled.
The recent marked weakening in the correlation between tree growth and
temperature means that past climate reconstructions are even more
reliable than previously thought, but forces scientists to rethink the
role of the vast northern forests in the global carbon cycle, said
Malcolm Hughes. Hughes, professor and director of the Laboratory of Tree
Ring Research at the University of Arizona in Tucson, is co-author of
the Nature paper. Currently he is doing field research in the northern
Rockies, but will return to the Tucson campus on July 20.
"The recent weaker correlation between tree growth and temperature
clearly affects the reliability of our reconstructions of the past.
Actually, it means past climate reconstructions (before the 1960s) are
better than we thought they were. And, as a result of this, it means
that we underestimated the differences between the present century and
past centuries," Hughes said.
In a global warming study published last March in Geophysical Review
Letters, Hughes and University of Massachusetts colleagues found the
1990s to be the warmest decade of the millennium, with 1998 the warmest
year so far. The contrast between this century and previous centuries
may be greater than thought, Hughes now suggests, because "our
calibration is contaminated partly by this recent weaker correlation."
The other major implication of the weaker relationship between summer
temperatures and growth has to do with the greenhouse effect.
"The northern forests of Alaska, Canada, Scandinavia and Siberia are
almost in the same league as the tropical forests in terms of their role
in the global carbon cycle," Hughes said.
"While this study is of subarctic forests in Siberia only, our tree
sites cover a big piece of real estate -- over 100 degrees of northern
longitude, or almost a third of the way around the Earth. Many
scientists are trying to figure out how the growth of forests will
change in a greenhouse-warmed Earth. But in all science, looking to the
past or the future, the present is our indicator. And what we are seeing
is a change in the mechanism."
Eugene Vaganov, director of the Russian Academy of Sciences' Siberian
Institute of Forestry, Hughes and other experts in Siberian environment
and tree physiology based their study on empirical evidence and
theoretical research. They used tree ring width and wood density
measurements from conifers growing at the northern Siberian timberline
and instrument-recorded weather data in computer simulations based on
Vaganov's mathematical models.
Vaganov, who leads the world's premier institute in Siberian forest
ecology, is active in many collaborations with Hughes and others at the
UA Tree Ring Laboratory, where the science of dendrochronology was born.
Russian and Arizona tree ring scientists joined forces in the late
1980s, at the end of the Cold War. Vaganov will be a visiting scholar at
the university in Tucson again this fall.
"One of the characteristics of the high-latitude Siberian forest is that
trees over most of this area are growing on frozen ground that thaws
only a foot or two deep in summer," Hughes said. The trees, primarily
larch, don't grow very tall, perhaps a maximum of five meters, he added.
Trees 600 and 700 years old may be only a quarter meter (10 inches) or
less in diameter.
"Basically, what's happening is really simple," Hughes said. "Average
temperatures don't climb above freezing until the first week of June. So
that's when the thawing takes place. And the more snow there is, the
later the ground thaws."
The snow acts like a blanket that keeps in the chill. Below the snow is
an almost infinite amount of solid ice. Only when the snow melts is the
ground exposed to the sun's radiant energy. Only when the soil reaches a
few degrees above zero can new growth begin, Hughes said.
The increased snow in Siberia may delay the onset of forest growth only
by a few days or a week, Hughes said. But that's a big slice out of a
short growing season that ends in late August, he added. "It turns out
that most growth takes place in June, and June temperatures are the
major influence on growth rates, so you may be reducing growth by 20
percent, simply by losing five or six days of prime growing time.
"As for the future of modeling changes in growth, this means that people
are going to have to have good projections of snowfall as well as summer
The Siberian forests are a rich and largely untapped archive of
information on the 20th century natural environment, Hughes said. They
might be studied in greater detail for a broader picture of what's
happening in nature, and can be used to cross-check information from a
handful of long-term ecological research sites in Alaska and Canada.