July 8, 1999 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 millennium.
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 temperatures."
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.
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