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The Days of the Shade Trees in Alaska

Interior Alaska is tough on trees. Transplanted trees from the Lower 48 give up quickly when exposed to the Interior's long, cold winter.

There was a time, though, when elm trees grew in Eagle, basswoods thrived in Tok and hickories sprouted in Fairbanks. Nobody was here to enjoy the hardwoods then, but Tom Ager, a geologist with the Global Change and Climate History Team of the U.S. Geological Survey in Denver, has the tools to discover information about them.

Today, interior Alaska has a climate that allows a handful of tree species to survive. White and black spruce, balsam poplar, aspen, willows and alders are the most common varieties tough enough to survive the Interior's cold temperatures in the winter and mad swings of daylight in the summer.

Ager uses the clues left behind by ancient trees and other plants to track their existence in Alaska over the centuries. Ager uses pollen-airborne grains that contain reproductive cells of plants as archives of ancient plant life. Although it quickly decays when exposed to oxygen, pollen that lands on water and sinks can endure for thousands of years.

Ager and a team of other researchers sampled the sediment from the bottom of lakes spread out over interior Alaska. Using a hollow tube about two inches in diameter, the scientists pulled up meter-long cores of lake bottom that contained ancient pollen grains. Scientists are able to determine the vegetative history of an area by marking a spot on the core where, for example, basswood pollen begins showing up in abundance. That part of the core is carbon dated, giving researchers an idea of when basswood trees began appearing around the lake.

Ager combined the dates of pollen grains with the temperature records that were reconstructed from oxygen-isotope measurements of microscopic fossils in the Pacific Ocean. Using the two, Ager found that oaks, hickories, beech, chestnuts, walnuts, wing-nuts, basswoods, elms, hollys, hazelnuts and sweetgums grew in interior Alaska during a warming period between 17 and 14.5 million years ago. During the time of the hardwoods, temperatures must have been about 25 to 30 degrees centigrade warmer than they are today, Ager said.

According to the isotope measurements on the underwater fossils, global cooling began about 14.5 million years ago. The pollen record shows a dramatic change in interior Alaska trees at the same time. The warm-weather hardwoods disappeared, leaving behind hardy birch, alder and pine. By about three million years ago, the forests were composed of spruce, larch, birch, alder, willow and pine. With the exception of pines, the forest of three million years ago greatly resembled what is here today. During the past 2.5 million years, changes have been extreme and frequent, Ager said.

Cold, dry periods, during which glaciers expanded from the five percent of the state they cover now to 30 to 50 percent, occurred for 100,000 years at a time. These glacial periods were interrupted by shorter periods (about 10,000-20,000 years) of warmer temperatures and more moisture. In the past 10,000 years (known as the Holocene), forests of spruce, larch, alder, poplar and willow have spread throughout interior Alaska. During warm spells in the distant past, spruce forests crept north of the Brooks Range and west to the Bering Sea coast, and trees grew at higher elevations than they do today.

Ager's results suggest that a warmer climate will mean an expansion of forested areas in Alaska. It's worth looking at, because trees in Alaska have proven themselves relatively quick responders; they are the wooden canaries in the coal mine of a changing climate.