Trees Living at the Limits of Survival

The white spruce has surrendered. My dog Jane and I walked north of the last white spruce a few days ago. As we climbed a hill onto Chandalar Shelf, we passed the last balsam poplar. The alders and willows duked it out for a while, but the willows triumphed as the farthest north shrubs on the trans-Alaska pipeline corridor.

No one would ever insult white spruce by labeling them as shrubs, no matter how short they grew. White spruce trees are always trees, and University of Alaska Fairbanks graduate Erika Rowland recently studied Alaska's northwest stands of the conifers in order to find what their movement might say about climate change.

Rowland journeyed to the Noatak River valley to find pockets of white spruce trees thriving along the Kelly and Kugururok rivers. Both rivers drain into the Noatak River, which flows through the western Brooks Range well north of the Arctic Circle.

To see where the white spruce were going, Rowland first looked at where they had been. In her master's thesis, Rowland states that white spruce came to Alaska from the Yukon Territories. After the climate warmed at the end of the last ice age about 10,000 years ago, white spruce trees spread northward to the arctic coast of the Yukon Territories. Slowly, the spruce crept into interior Alaska in the Yukon and Porcupine river valleys. By 9,000 years ago, spruce set down roots in the Tanana River valley. During the next 1,000 years, white spruce crept further south and west, appearing in the western Brooks Range about 8,000 years ago.

By dispersing their seeds farther northward, white spruce have moved north and west until stopped by altitude, cold temperatures, poor soil, or combinations of these. Rowland wanted to see if the trees had advanced in the recent warmer temperatures of the 20th century. Also, if the trees hadn't responded to the warmer climate, how long it had been since they stopped moving north?

Rowland did her detective work with the use of clues hidden in the earth. She removed a plug of sediment from the bottom of a lake to look for entrapped, ancient spruce pollen grains that would tell when the trees appeared in the area. She also analyzed peat samples, looked for fossilized tree remains and explored cutbanks of the Noatak and other rivers for recently exposed spruce logs.

Using radiocarbon and other dating methods, Rowland found the white spruce of the Kelly River valley were established at least 2000 years ago and the northernmost stands on the Kugururok River valley took root about 600 years ago, suggesting an expansion of about 30 km. over 1,000 years.

How did the warmer temperatures of the 1900s affect theses trees living on the edge of their range? Rowland found the trees didn't establish seedlings farther north on the tundra. Instead, the spruce stands became denser, with more trees than before occupying the same areas.

Though white spruce seems to have found its limits in Rowland's study site, she did report the results of other studies in which researchers found white spruce was on the march. In the Arrigetch Peaks region of the Brooks Range, trees were gaining a roothold above 700 meters elevation; on the Seward peninsula, healthy seedlings are sprouting west of the current treeline.