"Since European settlement began in about 1850, about 60 to 90 percent of the aspen forests have disappeared, taken over by conifers such as pines, spruce and fir," says Dong Ko, a recent Penn State graduate with a master's degree in geography.
"In just the last 50 years, there has been a 30 percent decline," Ko told attendees at the 86th annual meeting of the Ecological Society of America today (Aug. 7) in Madison, Wis.
Quaking aspen, the most widely distributed tree in the northern hemisphere, is found from coast to coast in a band that includes northern California, the Rocky Mountains, Wisconsin, upstate New York and New England. Beside being noted for their shimmering leaves and the golden color they turn in the fall, aspen are unusual because they grow as clones, putting out vegetative shoots rather than forming seeds from which saplings sprout.
"Much research exists on aspen groves in the intermountain West and especially about their decline there," says Dr. Alan Taylor, professor of geography. "But there is no published information on aspen stands from California. Nothing was known about them."
Assumptions for the decline of the intermountain aspen stands include the suppression of frequent fires, increased livestock grazing, and increased and concentrated herds of native herbivores including deer and especially elk. The same environmental factors were believed to affect California stands as well.
Ko and Taylor studied 20 aspen groves on the California side of the Lake Tahoe Basin. The Penn State researchers identified the ages of the trees in the stands and the types of trees found. Surprisingly, they found only 15 percent or five stands that showed a shift in composition toward coniferous, shade-tolerant species. In the remainder of the groves studied, 12 had stable aspen populations with evidence of self replacement and 3 showed regeneration of both aspen and conifers.
The researchers warn that the history of the Lake Tahoe Basin is unique. Most of the forests were clear-cut in the 1870s to supply wood to the miners of the Comstock Lode. The Basin's aspen stands are mostly around 120 to 130 years old.
"If the young age of the stands is not a factor, then aspen persists very well in this area," says Taylor. "The different behavior in different areas then makes this a much more complex problem."
To provide a glimpse at the complexities of forest behavior, especially in the light of clear cutting, fire suppression and other environmental changes, Taylor, who studies a variety of aspects of the Basin's forests, has produced an online exercise, "Lands in Transition, Complex Decisions in the Lake Tahoe Basin." This explains some of the complexity of the Lake Tahoe Basin forests and is supported by the U.S. Department of Agriculture's Forest Service, The program, which is an interactive learning program, was produced by Penn State's Gould Center for Geography Education and Outreach and is available online at http://www.gouldcenter.psu.edu/lit or on CD by contacting Mark Johnson at mgjohnson@fs.fed.us by email.
"Because aspen are clones and regenerate vegetatively, some of the groves are 8,000 years old," says Taylor. "Aspen have rarely been observed to regenerate from seed, however they did after the recent Yellowstone National Park fires."
The Penn State researchers suggest that it may take a major event, such as a massive fire, to force the trees to reproduce sexually. Also, because aspen are shade intolerant, they need open spaces to grow properly.
"Aspen may require fire to continue to survive in healthy stands and prevent other species, especially confers, from taking over," Taylor says.