Change in disturbance regimes -- rather than a change in climate -- is largely responsible for altering the composition of Eastern forests, according to a researcher in Penn State's College of Agricultural Sciences. Forests in the Eastern United States remain in a state of "disequilibrium" stemming from the clear-cutting and large-scale burning that occurred in the late 1800s and early 1900s, contends Marc Abrams, professor of forest ecology and physiology.

Moreover, Abrams noted, since about 1930 -- during the Smokey Bear era -- aggressive forest-fire suppression has had a far greater influence on shifts in dominant tree species than minor differences in temperature.

"Looking at the historical development of Eastern forests, the results of the change in types of disturbances -- both natural and man-caused -- are much more significant than any change in climate," said Abrams, who is the Steimer Professor of Agriculture in the Department of Ecosystem Science and Management.

"Over the last 50 years, most environmental science has focused on the impact of climate change. In some systems, however, climate change impacts have not been as profound as in others. This includes the forest composition of the eastern U.S."

To determine how forest tree species have responded to changes in disturbance regimes, temperature and precipitation over long periods of time, Abrams collaborated with Gregory Nowacki, a scientist with the U.S. Department of Agriculture Forest Service office in Milwaukee, on a study of the tolerance and sensitivity of trees to various factors.

"Many ecological phenomena combine to direct vegetation trends over time, with climate and disturbance playing prominent roles," said Nowacki, who received his Ph.D under Abrams. "To help decipher their relative importance during Euro-American times, we employed a unique approach whereby tree species/genera were partitioned into temperature, shade tolerance and pyrogenicity classes and applied to comparative tree-census data."