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Minor climate change can unravel a forest.

A modest climate cooling several hundred years ago upset the balance of tree species inhabiting southern Canada, suggesting that even changes spread over several centuries can dramatically alter forests and reduce their productivity, two Canadian scientists report.

Ian D. Campbell of Forestry Canada in Edmonton, Alberta, and John H. McAndrews of the University of Toronto studied how forests in southern Ontario responded to the little Ice Age, a global cooling that lasted from roughly 1200 A.D. to 1850 A.D. in Canada. Studies of pollen deposited in lakes during that period suggest that the predominantly beech tree forest gave way first to oak and then to pine trees after the start of the cooling. But while pollen can reveal the composition of an ancient forest, it cannot provide information about the land's productivity -- the number of trees present.

To test the impact of the climate cooling on woodlands, the researchers simulated the period using a forest model run on a supercomputer. Campbell and McAndrews mimicked the Little Ice Age by gradually lowering the man annual temperature in southern Ontario by 2 [degrees] CC over the 650-year cool span.

The simulation showed a succession o tree types resembling the pattern of change recorded by ancient pollen. Going beyond the pollen recordd, the computer experiment also provided a measure of the total amount of biomass within the forest. The biomass declined by 30 percent during the Little Ice Age, indicating that the cooling knocked the region' woodlands far out of equilibrium.

"Nobody had ever demonstrated before that such a minor climate change could have such a large impact," Campbell says.

The finding relates to concerns about future global warming, he notes, because it indicates that climate change does not simply alter the mix of trees in a forest by swapping cool-temperature species for warm, or vice versa. Instead, the slow cooling of the Little Ice Age caused a cascade of reactions that is taking centuries to play out.

The forest biomass declined steeply because the disappearance of beecch trees cleared lage sections. This prompted the spread of oak, which thrives in open spaces. Once the forest thickened, the oak died back, giving way to more shade-tolerant pine trees. By the end of the simulation, the year 2000, the forest still had not recovered its former productivity, the researches report in the Nov. 25 NATURE.

Climate experts debate how the Earth will react to the buildup of greenhouse gas pollution in the atmosphere, but most predict that the warming will proceed several times faster than the cooling modeled by Campbell and McAndrews.

"Most people are predicting fairly significant and rapid climate change. It's a warming instead of a cooling. But my research shows it was the rapidity of the change that was important, not the direction," Campbell says.

Some ecologists note that Campbell and McAndrews did not demonstrate that the model they used accurately represented how the real forest respondedto the Little Ice Age. In particular, the simple model did not incorporate wildfire and other types of disturbance that other researchers have shown to play an important role during shifts in climate.
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Title Annotation:research of how forests in southern Ontario reacted to Little Ice Age
Author:Monastersky, Richard
Publication:Science News
Date:Nov 27, 1993
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