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Photo of wilting leaves
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Can Our Forests Stand the Heat?

Scientists discover unusual die-off in sugar-maple leaves due to high spring temperatures

You might expect warm weather to be good for trees. But new research by professors and students in the Faculty of Forestry suggests that a balmy spring can be bad for temperate forests. The group, studying a patch of forest in the Haliburton Highlands, discovered that many sugar-maple leaves that developed during three record-hot days in May died without expanding to their full size; other leaves are stunted.

“In early June, when the canopy should be its greenest, the canopy was tinged brown,” says Sean Thomas, the Canada Research Chair in Forests and Environmental Change and a professor of forestry. “In my 10 years of working in this region I’ve never seen anything like this. The foresters I work with who have been there many decades have never seen anything like this.”

Thomas and his colleagues noticed the dying leaves in a 13.5-hectare research plot near Algonquin Provincial Park. Work began on this particular patch of forest – the first of such research “mega-plots” in North America – in 2007 in an effort to understand temperate forests through intensive study. It’s a method that was pioneered 30 years ago in Panama and Malaysia to study tropical forests, and to yield insight into a little-studied ecosystem.

But Thomas says that we also have a lot to learn about temperate and boreal forests. Although scientists have studied and managed these forests for decades, their focus has often been on growing healthy trees quickly for logging. “Most of the history of traditional forestry research has been narrowly applied,” he says.

Researchers at the Haliburton mega-plot are trying to learn everything they can about the basic biology of the forest. The first step was to record, measure and map every woody-stemmed plant with a diameter of one centimetre or more – about 47,000 plants in all. This work was completed last summer, and a team of graduate students are now analyzing the data. The census will allow them to track the growth and mortality of every tree in the forest over years.

Scientists working on the plot use a network of 180 temperature sensors and a nearby meteorological instrument tower. The 30-metre-high tower, run in collaboration with Jennifer Murphy, a professor of chemistry, and Nate Basiliko, a professor of geography, uses special equipment to measure the exchange of carbon dioxide, methane and other gases between the forest and the atmosphere.

Thomas says the death of new leaves is an effect few would have predicted for warmer springs; the researchers have ruled out other causes, such as disease, pests and rainfall. The finding is a good example of the sort of unexpected consequences of global warming that the forest plot and instrumentation will help researchers understand. For instance, if warmer temperatures are going to kill leaves more often in the future, and if the death of those leaves slows forest growth or reduces the intake of carbon dioxide, then global warming might be quicker and more severe than currently predicted.

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