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UA study: North American forests won't nip climate change in the bud

New research from the University of Arizona is challenging conventional wisdom about forests and climate change.

The traditional thinking goes like this: Forests in colder climates such as Alaska and Canada will do better as temperatures warm because they’ll experience longer growing seasons. As they grow, these boreal (northern) forests will absorb increasing amounts of the greenhouse gas carbon dioxide, which in turn will slow down global warming. The effect is called boreal greening.

But a team of researchers at UA came to the opposite conclusion, determining that most of the boreal forests in North America will likely do worse as the climate warms.

“That’s one of the most surprising results,” said Valerie Trouet, an associate professor with the Tree-Ring Research Laboratory. “We don’t see that the trees in the boreal forest are going to do better. On the contrary, there will be the opposite effect.”

"We don't see any greening in our results," said Trouet. "Instead, we see browning. The positive influence that warmer temperatures are believed to have on boreal forests, we don't see that at all."

What sets this study apart from earlier ones is the massive amount of data collected by the researchers. The team studied some 2 million tree ring samples collected from 1,457 sites in forests across North America. By comparing the tree rings to 50 years of climate data, they learned how trees react to changes in temperature and precipitation. They used that information to predict how trees will respond to a warming climate in the future.

“What we found is that for a large percentage of the forests in North America, we see reductions in the growth rate of trees,” said Prof. Brian Enquist of the Department of Ecology and Evolutionary Biology. “It means that future climate change impacts could be more serious than we thought.”

It turns out that boreal greening may be a short-term phenomenon. At some point, the climate becomes so warm that forests are exposed to temperatures they’ve never experienced and are not evolutionarily prepared to handle. That’s when, the research shows, that the growth rate will begin to decline and forests become more susceptible to wildfires, diseases and insects such as the bark beetle.

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A warming climate is expected to increase the growth of trees in California, Oregon and Washington, the coast of Alaska, and along the Gulf Coast. Trees in these temperate regions are less likely to experience big climate changes.

But everywhere else, the news is not good.

The trees of Tucson and most of Arizona are in one of the regions expected to be hardest hit by climate change.

“If you drive up Mt. Lemmon, you’ll see many dead trees,” said Enquist. “The mortality of trees has been increasing and this is corresponding to a series of record hot and dry years starting in 2000.”

“What this means,” said Trouet, “is that the boreal greening we were counting on doesn’t look like it’s going to happen. We’re going to have to take even more drastic actions if we want to curb climate change.”

"In Alaska, for example, where trees have been projected to respond positively to warming temperatures under the boreal greening effect, we see that trees are now responding negatively instead," said Margaret Evans, a UA assistant research professor who was a senior author of the study.

"Trees in very high latitudes are limited by cold temperatures, so yes, in warmer years they grow more, but there is a tipping point, and once they go past that, a warmer climate becomes a bad thing instead of a good thing," Evans said in a written statement.

In addition to Evans, Trouet and Enquist, the team included Noah Charney from the Department of Ecology and Evolutionary Biology as well as researchers in Montana, Pennsylvania, Switzerland, and Poland.

Their paper, “Observed forest sensitivity to climate implies large changes in 21 century North American forest growth,” was published in the journal Ecology Letters.

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A trail on Mt. Lemmon.