Trees tell the tale of our warming climate
USU • Analyzing tree rings shows a decade of stress.
Published: September 6, 2012 02:51PM
Updated: October 22, 2012 11:35AM
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Trent Nelson | The Salt Lake Tribune A student counts rings on a core sample taken from a tree near Logan, Utah. Researchers at Utah State University are looking at how tree rings tell the story about climate change in northern Utah, Wednesday, August 10, 2011.

So beetles are exploding in the West's forests and once-hardy trees cannot seem to fight back.

Why?

The reasons all point to climate change. While it's true rising temperatures themselves haven't killed the trees, there's ample evidence the heat creates conditions that make forests unable to fend off growing threats.

It's as if the Earth is behaving like a kid hyped up on too much sugar, supercharging ordinary behavior and spurring more tantrums, rowdiness and restlessness.

In the global climate system, the excess energy comes from greenhouse gases - infusions of carbon dioxide released into the atmosphere when we burn fossil fuels and other gases produced by human activities. As they build up, these gases amplify ordinary cycles into extraordinary ones. The result, scientists say: Texas-sized droughts and Mississippi-sized floods to go with the melting Arctic ice, rising seas and choking forests.

How do we know the climate is more hyper today?

The forests themselves supply much of the evidence, thanks to tree rings that provide a history of the world's climate.

Scientists can piece together the story by tracking the patterns of light and dark in the rings. The light part tells what happened early in the season; the dark records what happened late in the season. Then, by crosschecking their findings with thermometer readings, computer models and other scientific tools, they can glean climate trends.

What they have learned so far about northern Utah is that average high temperatures have increased by one degree in the past 60 years. And average low temperatures have jumped two degrees, according to the Utah Climate Center.

At Utah State University, several research teams have stepped up their study of tree rings to improve their understanding of northern Utah's climate.

"What the tree rings tell," says geologist Tammy Rittenour, "is what the natural conditions are."

Rittenour has begun to gather pencil-thick cores from climate-stressed trees in the Bear River and Wasatch ranges to help reconstruct the past climate. Her team also has been gathering cores from trees in the Raft River Range, the north fork of the Logan River and Mount Naomi. The researchers are looking for trees that have endured some wear and tear, ones that tell the year-by-year stories of long and short growing seasons and of wet and dry years.

They look at several kinds of tree species - including fast-growing Douglas fir and slow-growing juniper - and meticulously count the rings through microscopes. The fat rings indicate wetter years and skinny or nonexistent ones point to drought.

In the data gathered so far, Rittenour sees a distinct pattern of stress in the past decade.

"We are particularly sensitive to droughts," she says.

At the Utah Climate Center, also based at USU, tree-ring reconstructions from the region have helped link climate patterns in the Arctic with those along the Wasatch Front, including high-pressure systems that create pollution-trapping inversions.

Tree-ring data also helped the center identify the previously unknown relationship between wet-dry cycles of the Great Salt Lake and climate patterns in part of the Pacific Ocean.

Making global connections like these is important because generalizations about trends in the region don't always fit Utah. Sometimes temperatures and precipitation follow the patterns of the wet Northwest. Other times they are influenced by the dry Southwest. Understanding those patterns could help Utah devise a useful action plan for protecting forests and farmland as the Earth warms.

Roger K. Kjelgren, a plant physiologist at USU, noted that studying northern Utah tree rings also yields practical value. Focusing on junipers up to 700 years old, he uses tree rings to help the state's water managers plan for the future.

"They need to know what to plan for," he says. "Trees are one of the answers."

fahys@sltrib.com