Global warming? The Fraser fir, everybody’s favorite Christmas tree, has been there, done that.
Once spread over the South, the fir (Abies fraseri) responded to the region’s warming after the last Ice Age by retreating to the cool mountains of North Carolina.
An Appalachian State University professor and his graduate students have seized on the heat-sensitive trees as the perfect subject for observing how trees in general may respond to 21st century global warming.
Local tree farmers, biology professor Howard Neufeld said, actually “planted our experiment for us.”
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Some 1,500 farmers, mostly in the mountains, grow 50,000 of the trees; the state’s $100 million industry is second only to Oregon’s.
There’s little farming acreage available at the trees’ natural altitude of 5,000 feet or above, so they’re all at 4,200 feet or below, Neufeld said.
Growers made available 15 trees divided among three sites, two outside ASU’s hometown of Boone, and one nearby in east Tennessee.
All the trees are the same age – about 10 years old – and genetic makeup. But they’re planted at three different elevations: 2,200 feet, 3,300 feet and 4,200 feet. Each site is separated from the next by an average of 2 degrees Celsius.
Streamlining nature’s schedule
Trees are notoriously slow-growing; it takes 10 to 12 years to get a fir to Christmas-sale size, and 25 or more for trees being raised for lumber.
But with trees at different elevations at his disposal, Neufeld doesn’t have to wait for a set of trees to show the effects of changing temperatures over time.
He and his grad-student researchers began the multi-year experiment last year, but study so far has yielded hints rather than conclusions, Neufeld said.
He plans to follow up on them, and also complete a study of tree rings, whose growth is another key to climate change. When farmers sold trees last season, they sliced discs from the severed trunks and handed them to Neufeld to study.
On-site, the study trees look like their fellows arranged over mountain hillsides in a diagonal pattern. They’re pleasantly bushy, with the Fraser fir’s famous blue-green branches and heady aroma.
Look closely, however, and you’ll see plastic bands around the lower trunks for growth measurement, and probes sticking into trunks to monitor water uptake from roots.
On several different days, graduate student Scott Cory was with them from before dawn until after nightfall, enclosing the ends of branches and inserting them into a measuring device called a needle chamber.
The needle chamber records carbon dioxide entering the needles during the day and carbon dioxide and moisture given off at night. A comparison of the two shows how much carbon dioxide is being used up in photosynthesis, one indication of the tree’s growth.
Bring on the porridge
By the end of the school year, students were calling the middle elevation – 3,300 feet – the “Goldilocks” zone. Its trees seemed to be taking in just the right amount of water and carbon dioxide daily.
Those trees had the largest trunk size among the three test groups. They and the highest elevation trees, at 4,200 feet, had about the same high daily photosynthesis rate.
But those at the lowest elevation seemed to be struggling.
They were more stressed for water. And Cory, by checking every three hours, found that, like humans avoiding the midday sun, they knocked off work before noon.
Their daily photosynthesis rate was one-quarter below the other trees’.
An adaptive mechanism that closes stomata (leaf holes) to conserve moisture in times of water stress may be backfiring, Neufeld said: It also closes out the carbon dioxide needed for photosynthesis and starves a tree.
But, he cautioned, the researchers don’t yet know everything. Because the lower-level trees start photosynthesis earlier in the year, their total year’s growth may equal the others. And by season’s end, those heads-above-the-clouds high-elevation trees may have fallen behind.
Heads in the clouds means more fog and less sunlight, something not yet measured.
In spite of the fact that North Carolina’s average temperature has exceeded the 1901-2000 average in 18 of the last 25 years, Neufeld said the Southern Appalachians haven’t yet had any appreciable warming.
But temperature trends take a number of years to become apparent, he said. “We may see warming start in the next several decades.”
Chillin’ at Christmas
While Appalachian State researchers study Christmas trees’ reaction to climate change, some N.C. State scientists are studying how tree farmers may adapt.
John Frampton and colleagues for several years have been collecting seeds from lower-elevation Fraser firs that seem to withstand higher temperatures well.
And they’ve traveled to Turkey for seeds of fir varieties that tolerate heat. They’re growing in test plots in North Carolina.
“My wife and I actually had one last Christmas. It was pretty,” Frampton said. Needles, however, tended to fall off, he said.
Needle retention, which requires several hard freezes during the winter, is a Fraser fir trademark.
With Fraser firs, “If we get to a point we’re not getting enough cold treatment, one option may be to artificially chill them in a refrigerated storage unit,” Frampton suggested.