Heat without rainfall won’t prod trees higher
Altitude not only answer for conifers in climate change
A $7 million governmentbacked climate change experiment over a decade in the Colorado high country — using scaffolding and heat lamps to simulate global warming — has found hotter air alone won’t be enough for stressed conifers to survive by shifting to higher elevations.
“By just heating the air and land more, you might invite trees up higher, but they won’t be able to make it unless you also create the rainfall up there. … Rainfall becomes the limiting factor,” said University of Colorado evolutionary geneticist Jeff Mitton, a leader of the project done with University of California and U.S. Geological Survey scientists at elevations around 11,400 feet.
“So are trees at lower elevations going to start dying off as aspen are starting to do? Will they grow up higher? Not necessarily. If we heat them at higher elevations, but do not increase rainfall, it does not look like trees will be able to move up.”
Funded by the U.S. Department of Energy, the project stands out as one of the larger experimental climate change studies involving trees. The findings are expected to help scientists and federal agencies anticipate what climate change will mean in mountains around the world. Trees play key roles retaining snow, which supplies water for the western United States, and absorbing carbon emissions from human industry and vehicles.
Scientists said they didn’t realize how important water would be in tree adaptation. Many have understood that, as climate change leads to higher temperatures, trees generally would “follow their climate” and shift from high-stress lower-elevation warmer zones to higher elevations, where temperatures currently are too cool, causing alpine tundra treelines to climb.
The peer-reviewed results were published in December in the journal Global Change Biology. A team from the DOE’s University of California lab, UC-Merced, the USGS and CU planted Engelmann spruce and limber pine conifer seeds. The scientists installed infrared
heaters, suspended on the scaffolding, on plots above CU’s Mountain Research Station on Niwot Ridge west of Ward.
Few trees currently grow around treeline, around 11,200 feet elevation here, because of wind, cold temperatures and insufficient soil.
The scientists used the heaters to raise temperatures by 1.5 to 4 degrees Celsius. They monitored effects on trees planted in plots between 11,200 feet and 11,400 feet, where they had hypothesized conifers would migrate in response to global warming.
In plots where temperatures stayed the same, trees did not grow. In plots where temperature increased but water was not added, conifers began to grow but died within two years. Where seeds were planted, temperatures increased and water was applied, trees grew for more than three years and survived.
There’s no practical way to reverse the impact of climate change on trees because global concentrations of carbon dioxide in the atmosphere have risen to around 400 parts per million, higher than in 400,000 years, Mitton said. A preponderance of scientists agree that human activities since industrialization around 1850 have caused the increases in carbon dioxide and other heat-trapping gases. Many species, an estimated 3,000 a day including microbes and insects, are disappearing.
Even if Americans continue to reduce greenhouse gas emissions, the increasingly affluent populations expanding in other countries emit more gases — boosting overall emissions, Mitton said.
“We talk a lot and we wring our hands, but we are not accomplishing much yet. Carbon emissions around the world are going up,” he said. “And what we have found is that, as it gets warmer and seeds are released in alpine areas, trees might be able to go into higher elevations. But that can only happen if the amount of precipitation is increasing. And, in Colorado, there is no consensus on whether precipitation will increase or decrease.”
At high elevations, fierce winds trim limber pine, Engelmann spruce and subalpine fir to ground-hugging shrubs called krummholz or elfinwood.