Research raises questions about trees’ reaction to climate change
05 March 2018 | Adaptation
Heat waves that are expected more often in coming decades could disrupt one of earth’s most reliable defenses against climate change, researchers including a University of North Florida professor have concluded.
The scientists — a UNF biologist, Michael Aspinwall, among them — found that trees that normally suck up carbon in the atmosphere, dampening climate change, slow down or stop their absorption when temperatures rise too much.
Forests can be virtual vaults for locking up carbon, so how much difference a change like that can make is still an open question.
But the subject was barely on the radar when a team of scientists working in Australia — including Aspinwall, who joined UNF’s faculty last year — decided to turn up the heat in a forest of eucalyptus trees.
The test produced two surprises about the trees, which hadn’t been expected to fare well.
“We thought they would just get fried,” said Aspinwall, who studied trees about 20 feet tall that had each been grown from seedlings inside tube-shaped, see-through enclosures set up in a forest near the Hawkesbury Institute for the Environment at Western Sydney University.
Scientists controlled the temperature and water supply inside the enclosures and measured carbon dioxide levels in the air.
After a month of dry conditions inside the enclosures, researchers created something like a 110-degree heat wave, and watched what would happen after four days of unhealthy heat.
Not that much changed, though, because the trees did what the people tending them would do: They sweated. Using a process called transpiration, trees sucked water up from their roots to form tiny droplets on eucalyptus leaves that controlled the plants’ surface temperature better than anyone had expected.
“They proved us wrong. They handled it just fine,” Aspinwall said.
Sweating trees evidently didn’t absorb carbon as well as they normally do, however, which could be the bigger surprise whose impacts will take time to understand.
Trees’ ability to absorb carbon through photosynthesis, the process that nourishes plants and produces oxygen, is what made climate change activists tout big forests as “carbon sinks” that must be protected.
But the scientists’ report, published by the journal Global Change Biology under a whopping 20 researchers’ names attached, said the heat wave created inside the trees’ enclosures “reduced midday canopy photosynthesis to near zero.”
Mathematic models that are used to predict climate impacts are based on assumptions about how much carbon plants will soak up hour after hour, day after day.
If heat wave measurements keep showing the same pattern of slow, sometimes nearly non-existent carbon collection, there could be a lot of changes to the models, Aspinwall said.
That will likely be a bigger issue as time passes. A report from the federal government’s U.S. Global Change Research Program said last year officials had “very high confidence” that in this country, “heat waves will become more intense. The number of days below freezing is projected to decline while the number above 90 [degrees] will rise.”
On the good side, if plants that were expected to wither or die in heat waves can survive until the heat breaks, they’ll still be able to load up carbon sinks and moderate potentially climate-altering changes.
But someone has to figure out where the models need to change.
“Those people who are working on these models ... are going to have to reconsider some of their underlying assumptions,” Aspinwall said. “They’re ... going to have to account for these trees.”