Climate warming experiment finds unexpected results

January 4, 2019 by Emily Pontecorvo, American Geophysical Union
Dried leaf samples are bagged and numbered before being returned to study sites in Puerto Rico’s El Yunque National Forest. Heaters warmed experimental plots to four degrees higher than the ambient temperature of the tropical forest. Credit: Stephanie Roe

Tropical forests store about a third of Earth's carbon and about two-thirds of its above-ground biomass. Most climate change models predict that as the world warms, all of that biomass will decompose more quickly, which would send a lot more carbon dioxide into the atmosphere. But new research presented at the American Geophysical Union's 2018 Fall Meeting contradicts that theory.

Stephanie Roe, an ecology Ph.D. student at the University of Virginia, measured the rate of decomposition in artificially warmed plots of in Puerto Rico. She found biomass in the warmed plots broke down more slowly than samples from a control site that wasn't warmed.

Her results indicate that as the climate warms, forest litter could pile up on the ground, instead of breaking down into the soil. Less decomposition means less carbon dioxide released back into the atmosphere. But it also means less carbon taken up by the soil, where it's needed to fuel microbial processes that help plants grow.

"These results could have significant implications on the carbon cycle in a warmer future," Roe said.

Roe said there are few empirical studies of how tropical forests will respond to climate change. She set out to address this gap in June of 2017, when she and her research team travelled to El Yunque National Forest in Puerto Rico. They landed at a site called TRACE—the Tropical Responses to Altered Climate Experiment.

TRACE is the first-ever long-term warming experiment conducted in a tropical forest. It was established by the US Forest Service in 2016 for research like Roe's. The site consists of three hexagonal plots of land enclosed by a ring of infrared heaters raised four meters above the ground, and three more plots enclosed by fake heaters that are used as the "control" forest.

Roe collected leaves from the plots, dried them out in the lab, and then returned them to the plots randomly. In addition to the , she also included black and green tea, and popsicle sticks to represent woody biomass, to see how different materials would respond to the warming.

The heaters were programmed to continuously heat the plots to four degrees higher than the ambient temperature of the forest. The experiment was supposed to run for a full year, but at the beginning of October, Hurricane Maria swept across the island, destroying the TRACE sites.

Roe was back in Virginia when the storm struck. She had collected samples from the first few months of the experiment, and they were already showing signs of significant decomposition, so she decided to go ahead with the analysis based on what she had. And the results were not what she thought they would be.

"We would expect that microbes tend to work faster, like their metabolisms increase, with warmer temperatures," Roe said. "So we would expect to see an increase of activity of microbes and other decomposers to decompose the litter."

But instead of seeing faster rates of decomposition, Roe observed the warming produced a drying effect in the plots, which slowed decomposition. "What we found is actually it went the other way because moisture was impacted so much," Roe said. Moisture in the litter from the treatment sites was reduced by an average of 38 percent.

Roe pointed out that the increase in frequency and severity of storms in the region could amplify this effect. Hurricane Maria reduced significant portions of the tree canopy in El Yunque, allowing a lot more sunlight to reach the forest floor that can dry up the litter.

The results Roe shared are preliminary and not yet published. Her next project is to do further analysis of the nutrients in the litter and of the microbial communities to see if there are other factors that could explain the unexpected slowdown in .

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8 comments

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Scroofinator
1.8 / 5 (5) Jan 04, 2019
Her results indicate that as the climate warms, forest litter could pile up on the ground, instead of breaking down into the soil. Less decomposition means less carbon dioxide released back into the atmosphere. But it also means less carbon taken up by the soil, where it's needed to fuel microbial processes that help plants grow.

Hmm, a positive feedback in the carbon cycle that is beneficial.
But instead of seeing faster rates of decomposition, Roe observed the warming produced a drying effect in the plots, which slowed decomposition. "What we found is actually it went the other way because moisture was impacted so much," Roe said. Moisture in the litter from the treatment sites was reduced by an average of 38 percent.

So because we didn't understand the water cycle, the accepted theory may be wrong.
frflyer
4.4 / 5 (7) Jan 05, 2019
"So because we didn't understand the water cycle, the accepted theory may be wrong."

And what are the long term effects of lower moisture in the forests? Not likely beneficial. Sure there are uncertainties in climate science, but none of them them threaten to undermine the fact that humans are causing global warming. Uncertainty is not your friend. It cuts both ways.

Tropical forests need decomposition to keep up the supply of nutrients. Tropical rainforests hold most of their nutrients in the trees, whereas temperate forests hold most of the nutrients in the top soils, which are also deeper.

KenMitchell
3 / 5 (1) Jan 05, 2019
Makes perfect sense. If you heat an area without adding water, the RELATIVE humidity will FALL - because warmer air could hold more water.
Pete EE
5 / 5 (2) Jan 06, 2019
If you heat a plot of land, you will dry it because the nearby air will absorb more moisture.
If you heat an earth, you will have a higher humidity earth because the air will absorb more moisture.
Oops.
humy
4.3 / 5 (3) Jan 06, 2019

If you heat an earth, you will have a higher humidity earth because the air will absorb more moisture.
Oops.
Pete EE

No, that is not necessarily true and you haven't understood the physics.
Humidity, which means what is called "relative humidity" in this context because it is that and not what is called "absolute humidity" that is what effects evaporation rates, isn't just effected by the amount of moisture in the air but the temperature so it is possible to have more moisture in the air but lower humidity despite that because the temperature has gone up to more than counteract the effect of increase amount of water vapor.
See:

https://en.wikipe...humidity
"...Relative humidity depends on temperature and the pressure of the system of interest. The same amount of water vapor results in higher relative humidity in cool air than warm air...."
Pete EE
3 / 5 (1) Jan 06, 2019
Responding to humy
I want to begin an analysis with the facts that are certain.

In the context of this experiment (though the details aren't clear) it seems that a plot of land is exposed to ambient air. That means that an IR heater will increase the temperature of the land, vegetation and local air. The absolute humidity will remain constant. The relative humidity (which, as you say, is more relevant to evaporation.) will drop.

In the context of a warmed earth, my best first approximation is that relative humidity is what will remain constant. (Therefore absolute humidity will rise to match.) Hence my "oops" comment. The experiment had a constant absolute H but they wanted constant relative H.

It seems that they needed a physicist, or at least an engineer, in experimental design. (Biologists! amirite?) Counter-intuitively, this experiment would have been more realistic in a lab, or better yet in two greenhouses.
torbjorn_b_g_larsson
5 / 5 (1) Jan 07, 2019
Even I, who is not into plant experiments, know that these types of short term experiments are problematic and say little. Here is a contextual - plants, warming climate - find from earlier this very year:

"It has been held that the first version, C3, was the most sensitive to environmental carbon dioxide (CO2) levels, and that the plants using it would produce more biomass as those levels rise.

Results of a 20-year experiment, however, have overturned such certainties, revealing the biomass boost to be only a short-term effect that reverses over longer time-scales.

The findings, published in the journal Science, carry important implications for our ability to predict the full ecological impact of rising CO2 levels and ensuing climate change."
https://cosmosmag...ynthesis

On humidity speculation: I would wait for the publication, if any, and see if they controlled for such factors.
antigoracle
1 / 5 (2) Jan 07, 2019
Even I, who is not into plant experiments, know that these types of short term experiments are problematic and say little. Here is a contextual - plants, warming climate - find from earlier this very year:

"It has been held that the first version, C3, was the most sensitive to environmental carbon dioxide (CO2) levels, and that the plants using it would produce more biomass as those levels rise.

...a 20-year experiment, however, have overturned such certainties, revealing the biomass boost to be only a short-term effect that reverses over longer time-scales.

The findings,... carry important implications for our ability to predict the full ecological impact of rising CO2 levels and ensuing climate change."
https://cosmosmag.....HEE...

If this Chicken Little jackass had bothered to read that paper, he would have seen that blaming CO2 was pure conjecture. That's AGW Cult "science".

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