Plant Food: Does Carbon Count?

Admirable NYT article on forests misses one important point

On Saturday, The New York Times ran a front page story about the state of the world’s forests, their role in mitigating climate change, and the ways in which climate change will, in turn, affect them.

It was a terrific piece of long form journalism, but it glossed over an important point about carbon dioxide and plant growth.

The article, by environmental reporter Justin Gillis, ran almost 4,500 words and started by presenting a frightening tableau of devastation befalling forests around the world. Scientists still don’t fully understand how climate change will impact woodlands in years to come, but a common denominator in current “die-offs”—from the Americas, to Africa, to Asia—seems to be heat and water stress associated with global warming. The point, at which the article quickly arrived, is that:

If forests were to die on a sufficient scale, they would not only stop absorbing carbon dioxide, they might also start to burn up or decay at such a rate that they would spew huge amounts of the gas back into the air — as is already happening in some regions.

Gillis emphasized that “the point of no return has not been reached yet — and it may never be.” An assessment of forests’ carbon-storage capacity published in Science in August found that, on balance, they are still packing away more than they emit via destruction and decay. “One major reason,” Gillis reported, “is that forests, like other types of plants, appear to be responding to the rise of carbon dioxide in the atmosphere by growing more vigorously.”

The piece immediately went on to explain that:

Climate-change contrarians tend to focus on this “fertilization effect,” hailing it as a boon for forests and the food supply … [and] likely to continue for the foreseeable future, ameliorating any negative impacts on plant growth from rising temperatures. More mainstream scientists, while stating that CO2 fertilization is real, are much less certain about the long-term effects, saying that the heat and water stress associated with climate change seem to be making forests vulnerable to insect attack, fires and many other problems.

Gillis deserves credit for mentioning and countering the popular argument that carbon dioxide is plant food, and therefore nothing to worry about, but his rebuttal could have gone one step farther. At The Energy Collective, Simon Donner, an assistant professor in the Department of Geography at the University of British Columbia, aptly noted that:

The CO2 fertilization effect is limited, because plants require more than just CO2 to do their job: photosynthesis. Water is certainly a limiting factor, but nutrients are just as important. In experiment after experiment, scientists find that the CO2 fertilization effect is short-lived without additional inputs of nutrients, particularly nitrogen.

That’s true, although some of the most recent research in the field could be confusing. For instance, a study published in the journal Ecology Letters found that “Increases in the flux of carbon belowground stimulate nitrogen uptake and sustain the long-term enhancement of forest productivity under elevated CO2.”

Compared to the timescale on which climate changes unfold, however, the productivity is not long-term at all. The research, by a diverse team of ecologists and biologists, examined twelve years of data collected during a project in the Duke University forest in North Carolina which compared trees in the normal environment to others in one spiked with CO2. According to Dr. William Schlesinger, one of the co-authors, who was dean of Duke’s Nicholas School of the Environment until 2007, researchers had predicted a more ephemeral CO2 fertilization effect.

“We were surprised that the growth rate didn’t taper off even faster than it did,” he said, “and there is some indication that trees allocate when they have this extra carbohydrate from higher rates of growth at elevated CO2, they allocate a fraction of that to root growth below ground, which can help mobilize some nitrogen from the soil, and essentially they’re growth rates don’t decline as fast you might expect.”

Nonetheless, Schlesinger added, “they do decline, and the mobilization of nitrogen can’t go on forever because there’s only a certain amount of nitrogen that’s in the soil, but we were looking for explanation for why the decline wasn’t more dramatic.

“Another thing we saw was that while the trees in the high CO2 plots tended to have more leaves and tended to be growing faster, because of their greater leaf area, they dried out the soil faster, so between rainstorms, they went into drought condition earlier and stayed there longer.”

The paper in Ecology Letters emphasized, furthermore, that experiments in other environments had found an even more transitory effect, or none at all. Unfortunately, scientists can’t really say how much of the world’s biomass will fall into each category as the world warms, said Schlesinger, who is now president of the Cary Institute of Ecosystem Studies, an independent nonprofit research center in Millbrook, New York. As far as the fossil carbon being absorbed by forests is concerned, however, “the general feeling now is that while some of that is being driven by a fertilization effect, the bigger effect is actually regrowth of forests on abandoned agricultural land” and other areas where trees have been cleared.

“There aren’t many global greeners running around the way the Idso brothers [Craig and Sherwood] are,” Schlesinger said. “One of them was quoted in that New York Times article. They’ve been saying for years that the CO2 is the best thing for planet Earth—plants will grow abundantly, crops will be overflowing and, you know, it’ll be the Garden of Eden.”

Lamentably, we won’t be getting any more data from Duke’s forest. Its project closed down in 2010; along with a couple other sites, it was part of the Department of Energy-sponsored Free Air Carbon Dioxide Enrichment (FACE) program. Other FACE experiments continue worldwide, however, including one at the Oak Ridge National Laboratory in Tennessee, which drew coverage from the NBC affiliate in Knoxville this summer (unfortunately, rather than explain the work’s import, WBIR-TV pursued an anodyne angle about career development for college students).

Scientists consider the open-air design of FACE projects an improvement upon older, controlled-environment, chamber-based experiments. More research in wider variety of environments to understand what a warmer world will affect forests, however. It would be useful to study carbon enrichment in the tropics, for example, Schlesinger said, but the project are logistically complicated and expensive to run.

Ecologists David Lobell and Christopher Field recently published a paper in Global Change Biology expanding upon the challenges associated with developing good experimental designs for studying the fertilization effect. And for background reading on forests and carbon, it’s worth checking out the publications of Dr. Yiqi Luo at the University of Oklahoma.

These sources contain more details than are likely to find their way into most news stories, but reporters should be bear them in mind for the next time a source tries to tell them that CO2 is just plant food.

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Curtis Brainard writes on science and environment reporting. Follow him on Twitter @cbrainard. Tags: , , , ,