Trees That Love Global Warming

Every day an 18-wheel tanker truck pulls up alongside a lush forest near Duke University in North Carolina. Within a short time, the truck’s cargo of dreaded carbon dioxide gas begins flowing through a series of pipes and onto a forest rich with loblolly pines and small hardwood trees.

For four years now, scientists at Duke have inundated the forest with carbon dioxide, the principle greenhouse gas that is expected to wreak havoc on the planet in the decades ahead by elevating temperatures, causing sea level to rise, and severely altering vegetation around the globe.

Why, one might ask, would these good people deliberately subject the forest to such harsh treatment?

The goal of the project is to replace theory and conjecture with hard facts about the impact of increased levels of carbon dioxide, produced primarily by the burning of fossil fuels. Those facts are hard to come by, because the effect will be decades long, and it’s not easy to nail down evidence in such a complex arena. So the Duke researchers are addressing one fundamental question: What effect will elevated levels of carbon dioxide have on plant life?

Pines Thrive

The preliminary answer seems to be that at least some of the trees in the forests will love it, growing more rapidly, reproducing more robustly, thriving at a time when some parts of the globe will slip perilously into a rising sea.

“It’s really dramatic,” says Shannon LaDeau, a doctoral candidate at Duke who is running part of the long-term experiment.

The pines are growing about 25 percent faster than pines just outside the experiment, and they are twice as likely to be reproductively active. “They are making three times as many cones” which carry and incubate their seeds, she adds.

So if the trees there are doing so well, why is the world in an uproar over global warming? Because the Duke experiment addresses only one part of a problem that is extremely complicated.

What’s good for the loblollies is devastating to other living organisms, including coral reefs. Researchers at Columbia University’s Biosphere 2 in the Arizona desert have found that the atmospheric level of carbon dioxide that we might expect in a few decades will dissolve the reefs like an ice cube in boiling water.

So we can expect some good and some bad effects from global warming. Arid regions that lack water for agriculture may get a lot more rain, but low-lying regions will most likely slip below sea level.

And the forests, while robust in some areas, will almost surely change.

“You’re certainly going to change the competitive dynamics between different species,” LaDeau says.

“We could have a change in forest composition, dominated by those species that can use carbon dioxide efficiently at the expense of others,” adds William H. Schlesinger, professor of botany and the principal investigator on the project.

Localized Greenhouse Effect

The Duke experiment is an interesting marriage of technology and science. The carbon dioxide is pumped into a series of pipes surrounding a plot of land about 90 feet in diameter.

“These are big pipes that extend above the canopy of the pine forest,” LaDeau says. The level of carbon dioxide is continuously monitored. When the level drops, the system delivers more gas, and if it rises too high, it simply shuts down.

“If the wind comes out of the west, it turns on the pipes on the west side,” she adds, keeping the level precisely the same, even on a windy day.

Schlesinger says he wasn’t surprised that the trees in the plots grew dramatically during the first year of the experiment, 1996, because trees absorb carbon dioxide through photosynthesis. But the trees continued to grow about 25 percent faster than trees outside the plots over a three-year period, and that, he says, was surprising.

“Basically, the trees woke up one day to 560 parts per million carbon dioxide, and that was essentially newfound wealth in terms of photosynthesis, but you can’t sustain that rate in photosynthesis in greenhouse experiments without adding fertilizer,” Schlesinger says.

He added that they did not add fertilizer to the soil and it’s still too early to tell if that growth rate will be sustained through the current year, the fourth year of the project.

One unintended result of the research was to fuel the vigorous political debate over global warming. Some scientists argue that the dire predictions will never come true, partly because forests will expand rapidly and absorb any significant increases in carbon dioxide.

Good for Some, Bad for Most

“Our calculations suggest there will be some of that, but at the very most we can only get half the emissions of carbon dioxide from fuels into the forests,” Schlesinger says. “The other half would accumulate in the atmosphere and lead to global warming.”

He also points out that loblolly pines are not typical. They are among the fastest growing trees on the planet, and as the Duke experiment reveals, they are very responsive to elevated levels of carbon dioxide. So it would be a terrible mistake, he says, to extrapolate from the Duke experiment and conclude that nature will heal this wound.

The impact of carbon dioxide on plant life is just beginning to be understood. A similar project in Tennessee has found that hardwoods also experienced a 25 percent jump in their growth rate during the first year, but that is a very young experiment and it remains to be seen whether that growth will be sustained.

Young hardwoods, mostly oak and hickory, near the base of the loblolly pines in the Duke experiment are also growing rapidly. They should take over the forest in about 50 or 60 years, Schlesinger says, smothering out the loblollies. Unless, of course, the pines win that battle by sucking up enough carbon dioxide to beat out the competition. That’s the sort of thing ecologists worry about, because it could upset the balance of nature that allows many other creatures to survive.

No matter how it pans out, Schlesinger says, the forest of the future will probably look much different than it does today.

“There will be a different composition, and the diversity may be lower,” he says. But there’s one thing he’s pretty sure of. Some of those trees are going to grow like crazy.

Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.