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^{Carolyn M. Emerson}

At Dave Evans’s study site in the Mojave Desert, just north of Las Vegas, nine rings of PVC pipe supported by thin posts hang above the creosote bushes and burro-weed. Twenty-five yards across and about five feet off the ground, the white rings look like something out of a science-fiction show, a signal to alien landing craft, perhaps. In fact they are part of one of the longest-running experiments ever attempted to try to assess the effects of extra CO₂ on natural ecosystems.

The experiment ran for 10 years. Six of the rings served as controls, three with ambient air blown over the plants they surrounded and three “silent,” with no blowers. In the three experimental rings, the pipes blew enough CO₂ into the area enclosed by each ring to raise the level there to 550 parts per million.

While his colleagues from the University of Nevada assessed how the plants responded, Evans examined the soils and underground organisms. He found that Mojave soil reacted very differently than Gill’s grassland soil. In areas of high CO₂, it had more nitrogen available for plants and soil microbes to use. But there were a couple of catches. In normal years, most of the extra nitrogen was scarfed up by soil organisms, not plants. And wet years were something else again.

As anyone who has visited a desert during a wet year knows, rain works wonders. Plants that may not have been seen for a decade or more flourish and bloom. Evans’s colleagues found that in wet years the high-CO₂ rings showed a lot more plant growth than the rings without added CO₂. But it wasn’t the creosote and other large native plants that grew more, or the small native annuals that nature buffs might go to the desert to see.

“The biggest increase was in invasive annuals,” says Evans. “It’s the invaders that were responding.” Areas within the high-CO₂ rings were choked with red brome, a close relative of the cheatgrass that has spread through Washington in recent decades. Areas in control rings and outside the rings had very little of it. It was the combination of extra CO₂ and extra water that allowed the red brome to flourish.

Evans says invasive species can rapidly exploit new nutrient supplies. The Mojave’s native plants can’t make the same kind of quick score. They specialize in endurance and the ability to make do with a little. “It’s in it for the long haul,” says Evans of creosote, which can live for a thousand years.

The main threat from invasive species in the Mojave may come at the end of the growing season, says Evans.

“Red brome is good fuel for wildfires. The grass dies and dries in late summer. So one consequence of elevated CO₂ in arid ecosystems could be an increase in fire frequency or intensity, due to the presence of these invasive annuals. That could kill the system, because the native plants aren’t tolerant of fire at all.”

Evans sees the long-term effects of that whenever he visits the site on the Hanford reservation where he did his doctoral research. Twenty years ago, he says, the landscape was sagebrush interspersed with bunchgrasses that offered good grazing opportunities for cattle and wildlife.

“Then cheatgrass came in. It all burned 10 to 15 years ago, and now all we see is the cheatgrass. The sagebrush hasn’t come back.”

And the burning continues. This past August, fire charred more than 20,000 acres of cheatgrass-dominated land at the Hanford reservation.

When they took a closer look at what was happening in their soils, Gill and Evans found subtle but significant changes. In Gill’s tunnel, plants in the middle and in the high-CO₂ segments of the tunnel grew more and deeper roots.

“They end up building fewer and fewer leaves and more and more roots, trying to tap into the nitrogen that’s available,” he says. That could pose a problem for grazing animals, both domestic and wild. The total size of plants will be the same, but more of each plant will be below ground, where grazers can’t reach it.

The search for more nitrogen affects the Community of soil microbes too, says Gill.

“There’s pretty good evidence that bacteria aren’t very good at it," he says. "They’re small, they’re individual cells, they need their carbon source and their nitrogen source adjacent to one another. But if you’re a fungus, you’ve got long mycelia, and so you can tap in [to deep sources of nitrogen]. So what we see is that as CO₂ increases, you get a shift from a bacterial-dominated community to a fungal-dominated community.”

The same thing happens in most forest systems that have been studied, and Evans also found it in desert soils. During the last three years of his Mojave experiment, he supplied the test rings with a different isotope mix of CO₂ that allowed him to trace where the extra CO₂ ended up.

“What we found is that mainly the fungi are seeing this new carbon,” says Evans. Bacteria took up very little of it.

Gill says the shift toward fungi, at the expense of bacteria, could send ripples through the whole biosphere.

“It changes the patterns in which nutrients are being made available,” he explains. “It changes the nature of the organic material within the system.”

The details may be complicated, but the conclusion is simple: CO₂ that we have put into the air is changing the way the soil works.

Then there’s the look back offered by the “old” end of Gill’s tunnel. It’s one of the few scientific studies of ecosystem response to climate change that doesn’t take today’s conditions as its starting point.

“People think that today is sort of the standard that we’re working off of,” says Gill. “I think we often fail to recognize that today is very different from prehistoric conditions. Plants lived [in an atmosphere that contained] from 220 to 260 parts [of CO₂] per million for a long time, for 10,000 years, since the last interglacial, and they evolved to deal with that. In a hundred years, we’ve increased [the concentration of CO²] by 150 parts per million—a huge change.”

Exposing plants and soil to a preindustrial level of CO₂ provided startling results. From the low-CO₂ segments of the tunnel to the middle, where CO₂ is at current levels, “we see lots of changes in the way the plants photosynthesize, the rate at which they lose water, how they use the nitrogen, and the microbial community [in the soil],” Gill says. “It’s very, very sensitive over that initial range. As you move from preindustrial levels up to where we are today, there’s pretty good indication that there’s a net accumulation of carbon—that these ecosystems did act as carbon sinks, slowing the rate of greenhouse gas accumulation.

“But then when we start [at today’s level] and move up to 550, what we find is that they aren’t nearly as sensitive, they aren’t as well-suited to absorbing elevated CO₂.”

In other words, grassland ecosystems have already adapted to today’s higher CO₂ levels—and may not be able to adapt further to the even higher levels they’ll be exposed to in coming years. With similar changes going on in other ecosystems, the implications for us are grave. We can no longer count on earth’s resilience to bail us out, any more than the Maya could reclaim their lost soil by offering prayers to the gods.

“The ability of native ecosystems to function on our behalf, at least from a carbon-balance standpoint, is not likely to continue as strongly as it has in the past,” says Gill. “As far as forecasts go, consider the future to be worse than you thought.”

So what do we do about climate change? What can we do?

Perhaps the clearest message from Gill’s and Evans’s work is that we have to stop putting so much CO₂ into the atmosphere. As the saying goes, when you’ve gotten yourself into a hole and you’re trying to get out, the first step is to stop digging.

Gill says biodiesel and other new energy technologies could make a big dent in our CO₂ output and give us a chance to address the damage already done. Carbon sequestration programs, which encourage people to plant trees to offset the CO₂ they generate by driving a big car or running a business, are more problematic. They’ve gotten a lot of attention as a possible cure, but nitrogen limitation and other constraints make them a short-term solution at best.

“You may be able to tweak the system so that over the scale of 50 years, 100 years, you increase carbon storage, but that’s only if there’s abundant nitrogen and enough water, and we haven’t paid nearly enough attention to that,” says Gill. “We can’t just look at this in terms of carbon sequestration. We have to start looking at the interconnections.”

Follow the links to learn more about research by John G. Jones, Rick Gill, and Dave Evans.

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The shift toward fungi, at the expense of bacteria, could send ripples through the whole biosphere. The details may be complicated, but the conclusion is simple: CO₂ that we have put into the air is changing the way the soil works.