For the past nine years, an international group of scientists have been studying the effects of climate change on three familiar northern trees—the aspen, paper birch and sugar maple.

    The experiment, funded by the federal Department of Energy, the Office of Science (PER) and several academic and research institutes, is located 15 miles west of Rhinelander, on 80 acres of a 520-acre research facility in Harshaw.

    The Aspen FACE (free-air carbon dioxide and ozone enrichment) project was established to test the effects of elevated carbon dioxide and ozone levels on northern forests. FACE is seeking renewed funding from the Department of Energy for another three years—most likely to be its last renewal, at least for this phase of the experiment.

    Much has been learned in the past decade. Already the project has been pronounced very successful. Twenty-two separate research organizations from eight different countries are providing more than 50 scientists each year who are performing experiments and publishing their findings in the world’s leading science journals. In April, scientists from around the world will convene again in Rhinelander, as they did in 2004 and 2007, to attend a Joint FACE Meeting and share experiment findings.

    “This is one of the major climate change experiments in the world,” says project leader Neil Nelson. “It’s the longest-running open air experiment of its type. Only two other facilities researching forests have this kind of open-air experiment with elevated CO2, and we are the only facility that is testing ozone.”

    Borrowing a metaphor from his colleague, Mark Kubiske, Nelson explains how FACE is “a lot like marine biology, in the sense that they build a ship and scientists buy time to use it. Here we’ve done much the same by providing a research platform on which scientists from around the world have built their careers.”

    Once the current treatment phase of the experiment ends, Nelson says, scientists will spend another year and a half doing final tabulations on tree growth and survival and biomass production, as well as archiving plant material for future use and creating database information in a useful form for future scientists.

    The treatment phase has involved elevating CO2 levels to a target of 200 parts per million above the current atmospheric level, to a concentration of about 560 ppm; and increasing O3 levels to about 50 parts per billion—a 50 percent increase over current atmospheric levels. Both levels were chosen based on predictions for what levels will look like in 2050.

    It’s a timely experiment. Six years ago, CO2 levels were at 360 ppm. They are now at 380 ppm—a fast increase, especially considering that 100 years ago CO2 levels were at 280, and 1 million years before that, at 180.
   
    Ground-level ozone, the kind that causes a multitude of health problems, varies with location and air currents. It is formed when nitrogen oxides and volatile organic substances from urban areas combine on sunny days. The Aspen FACE experiment tests what impact increased ground-level ozone has when distributed by air currents over forested regions.

    What has been learned in the past nine years?

    Mark Kubiske, the project’s lead research plant physiologist, says the biggest surprise from the botanical point of view has been discovering the effect of elevated ozone levels on aspen and birch was different from predictions. “We looked at how the five different genotypes of aspen we tested interact and how aspen interacts with some of the other tree species,” he says. “One of our aspen genotypes grows better in high ozone levels than it does in background (normal) air or in elevated CO2.

    “We think it’s a matter of competition,” Kubiske continues. “The genotype competitors became weaker in high ozone, allowing this one genotype to perform better. The paper birch generally is more tolerant of ozone than aspen.” So, finding an aspen genotype that competes better in elevated ozone, he says, could help the forest management industry, since aspen is the workhorse of the fiber-using industry in the Northeast and North Central regions of the U.S.

    Bill Mattson is the project’s supervisory research entomologist. Slated to retire later this year, Mattson has been awarded the highest honor by the Entomological Society of America for his career accomplishments. His research on the effects of CO2 and ozone on insect populations has been published in leading journals.

    Mattson has been monitoring two dozen insect species. “We’re trying to anticipate if the insect community will get out of balance and result in epidemics or outbreaks that would result in damage productivity of forests,” he explains. “We predicted that elevated CO2 would change the relationship of insects to plants, primarily the above-ground species like caterpillars, ladybugs and beetles. As you increase the CO2, we know that leaf tissue diminishes in quality, and this is what the insects live on. But we haven’t seen any evidence of negative effects on insect populations so far.

    "On the other hand," he continues, "with elevated ozone treatments, we’ve seen a dramatic increases in the number of attacks of trunk-boring beetles on paper birch. The consequences are lethal, since the beetles crawl under the bark and girdle it. As ozone rises, the bronze birch borer will thrive and increase birch mortality.

    “We think ozone interferes with a plant’s ability to uptake water and its capacity to use water,” Mattson adds, “and this lowers resistance to attacks by bronze birch borers. Probably, this is the most obvious example we’ve found over the past 10 years of how these gaseous treatments may change the relationship between insects and the plants they live in.”

    What is still unknown, Mattson says, is the effect of CO2 and ozone elevation on below-ground life, the insects that feed on the dead leaf matter and fine root systems. “There has been some work done by other scientists indicating that below-ground insects are affected by elevated CO2 levels,” he said. “The plant litter may not be processed as quickly with elevated CO2. We know less about the effect of higher ozone. There are specialists working on fungi, but we don’t know what their results are.”

    In general, the experiment has shown that aspens and paper birch grow faster under elevated CO2, with little negative impact—with the caveat, however, that CO2’s impact on the insects that recycle leaf litter into nutrients may not be so positive. Higher ozone, on the other hand, clearly lowers a tree’s resistance to attacks by lethal insects.

    Kubiske says one of the big questions for future scientists is to learn how sustainable is the growth stimulation under elevated CO2. “One of the reasonable hypotheses is that as trees grow bigger faster, and leaf litter chemistry changes, trees will run into something that limits their growth—like the availability of nutrients, or a change in litter chemistry changing the way nutrients are recycled and leading to a decrease in CO2 stimulation.” 

    Experiments were done, he says, with CO2 doubled over its current levels—to 650 to 700 ppm. “That was the saturation point,” he says. “We got to a point of CO2 concentration where trees no longer respond to it, like a sponge that’s full.”

    Kubiske says that in the past eight or nine years, scientists around the world studying CO2’s impact on forest growth have pooled their research and written a paper for the National Academy of Science. “For at least nine years, we have found that trees seem to grow faster and we haven’t found any limitation on this growth from nutrients or decrease in water uptake.” There does seem to be a change in the microorganism communities below the soil, however. “Those communities,” he says, “tend to look more like those of a mature rather than a younger forest.”

    Kubiske says he looks at the elevated CO2 issue a bit differently than most scientists. “Green plants and all they co-exist with have evolved over time. If you look at most of the green plants today, you find they have an excess capacity to take up carbon dioxide. That’s why they grow faster when you give them more CO2. Their physiology allows that; it evolved in the distant past, when CO2 was higher. So my response is, if green plants have excess capacity to make use of excess CO2, why not the whole system have this capacity? This isn’t the kind of thinking that has steered the scientific community, who has been looking for a more negative consequence of elevated CO2.”

    Mattson, on the other hand, wonders if the green plants covering the earth today truly evolved in atmospheres of elevated CO2. “Many of today’s plants have never experienced elevated CO2. Yes, if you go back 22 to 55 million years ago, to the end of the dinosaur era, CO2 may have been in the 2,000 ppm range, but in the last one to 10 million years, CO2 has been relatively low. Until now, that is. We’re dramatically pushing out of that low range.”

    Kubiske responds. “I agree with Bill. In the deep distant past, the world was a much different place, but that was when the foundation for all of today’s organisms developed, and some of the basic biochemistry is unchanged. The question is, why do plants have the capacity to respond to higher CO2? Same for ozone—many plants are well-protected against a potent oxidizer like ozone, yet ozone in the lower atmosphere is not something that readily occurs, so why do plants have that defense ability?”

    Even though the project has been going on for nearly 10 years, all three scientists emphasize that it is a very short time for studying long-term effects of climate change. “One experiment going on now,” Kubiske says, “is what effect elevated CO2 has on water uptake, the manner in which leaves lose water. Higher CO2 causes more leaf pores to close and reduce water uptake; on the other hand, it stimulates the growth of more leaves. We don’t know how these two things combine.”

    Another interesting observation over time, Kubiske says, has been that under CO2 elevation, the amount of sunlight a tree gets is a key variable. “We’ve found that a cloudy, rainy summer produces less growth, and a sunny, drier summer produces more growth. It’s counter-intuitive.” He adds that only more time will tell whether a continued drought of deepening severity would produce enough water deficit to affect tree growth.”

    Mattson notes that in an “incredibly short-term experiment like this one, we don’t learn much about the impact of big events, like a big fire, a big insect outbreak, a drought. These events are missing from an experiment like this, and they are completely unpredictable.”

    Nelson concurs that more time is needed for understanding the forest’s long-term reaction to climate change. “We want to take the results we have and scale up to the landscape and regional levels to see what they mean. We need more information on genetics for this. We know some genotypes within a species respond very differently. That’s the way research is. There’s always new information you need to have to do what you want to do.”

    Some long-term predictions have come out of climate change research like the FACE experiment. “A group working here published a paper a year or two ago on how Wisconsin will change in the next 100 years,” he says. “Depending on which scenario of energy use you look at, the climate here could be anywhere from what you find today in Missouri to Arkansas. That’s the challenge the forestry community is facing.”

    “If that prediction is right,” Nelson says, “we have species that won’t survive. The paper birch, white spruce, balsam fir, northern white cedar, hemlock…those will be replaced by more southerly species moving north.”

    For more information about the Aspen FACE experiment, visit www.aspenface.mtu.edu.