This story first appeared in the SERC quarterly Newsletter summer 2006
With rapid climate change and the continuing increase
in atmospheric carbon dioxide no one can say with any certainty what the future world will look like.
Could the temperate zones of North America come to resemble tropical forests? Will crop production increase as plants grow more vigorously in high carbon dioxide (CO2)? Will opportunistic invasive plants gobble up more real estate in new territories, wiping out native organisms and reducing biodiversity? While these answers remain elusive, a few things are certain: CO2 is rising, largely from the burning of fossil fuel; global temperature is rising dramatically; and the response of plants to these changes is going to play a significant role in how the world fares.
Our lack of understanding about how all the world's plants will respond to rising CO2 and the complex interactions within ecosystems leaves a lot of room for debate about what's to come. Some would say that plants can take up extra CO2 and thereby buffer us from the ill effects of rapid increases. Others would prefer not to focus on the ability of plants to take up excess CO2 for fear it will weaken the case for reducing CO2 emissions.
Bert Drake, who has been running the world's longest continuous field study on plant response to rising CO2, says both sides are missing the point. "The fact that plants may provide an enormous sink for excess carbon gives us hope that we may learn to use the biosphere to help us mitigate the impacts of rising CO2." He said, "But we don't know nearly enough about which plants can take up the most CO2 and the fate of the CO2 they absorb."
"Different plants have different capabilities to take up excess carbon," he said, "but we don't know why, and just because plants can take up increased carbon, doesn't mean they always do." When they do, the consequences are not always as we would like, as a recent widely publicized study illustrated when it announced that poison ivy outpaced many other plants and became more toxic to humans when exposed to increased CO2.
The very concept that increased CO2 would stimulate plants to grow and take up more carbon indefinitely was discounted just 20 years ago when Drake began his field study. Most scientists believed plants exposed to increased atmospheric CO2 would increase CO2 uptake and grow rapidly at first, but that the response would level off as they acclimated to the new conditions. In fact, they found that for the first five years, plant response to a high CO2 environment remained constant. They anticipated that it would begin to decline as time went on.
That's where most studies would have ended. Unlike medical studies, in which patients are often followed for decades before a given treatment is even considered usable, for most environmental studies three to five years is considered long-term. Drake had the foresight and the funding from the Department of Energy, however, to keep going. To everyone's surprise, the plants' ability to continue to take up excess CO2 increased as the study went on for the next 15 years.
Ten years ago, Drake opened another study site in a scrub oak system in Florida to address the same issues with different plants in a different environment - one dependant on fire. Between the two sites about 100 scientific papers have been written, some of them among the most quoted papers in the field. They've addressed a number of fundamental questions, and according to Drake, what we now know is this: The key environmental factors affecting plant response to high atmospheric CO2 are: rainfall, which impacts water availability and salinity, sea-level rise, which results in flooding, and availability of nitrogen. While plant growth has varied throughout the study as these elements have fluctuated with respect to one another, the overall trend has been that plant growth is higher in the study sites with more CO2 than in those without. "Over the long term, increased CO2 releases the plant from stressors such as lack of water or nitrogen or too much salt," Drake said.
For Drake, the last two decades of research have revealed an important message if we as a society are willing to invest the time and money to understand it. Just as poison ivy seems to have a super-charged ability to take up CO2 scientists suspect that other plants, particularly some invasive species, may share this trait. Phragmites for instance is a highly invasive wetland grass that is reducing biodiversity in marshes and impacting wetlands around the country from Washington in the Pacific North west to North Carolina in the South east. Drake and his colleagues are now planning experiments to find out if Phragmites may be a super-responder to CO2. If it is, then we can expect it to continue to spread as CO2 rises in the years ahead. "That's one of the negative outcomes of not understanding all of this well," Drake said. "But what if we could turn this story on it's head," he continued, "What if we said 'well, if this plant really responds to CO2, let's start growing it. Let's dedicate some land to growing it and harvesting it and burning it instead of coal to generate electricity.' Is it conceivable, would it work out? I don't know." Drake points to the use of switchgrass in special stoves in Canada and the increasing popularity of pellet stoves in the U.S. to replace fossil fuels.
The importance of such a resource is not just that it's renewable and reduces our dependence on non-renewable sources, but it can produce a zero net gain—or near zero net gain—in atmospheric CO2. Instead of releasing the carbon that's locked up in fossil fuels such as coal and oil, a plant-based renewable energy program has the potential to simply cycle carbon--releasing the same amount of carbon that the current crop of energy plants is taking up.
For Drake, it's just one idea of many that may help turn a story of a liability into an opportunity. He is among a growing body of researchers becoming increasingly interested in finding out how we can use plants to store more carbon and to provide a viable alternative to fossil fuels. The potential for plants to store excess carbon and provide renewable energy may be all around us, but we don't know which plants do it best, under what conditions, or why. "We've got to know the rules that plants have evolved for taking up extra carbon," he said, "and we don't. We don't understand it well enough to predict which plants are best for our purposes."
With the 20-year anniversary of his study site at SERC and his widely referenced contributions to the field, Drake is taking the opportunity to speak out around the country about climate change and about the potential for using plants to help solve the problem. "We need to know more," he said, "we don't know enough, but with continued research, perhaps we can find out."
View a video interview with Dr. Drake
For more information, or to reach Dr. Drake, please contact SERC science writer Kristen Minogue.