Seashore Mallow can survive one drought after another. It's a perennial. And it feels right at home in soil so salty it would kill most food crops. And most importantly, it has oil-rich seeds that could someday become an important source of biodiesel.
No one knows yet if Seashore Mallow will actually produce a reliable, affordable fuel.But many researchers believe that the future of biofuel will rest largely on such plants, which -- like agave, sorghum, miscanthus, and prairie cordgrass -- are rugged enough to withstand an increasingly harsh climate.
From the hurricane-battered East Coast to the drought-strickenMidwest, 2012 was a reminder that bioenergy crops of the future may need to withstand unusual floods and droughts. Just look at what happened to corn, the country's source of bio-ethanol. From Kansas and Nebraska to Illinois and Indiana, cornstalks withered in blistering heat under blue skies that rarely even hinted at rain. Whether they were aiming for harvests of sweet corn, cattle feed or corn intended for ethanol, some farmers ended up with little more than dead plants and hefty insurance claims.
Nonetheless, Midwest researchers are working on drought-resistant corn, and along with California and the Southeast – the region is Grassoline Central for many scientists and farmers working on cellulosic biofuels – renewable “green” fuels made not from food like corn but from the stalks and leaves of woody plants.
If you drove around certain parts of the Midwest this summer, you might have seen patches of energy grass or tall, green sorghum flourishing amid the ruined corn and soybeans.
At the same time, biofuel researchers are investigating energy crops that can thrive in soils that have been flooded or inundated by seawater – something particularly important as sea levels rise. Candidates include shrubs accustomed to sea spray, such as the Seashore Mallow, along with salt-tolerant grasses and even trees. (See our sidebar for more on research on Seashore Mallow and other salt-resistant plants.)
Climate change, in short, points to an urgent premise: Biofuel plants of the future should be hardy, easy to grow, and full of energy potential, but they also need to be grown more efficiently and with fewer resources. This means looking at the earth’s abandoned, degraded, or marginal land – land which, in many cases, is too arid or saline to grow traditional food crops.
Some of these advanced biofuel crops have never really been systematically cultivated before, raising questions about optimum breeding, planting, and harvesting. But researchers see opportunity in that uncertainty. Here’s a look at a few especially tough plants that show promise as advanced biofuel sources:
Giant Miscanthus (Miscanthus x giganteus)
Any variety of grass that can grow more than 12 feet tall is bound to get some attention. When that plant can reach such heights on farmland with no fertilizer, it’s a potential game-changer. Of all of the grasses that have been suggested as a lignocellulosic source of biofuel – that is, biofuel made from the woody cell walls of plants – miscanthus seems to hold particular promise, says Dr. Evan DeLucia, head of the department of plant biology at the University of Illinois at Champaign.
As DeLucia explains, miscanthus has many appealing features beyond its prodigious height. As a sterile hybrid, it doesn’t produce viable seeds, which means it’s not likely to invade neighboring fields. The plants in Illinois seem to be largely resistant to local insects and disease. Importantly, it doesn’t seem to require nitrogen-based fertilizers, a major source of groundwater pollution, and nitrous oxide, a potent greenhouse gas. “There are plots in England that have been growing for 15 years without any added nitrogen,” DeLucia says.
The reason: Many perennial grasses, like miscanthus, funnel their minerals and carbohydrates during the winter months to underground stems called rhizomes. “At harvest time, the leaves are essentially paper,” he says. Harvesting biomass during the winter months allows the plants to hold on to large amounts of minerals, reducing or eliminating the need for fertilizer, he explains. (The rhizomes also sequester significant amounts of carbon.)
Perhaps most of all, miscanthus is hardy enougn to withstand periodic drought – including the drought of 2012. "We had an accidental experiment that summer," DeLucia says. While the hot, dry summer stunted the growth of native prairie grasses and nearly wiped out the corn on experimental plots, the stands of miscanthus actually put on more biomass in 2012 than in previous years. "Miscanthus was the clear winner," he says.
In theory, miscanthus’s combination of size and toughness could prove quite valuable in the energy markets of the future. In a study published in a 2011 issue of Frontiers in Ecology and the Environment, DeLucia and colleagues used computer models to estimate the potential benefits of growing miscanthus instead of corn for ethanol. According to the models, replacing just 30 percent of the least productive ethanol corn crops with miscanthus could increase ethanol production by 82 percent while reducing greenhouse gasses and dramatically improving groundwater by cutting back on nitrogen pollution.
But miscanthus has some drawbacks, too. It grows from rhizomes that have to be planted one by one, which is unusually time consuming and expensive. The biggest problem: Although there are some plans in the works, there is currently no facility in the United States ready to turn stalks of miscanthus—or any other lignocellulosic material—into ethanol on an industrial scale. “When that market becomes available, miscanthus should really take off,” DeLucia says.
Prairie cordgrass (Spartina pectinata) and switchgrass (Panicum virgatum)
Miscanthus isn’t the only energy grass causing a buzz in the biofuel world. Prairie cordgrass,, a hardy grass that grows up to 9 feet tall in the marshlands and tidal flats of North America, Europe and Asia is certainly worth a look, too, says Dr. D.K. Lee, Ph.D., an assistant professor of crop sciences at the University of Illinois. It combines some of the drought resistance of miscanthus with some of the salt tolerance of seashore mallow. It can also withstand relatively cold temperatures, so much so that it could possibly take hold in Canada. For now, Lee says that it shows the most promise for lands that are prone to frequent flooding. Although it can withstand a good dry spell, he says, it’s unusually well-equipped to grow in water-saturated soils.
Another energy grass – switchgrass -- has been growing on America’s plains and prairies for thousands of years. By necessity, it has evolved to withstand extremes of temperature, wind, and most of all, periods of drought. “It’s highly adapted to the prairie, where moisture is scarce,” says Fred Allen, PhD, professor of plant sciences at the University of Tennessee in Knoxville. As Allen explains, switchgrass has an extensive root system that can draw water from the soil. Like cordgrass and miscanthus, it also uses C4 carbon fixation, trapping four carbon atoms instead of three – a metabolic strategy that helps it stand up to the heat.
Like miscanthus, this native grass reaches impressive heights. The lowland variety that grows in Tennessee and other areas of the south averages about nine feet, but Allen has seen some individual plants that reached over 13 feet – all of which can add up to some serious biomass. Allen says an acre of lowland switchgrass can produce six to eight tons of biomass in one harvest, and that’s on relatively marginal land. Fertile land could yield up to 14 tons. “I think switchgrass could compete with miscanthus in terms of tonnage in our type of climate,” Allen says.
Like many other parts of the country, Tennessee became a laboratory of extreme weather during the summer of 2012. “We had a days that were over 100 degrees in June, and there was very little rain,” Allen says. While fescue grass quickly turned brown as a biscuit in the Tennessee heat, he says, switchgrass stayed green. “The drought definitely reduced the yield,” he says, “but not nearly as much as it did for corn.”
The deserts of the American southwest may not seem like a promising place to raise biofuel feedstocks—or much else for that matter. But some plants feel right at home here.
Members of the agave family naturally thrive in landscapes that are too hot and too dry for most forms of agriculture. This is partly because the agave plant has a metabolism that cuts down on water loss.
Known as CAM, the process lets agave absorb carbon dioxide during the cool nighttime and store it to use in photosynthesis in the daytime. During the heat of the day, agave keep their stomata, or the openings in their leaves, tightly closed, thus saving an enormous amount of water.
Agave is perhaps best known as the plant that gives us tequila, a liquor made from the sugar that collects in the base of the stems. But in the future, the plant could be used not only to make tequila but to produce a different kind of alcohol—the kind that fuels cars, not college parties.
The process of distilling fuel-grade ethanol from agave is essentially the same as the process for distilling tequila, says Dr. Sarah Davis, Ph.D., assistant professor of environmental studies in the Vionovich School for Leadership and Public Affairs at Ohio University. As Davis and colleagues reported in a 2011 issue of GCB Bioenergy, it’s possible to obtain a liter of 40 percent ethanol from 5.5 kilograms of the sugary stems, which are called piñas. Theoretically, a tequila-processing plant processing 400 metric tons of piñas every day could produce 61 million liters of 100 percent ethanol in a year. And that doesn’t include the millions of liters of second-generation fuel that could be produced from leftover leaves.
Agave plants have an amazing ability to build up biomass in less-than-ideal conditions. Previous studies have found that the species A. tequilana can produce impressive yields of 26 metric tons of biomass per hectare per year in semiarid region of west central Mexico, and that’s without any extra irrigation.
Davis is currently growing experimental plots of three agave species southwest of Phoenix and is investigating the possibility of growing the plants in less obvious places, including northern California. But as Davis points out, there are already roughly a half-million hectares of land in Mexico, Africa and other parts of the world where agave was raised for natural fibers. That industry has collapsed after the introduction of certain synthetic fibers, perhaps opening the door for a new industry. “That land could be reclaimed and repurposed for biofuel,” Davis says.
Meanwhile, Daniel Tan, PhD, a researcher and plant physiologist with the University of Sydney, is growing test plots of agave in the deserts of Australia. His recent study, published in Energy and Environmental Science, have found that agave can produce five times as much energy (in the form of ethanol) as it takes to grow it. He has also pointed out that his agave doesn’t need to be irrigated, “and in the rainy season it grows very fast.”
The dry, hot midwestern summer of 2012 proved one thing: Sorghum is one tough plant, especially compared to corn and other common crops. “Driving around Nebraska, everything was about half dead,” says Dr. Ismail Dwiekat, associate professor of agronomy and horticulture at the University of Nebraska—Lincoln. “If you saw anything green, it was sorghum.”
Sorghum comes in many different varieties. Sweet sorghum, a crop commonly grown on marginal land in the tropics, produces both grain and syrup, the latter of which often end up in either sorghum “molasses” or alcoholic beverages. Grain sorghum is grown in semi-arid regions of Africa and elsewhere, including patches of the American Midwest. Sorghum grown in the Midwest is often used to feed cattle.
Both sweet sorghum and grain sorghum are already increasingly important sources of biofuel. Dwiekat notes that a bushel of grain sorghum can produce just as much ethanol as a bushel of corn—about 10.2 liters. But grain sorghum requires only about one-half to two-thirds as much water. “As long as you get 15 inches of rain a year, it doesn’t need any additional irrigation,” he says.
In 2012, the Environmental Protection Agency reported that, according to its estimates, ethanol made from grain sorghum at appropriately green facilities would likely qualify as an advanced biofuel because it would cut overall greenhouse emissions by at least 50 percent compared with gasoline. The agency is currently accepting public comment on sorghum and hasn’t officially given it the “advanced” designation, which would mean the ethanol could be sold at a premium Still, sorghum is already being turned into ethanol at plants that also process corn, and Dweikat predicts that production will increase in the near future.
Meanwhile, sweet sorghum growing in the tropics fills a unique role. “It’s the only crop that can produce both food and biofuel at once,” says Serge Braconnier, an ecophysiologist with CIRAD, an agricultural research center based in Paris, France.
By his estimates, a single hectare of sweet sorghum can produce six tons of grains for food during a four-to-five month life cycle. And there’s enough sugar in the stems to produce 5,600 liters of ethanol, which is about 15 percent less than a comparable hectare of sugarcane that takes 16 months to grow.
While sorghum is popular in Africa and is “the hottest thing in Brazil,” according to Dwiekat, it definitely has a long way to go elsewhere. From Nebraska to Asia, many farmers are reluctant to put it in their fields. “Sorghum is seen a poor man’s crop,” Dwiekat say. “But it was put on earth for a reason. We need a crop that’s suitable for the current environment. And corn isn’t one of them.”
That’s where tough plants come in. About 18 percent of the earth’s surface is semi-arid, including 600 million hectares of former farmland, and in the United States, farmers are paid to let millions more lie fallow. As plant chemistry and microbial biology experts Drs. Heather Youngs and Chris Somerville of the Energy Biosciences Institute point out, “Growing perennial grasses on the 13 million hectares of land farmers in the US are paid to keep out of production to support commodity prices -- combined with crop residues – could provide enough fuel to meet 65 percent of the demand for gasoline in the U.S.”
Of course, figuring out where – and how – to plant such crops can make a big difference. As Sarah Davis has pointed out, cutting down forest to plant miscanthus would contribute to greenhouse gas emissions, for example, but planting it on former pasture land could result in a net “sink” of greenhouse gases. Other factors such as tillage, drainage, and residue removal also make a difference, she explains. Miscanthus, for example, needs a relatively large amount of fresh water in arid zones, so cordgrass might be a better choice in those areas. In regions with more rainfall, miscanthus is a contender.
But if scientists can find ways to turn drought- and saline-resistant plants into energy on a wide scale, it just might be possible to fuel the world without putting too much strain on watersheds – or competing for farmland or forests. “In a world were arable land and water are increasingly scarce,” says Tan, “it’s important to find biofuels that won’t compete with food production for land and water.”