SEARCHING FOR SUSTAINABLE ENERGY,
by The Editors
Miscanthus and switchgrass are widely regarded as the leading grass species with C4 photosynthesis (carbon dioxide taken up from the air is incorporated into a 4-carbon compound; most efficient under high light and high temperature conditions) for bioenergy use in Europe and the United States. Risks of these grasses are less clear, but they should nonetheless be explicitly evaluated for environmental hazards. Of the 18 worst weeds in the world, 14 use C4 photosynthesis. Of these 14, 8 are perennial, 8 genera of grasses are represented, and 6 species are spread by rhizomes or stolons.
Several Miscanthus species have been documented as invasive, or having the potential to become so. In particular, the two parent species of Miscanthus x giganteus—M. sinensis (Chinese silvergrass) and M. sacchariflorus (Amur silvergrass)—are regarded as invasive. Presence of invasive species in the same genus is regarded as an important predictor of invasiveness in weed-risk assessments. Rendering these species sterile through breeding is not infallible. Triploidy does not guarantee continued sterility in either plants or animals, as was documented for hybrids of cordgrass (Spartina spp.) and carp. Moreover, seed viability is just one component of invasive spread. Species that spread vegetatively can still have ecological impacts. For example, genetic studies of giant reed show that long-distance invasive spread of this species in California waterways is almost entirely due to rhizome fragmentation.
The U.S. native, switchgrass, shares many traits with Miscanthus. In addition to sprouting from rhizomes, switchgrass can also produce seed, so its capacity for spread may exceed that of Miscanthus. Some switchgrass cultivars are strong competitors of native species in ecological restorations. In its favor as a candidate biofuel crop is its native status in parts of North America. Consequently, its suite of competing species, parasites, and herbivores may help keep escaped populations in check (regarded as a disadvantage in the context of biomass production). However, in regions of the continental United States in which switchgrass is not native, for example, the entire West Coast, those natural checks to invasion may not be present. Escape from co-adapted competitors and natural enemies may partly explain switchgrass invasions in Canada and Europe.
Of most concern is the poor record worldwide in eradicating or even controlling an introduced grass once it becomes an invader. The value of most crops mandates that invasive grasses be controlled on croplands with herbicides. Such action is prohibitively expensive on rangelands and in natural areas, such as national parks and reserves. Developing the most economical tool—biological control with a specific introduced herbivore or parasite on the invader—has been avoided because of the perceived risk of an agent expanding its host range to include commercial grasses, such as wheat, corn, barley, or rice. As a result, our defenses against a grass crop that becomes invasive are severely limited.
Biofuels have the potential to reduce both dependence on nonrenewable and foreign fossil fuels, and climate change associated with burning fossil fuels. Other environmental benefits of biofuels may include carbon sequestration, development of building materials, bioremediation, stabilization of soil, and creation of wildlife habitat. Non-native plant species have served as valuable crops throughout history. However, we have made mistakes in the past with well-intentioned species introductions that have gone awry. The emergence of invasion ecology as a discipline is largely attributable to the knowledge gleaned from these mistakes. The objective of this article is not to denigrate exotic plant species or the pursuit of bioenergy options; rather, it is a plea to exercise greater caution prior to the introduction of purportedly beneficial species by weighing the risks and benefits in an explicit, quantitative way. Knowing what we know about invasive species and their impacts, to not subject exotic biomass grass crops to greater scrutiny may be perceived as biofuels-wise, but is invasive species–foolish.
S. Raghu is an invasion ecologist with the Illinois Natural History Survey in Champaign. Adam S. Davis is a weed ecologist with the USDA Agricultural Research Service and is based at the University of Illinois at Urbana-Champaign. Both are interested in understanding invasiveness and mitigating impacts of invasive species, including species that were introduced as potentially beneficial species.
