SEARCHING FOR SUSTAINABLE ENERGY,
by The Editors
Carbon-negative biofuels, produced from plant biomass, offer a cost-effective renewable energy alternative to fossil fuels. Numerous grasses and woody species have been evaluated in this regard, but perennial grasses that grow from rhizomes are regarded as most economically promising. Some of these species include switchgrass (Panicum virgatum; native to the eastern and central United States), giant reed (Arundo donax; native to Asia), reed canarygrass (Phalaris arundinacea; native to temperate Europe, Asia, and North America), and Miscanthus (Miscanthus x giganteus; native to Asia). Switchgrass, giant reed, and reed canarygrass are reproduced both vegetatively and from seed, while Miscanthus is a triploid hybrid that reproduces only vegetatively. The following traits are thought to be important in selecting ideal grasses for biomass: efficient photosynthesis; rapid growth; long canopy duration; no known pests or diseases; perennial growth; sterility; low energy into maintenance relative to the production of energy-rich biomass; efficient use of light, water, and nutrients; and nutrient translocation to storage organs during the nongrowing season.
Extensive research projects are under way on the agronomic and energy-production potential of these perennial grasses. While these projects are yielding promising results in terms of biomass yield and energy production, key gaps exist in the knowledge.
Biofuel Grasses—What Is Not Known?
While there is extensive debate about the cost-effectiveness of ethanol production and environmental impacts of ethanol combustion, at least one significant ecological question has not received rigorous attention: the ecological risks posed by the grass species being considered as biomass crops. This is an important issue to consider before deploying these species across the landscape, because many of the characteristics deemed ideal in a bioenergy crop are also features commonly found among invasive grasses.
Biofuels or bioenergy crops offer significant potential for economic and environmental benefits (primarily for carbon sequestration), but moving them from their native ranges into novel habitats may also carry concomitant ecological risks of invasion. Executive Order 13112 requires that risk–benefit analyses be performed to ensure that the benefits outweigh risks and that a management plan be put in place to manage any risks identified.
Few scientific studies have been performed to date on ecological costs or risks of the grasses being considered as biofuels. Anecdotal claims of “low risk” for some species, based on experiences elsewhere, may indeed be valid; however, numerous species introductions with reputed economic and environmental benefits have had long-term economic and environmental costs due to invasive spread beyond the cultivated ranges. For example, the prevalence of wild Cannabis sativa (hemp, marijuana) across the midwestern United States today has origins in federal encouragement of hemp production during the 1930s to 1940s. Other similar plant introductions that have turned invasive include kudzu (Puereria lobata; from Asia) and Johnson grass (Sorghum halepense; from the Mediterranean region), both introduced for forage and erosion control.
Two candidate biofuel species that are likely to create negative ecosystem impacts are giant reed and reed canarygrass, exotic grasses with C3 photosynthesis (carbon dioxide taken up from the air is incorporated into a 3-carbon compound; most efficient under cool, moist conditions and under normal light). They are known to be invasive in some ecosystems in the United States. Giant reed chokes riparian areas and alters fire cycles, disrupting native communities as it spreads, whereas reed canarygrass is a major invader of wetlands, where it detrimentally affects wildlife habitat. Despite these known risks, giant reed is being promoted as a bioenergy crop with 15,000 acres in Florida and 30,000 acres in Alabama targeted by energy firms for cultivation. Large-scale planting of a known invasive species for biofuel use creates an unacceptable precedent, lowering the regulatory bar for species whose risks have not yet been evaluated.
