For Switchgrass Cultivated As Biofuel in California, Invasiveness Limited by Several Steps
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RESEARCH ARTICLE ▼ For switchgrass cultivated as biofuel in California, invasiveness limited by several steps by Joseph M. DiTomaso, Jacob N. Barney, J. Jeremiah Mann and Guy Kyser Jacob Barney Jacob The expected production of biomass- derived liquid fuels in the United States may require cultivation of millions of acres of bioenergy crops, including perennial grasses such as switchgrass. Switchgrass is not native to California and possesses many qualities in com- mon with other perennial grasses that are invasive. To evaluate the potential invasiveness of switchgrass in California, we conducted risk analysis and climate- matching models as well as greenhouse and field evaluations of switchgrass, looking at its environmental tolerance and competitive ability against resident Some characteristics that make for an outstanding biofuel crop will make the same plant a hardy, hard-to-manage weed if it escapes cultivation. Switchgrass can thrive if it escapes into an ecosystem riparian vegetation. We concluded that with an abundant, year-round water supply, so it’s best grown far from such areas. dryland regions of California are not consideration, perennial nonfood grasses 2005). These cultivars are often separated suitable to vigorous establishment and are the leading candidates. To be success- into upland ecotypes (Trailblazer, Cave invasion of switchgrass. However, ripar- ful in this role, these bioenergy grasses in Rock, Blackwell and Sunburst) and ian areas appear to be far more likely to will need to possess many agronomically lowland ecotypes (Alamo and Kanlow) support switchgrass populations. With desirable traits, including broad climatic (Pedroso et al. 2011). tolerance, rapid growth rates, high yields, Switchgrass is not native to California effective mitigation practices in place few natural enemies and resistance to and was, in fact, included for a brief time throughout the development, growth, periodic or seasonal soil moisture stress on the California Department of Food harvest, transport and storage pro- (Heaton et al. 2009). and Agriculture (CDFA) Noxious Weed cesses, it should be possible to minimize One of the leading candidates among List (CINWCC 2007) due to concerns bioenergy grasses is switchgrass (Panicum about its potential invasiveness. Although or eliminate the movement of seeds virgatum L.) (Parrish and Fike 2005; there was one documented report of an and vegetative propagules to sensitive Pedroso et al. 2011). Switchgrass is a pe- escape of switchgrass from cultivation in habitats. Consequently, we believe that rennial warm-season (C4) bunchgrass Orange County, California (Riefner and switchgrass is unlikely to become a sig- native to most of North America east Boyd 2007), there are no known records of the Rocky Mountains, where it was of its escaping elsewhere or causing any nificant problem in California, even with historically a major component of the tall- ecological or economic damage, despite widescale production. grass prairie. It was included in the initial its long-time use as a forage and conserva- screening for biofuel crops in the United tion species (Parrish and Fike 2005). Since States in the 1970s and was determined its removal from the CDFA Noxious Weed iomass crops in the United States are to be the model bioenergy species by List, it has been the focus of yield trials Bprojected to yield 136 billion liters the Department of Energy (McLaughlin throughout California (Pedroso et al. of biomass-derived liquid fuels by 2022 and Walsh 1998). This was primarily 2011). Because of the state’s Mediterranean (Heaton et al. 2008). The expectation is due to its broad adaptability and genetic climate, the yield potential is high; how- that this will require cultivation of be- variability (Sanderson et al. 2006). Over ever, the crop will require significant wa- tween 54 and 150 million acres of bioen- the past three decades, breeding efforts ter and nitrogen inputs. ergy crops. Furthermore, state and federal have developed several cultivars, many greenhouse-gas reduction initiatives of which produce dense stands, tolerate Online: http://californiaagriculture.ucanr.edu/ have incentivized widespread cultiva- infertile soils and readily regenerate from landingpage.cfm?article=ca.v067n02p96&fulltext=yes tion of biofuel crops. Of the crops under vegetative fragments (Parrish and Fike doi: 10.3733/ca.v067n02p96 96 CALIFORNIA AGRICULTURE • VOLUME 67, NUMBER 2 TABLE 1. Agronomic and invasive characteristics of a potential biofuel crop (switchgrass), a Invasive and biofuel grasses phylogenetically related weedy species (johnsongrass) and corn (an agronomic crop related to both) In an ideal system, biofuel crops should be cultivated in a highly managed Biofuel crop Weedy associate Agronomic crop agricultural setting similar to that of most Agronomic/invasive characteristics Switchgrass Johnsongrass Corn major food crops, such that the crop could C photosynthesis +* + + not survive outside of cultivation. Under 4 such conditions, the likelihood of escape Perennial + + –† and invasion into other managed or natu- Rapid establishment – + + ral systems would be very small. Unlike Highly competitive +/–‡ + – biofuel species, most food crops have been selected for high harvestable fruit or Drought tolerant + +/– – grain yield. This nearly always results in Reallocation of nutrients to roots + + – a loss of competitive ability, typically ac- No major pests or diseases + + – companied by an increase in the addition of nutrients and often pesticides. Prolific viable seed production + + + When a biofuel crop is grown for cellu- Ability to shatter and disperse seeds + + – lose-based energy, the harvestable prod- Source: Barney and DiTomaso 2010b. uct is the entire aboveground biomass. To * + = Plant demonstrates the characteristic. be economically competitive, such peren- † – = Plant does not demonstrate the characteristic. ‡ +/– = Some ecotypes of the plant demonstrate the characteristic. nial crops should be highly competitive with other plant species, harbor few pests and diseases, grow and establish rapidly, johnsongrass and only a few similar to managed systems should be assessed produce large annual yields and have a those of corn (table 1). While this is not before the crops are commercialized broad range of environmental tolerance, direct evidence that switchgrass will be and introduced into new regions. A pre- while also requiring few inputs per unit a significant invasive or weedy species, it introduction evaluation can be performed area of water, nutrients, pesticides and does suggest that the risk may be greater in tandem with typical agronomic yield fossil fuels (Raghu et al. 2006). Few spe- than for more typical agronomic crops. trials. This approach can both quantify cies fit these requirements better than Although cultivation of switchgrass ecological risk and assess the suitability rhizomatous perennial grasses, primarily and other biofuel crop species may ul- and economic performance of the species nonnative species (Barney and DiTomaso timately prove a net benefit to society, within a particular region. 2008; DiTomaso et al. 2007). However, the environmental risks associated with To assess the invasive potential of these qualities and traits are nearly iden- their potential escape into natural and any proposed biofuel species, including tical to those found in harmful invasive species (Raghu et al. 2006). For example, species such as johnsongrass (Sorghum halepense (L.) Pers.) and kudzu (Pueraria Barney Jacob montana [Lour.] Merr. var. lobata [Willd.] Maesen & S.M. Almeida) were introduced as livestock forage or for horticultural use but have escaped cultivation to be- come serious weeds in many areas of the United States. In selecting biofuel crops, a balance must be struck between high pro- ductivity with minimal inputs, on the one hand, and risk of establishment and sur- vival outside the cultivated environment (DiTomaso et al. 2007) on the other. Johnsongrass, like switchgrass, was first cultivated as forage, but it subse- quently escaped and has become one of the world’s most expensive weeds in terms of control costs (Warwick and Black 1983). It is currently listed as a noxious weed in 19 U.S. states. When compar- ing switchgrass to johnsongrass and to corn, a typical agronomic grass crop, it Traditional crops are bred to enhance production of food or fiber, but biofuel breeders emphasize is clear that switchgrass possesses many hardiness and quick, abundant growth—”weedy” characteristics. Here, researchers grow switchgrass growth traits similar to those of weedy in PVC towers, making it easier for them to monitor root growth and the effects of water stress. http://californiaagriculture.ucanr.edu • APRIL–JUNE 2013 97 switchgrass in California, we propose a TABLE 2. Protocol for evaluating the degree of risk that a biofuel crop (including cultivars and genotypes) seven-step evaluation protocol (table 2) will become invasive in a region (Barney et al. 2012a). These evaluations should be performed for all candidate Evaluating the risk potential of biofuel crops: qualitative and quantitative studies genotypes and cultivars as well as for 1. Conduct risk assessment using a science-based protocol to determine invasion potential qualitatively. transformed genotypes of native species because ecological interactions can vary 2. Determine the potentially invasible range using climate-matching analysis (e.g., CLIMEX) under various assumptions (e.g., drought tolerance) and scenarios (e.g., irrigation).