- Top of page
- Materials and methods
- Supporting Information
Government policies in the United States and Europe encourage an increasing proportion of energy to be derived from biofuels (Robertson et al. 2008). Dedicated bioenergy crops are hence being developed and deployed on a large scale for fuel production. In the United States, the Biomass Crop Assistance Program (BCAP) instituted by the United States Department of Agriculture Farm Service Agency (USDA-FSA) has initiated projects in bioenergy production focussing on extensive herbaceous plantings (50 000 acres per state by 2014) in Ohio, Pennsylvania, Arkansas, and Missouri (USDA-FSA 2011). There is pressure for bioenergy crops to yield large quantities of biomass while avoiding pitfalls associated with grain-based biofuels. By cultivating bioenergy crops on marginal lands unfit for food crops, it may be possible to avoid the ‘food vs. fuel debate’ (Tilman et al. 2009). The significant environmental and economic costs associated with irrigation and pesticide application may be mitigated by bioenergy crops that are drought tolerant, competitive with weeds and pest resistant (Heaton, Voigt & Long 2004). Such vigour is not without its costs, however: a downside of these traits is the potential for these plants to thrive outside of cultivation (Raghu et al. 2006).
Sheltering plants from stresses through cultivation can reduce environmental stochasticity, allowing for repeated sampling of recruitment sites and improving the likelihood of population establishment outside of cultivation (Mack 2000). The cultivation of bioenergy crops poses several risks, in terms of facilitating invasions. Propagule pressure is correlated with invasion success (Lockwood, Cassey & Blackburn 2005), and considering the large spatial scale of planned bioenergy plantings (USDA-FSA 2011), it is likely that propagule pressure may be high and sustained. Traits typifying an ideal bioenergy crop (e.g. fast growth, highly efficient nutrient/water/light use) overlap with those of weedy and invasive species (Heaton, Voigt & Long 2004; Raghu et al. 2006). Additionally, many bioenergy candidates have a history of invasion (Buddenhagen, Chimera & Clifford 2009), which is troublesome considering that prior invasion is one of the most reliable predictors of future invasion success (Hayes & Barry 2008).
Many leading bioenergy candidates are perennial grasses (Lewandowski et al. 2003), a group of plants also recognized for their world-wide success as invaders (D'Antonio & Vitousek 1992). Grasses may regenerate by both wind-dispersed seeds and rhizome fragments (Briske & Derner 1998). In the Midwest United States, Miscanthus × giganteus has been identified as a promising candidate bioenergy crop, yielding on average 29·6 Mg ha−1 (Heaton, Dohleman & Long 2008), and a focal species in the USDA-FSA BCAP program (USDA-FSA 2011). Miscanthus × giganteus is a naturally occurring triploid hybrid between the diploid Miscanthus sinensis and the tetraploid Miscanthus sacchariflorus (Stewart et al. 2009). Both parent species of M. × giganteus produce fertile seeds and after being introduced to the United States as ornamentals have escaped cultivation to form naturalized populations (Quinn, Allen & Stewart 2010). In Australia, naturalized populations of M. sinensis have been found in the Blue Mountains (Jorgensen 2011). In Europe, M. sinensis seedlings have been observed outside of cultivation (Pysek, Sadlo & Mandak 2002; M. Deuter, personal communication). Clonal production systems for sterile M. × giganteus have been designed where ‘mother fields’ are a source of clonal material, and biomass production fields are planted with rhizome fragments or rooted plantlets using specialized machinery (Jorgensen 2011). In the agronomic literature, M. × giganteus is typically indicated to only produce sterile spikelets due to its triploid genome; however, an anecdotal account of fertile seed production has been reported (Nielsen 1987). A greenhouse study conducted in Illinois screening over 7 million spikelets did not yield a seedling, supporting the assumption of sexual sterility (Matlaga, Schutte & Davis 2012). The Illinois clone variety of M. × giganteus has been the focus of the majority of production trials and the USDA-FSA BCAP program in Ohio, Pennsylvania, Arkansas, and Missouri (USDA-FSA 2011) and represents c. 90% of the commercial market (E. Heaton, personal communication). The Illinois clone variety of M. × giganteus has been found to have lateral vegetative expansion rates of 0·15 m year−1 in unmanaged arable lands in Illinois (Matlaga, Schutte & Davis 2012).
Concern about the invasive potential of this species is supported by invasive behaviour of species that combine strictly clonal reproduction with aggressive spread outside of their native range. For example, giant reed Arundo donax is a sexually sterile (Mariani et al. 2010) bioenergy candidate, which is highly invasive in California where it spreads via recruitment from rhizome mats and fragments dispersed through riparian systems (Khudamrongsawat, Tayyar & Holt 2004). Hence, an a priori assumption that sterile M. × giganteus has low invasive potential is not supported by current scientific data.
Drawbacks of a clonal production system, namely the high costs of vegetative planting and limited opportunity to improve yields through traditional breeding techniques (Quinn, Allen & Stewart 2010), have spurred efforts to engineer fertile varieties of M. × giganteus (Yu et al. 2009). A report in the agricultural media indicates that commercialization of a fertile-seeded M. × giganteus variety is imminent (Ross 2011). Several production scenarios have been proposed to prevent or limit escape of fertile M. × giganteus seed. Seed production fields, strategically placed in areas with low invasion risk, planted with M. sinensis Andress. and M. sacchariflorus (Maxim.) Hack. could generate predominantly triploid F1 M. × giganteus seed for planting in biomass production fields (Quinn, Allen & Stewart 2010). Additionally, functional sterility could be used where late flowering genotypes are planted in northern latitudes allowing little or no time for seed maturation prior to the onset of winter (Quinn, Allen & Stewart 2010). It is crucial to understand what threshold of seed viability must be avoided to prevent escape and spread of M. × giganteus.
Conducting comprehensive risk analysis of bioenergy crop invasion is a major challenge. The negative impact of an invasion is dependent on several factors, including population dynamics of the invader, expressed as rates of population growth and spread, as well as impacts on native communities and the cost of eradication (Parker et al. 1999; Neubert & Parker 2004). To minimize and mitigate the potential of invasive bioenergy crop cultivars, a nested-sieve framework for cultivar evaluation has been proposed. Under this framework, candidate bioenergy crops must pass through pre-entry sieves, including trait-based screening and demographic climate matching analysis, and post-entry sieves, including estimation of population growth in potential range, prior to widespread release (Davis et al. 2010). The invasion risk posed by M. × giganteus in Florida (Gordon et al. 2011) and California (Barney & Ditomaso 2008) has been deemed acceptable using Weed Risk Assessment (WRA) systems methods, primarily based on the species sterility. However, M. × giganteus was evaluated for introduction into the North Central region of the USA by the same method and received a score of ‘evaluate further’ (S. Raghu and A. Davis, unpublished observations). Considering that establishment of large-scale M. × giganteus plantings are underway in the United States (USDA-FSA 2011), the time for pre-entry screening methods like WRAs has likely passed.
An important next step in the safe deployment of bioenergy crops is to guide risk analysis and management with demographic models parameterized with vital rates estimated empirically in the proposed production area. To date, this has been accomplished on a limited spatiotemporal scale for a single oilseed crop (Camelina sativa) in the intermountain West of the USA (Davis et al. 2011). Here, we expand upon this approach using a spatial demographic model describing the population growth and spread of the herbaceous perennial bioenergy crop M. × giganteus. To evaluate the invasive potential of both a sterile cultivar currently in production and a fertile cultivar under development, we simulate feral populations with specific sexual and clonal recruitment scenarios to understand approximately where within M. × giganteus reproductive parameter space thresholds for increasing population size (population growth rate, λ, >1) and spatial extent (wavespeed, c*, >1) are located. The comparison of model projections for fertile and sterile lines of M. × giganteus not only provides information on invasion risk for this particular bioenergy crop, but also represents an opportunity to understand the role of plant reproductive strategy in biological invasions. Finally, the quantitative approach described here may serve as a model system for evaluating bioenergy crops and aiding in the development of non-invasive ideotypes. Here, we report the results of a quantitative risk analysis study, in which empirically derived parameter estimates were used to implement a model of M. × giganteus spread and establishment. Our modelling analysis was framed by the hypothesis that variation in sexual reproduction in this species makes a larger contribution to population growth rate than does variation in clonal reproduction.
- Top of page
- Materials and methods
- Supporting Information
Our analyses of the spatial population dynamics of sterile and fertile cultivars of M. × giganteus support the hypothesis that variation in sexual reproduction of this species influences population growth to a greater extent than does variation in clonal reproduction. The threshold for positive population growth (λ = 1) was exceeded at lower demographic rates for seed/seedling establishment compared with rates associated with rhizome fragment establishment. As a matter of concern for producers, under similar rates of population growth (λ), the fertile variety model displayed markedly faster rates of population spread (c*) compared with the sterile variety. These fundamental differences in potential population dynamics between sterile and fertile varieties imply that the invasive potential of these varieties, and management guidelines for safe containment, should be addressed separately.
Analyses of the demographic behaviour of the sterile cultivar indicated that the survival and sprouting of rhizome fragments in the soil may strongly influence population dynamics of M. × giganteus. Matlaga, Schutte & Davis (2012) experimentally investigated the within-year establishment success of 1–20 g M. × giganteus rhizome fragments in a field experiment in Illinois. Overall, 9·5% of fragments established and fragment size was a significant predictor of establishment success; small and large fragments had 0% and 42% establishment probabilities, respectively (Matlaga, Schutte & Davis 2012). Considering the overall rhizome sprouting rate of 9·5% (therefore p = 0·095), our model results imply that adult plants must produce several rhizome fragments per individual (rp = 2–4) to achieve a growing population under these conditions. At the upper end of rhizome sprouting rate observed in the field (p = 0·42), we observed an expanding population if one rhizome fragment was produced per every two individuals (rp = 0·5). Because the rhizome system of M. × giganteus is extremely tough and requires significant force to fragment, rhizome production rates of rp = 0·5–4 are possible only under cases of severe and frequent disturbance.
Based on these model results, we identify two types of growth environments that require special attention by producers of sterile M. × giganteus cultivars: field margins and riparian areas. It is possible that, on the margins of biomass production fields, machinery could provide regular and severe disturbance resulting in the fragmentation of M. × giganteus rhizomes and creating the potential for positive population growth. For production fields abutting waterways, flooding could also drive invasive spread if flow rates are sufficient to scour and destabilize banks, leading to rhizome fragmentation or even movement of entire rhizome mats, as has been observed for Phragmites australis and A. donax (Keller 2000; Khudamrongsawat, Tayyar & Holt 2004). We advise producers to create setbacks of at least 5 m from high watermarks, plant such buffer areas to a mowed perennial sward, and to monitor these field margins annually.
Observations of λ > 1 for fertile M. × giganteus cultivars, even in model scenarios with low seed viability, were consistent with recent work showing that reductions in fecundity of 95–100% are needed to slow the population growth rate of long-lived species below λ = 1·0 (Crone et al. 2011; Knight, Havens & Vitt 2011). A growing number of reports suggest that population growth of long-lived species is sensitive to adult survival but not fecundity (Ramula et al. 2008; Knight, Havens & Vitt 2011).
These results offer concrete guidance for the development of bioenergy crop cultivars with lower invasive potential. Breeding programmes aimed at developing fertile M. × giganteus cultivars should explicitly estimate the viability of seed produced in biomass production fields as well as seedling survival, seed survival and germination success within nearby habitats. We recommend that agronomists do not exceed the demographic rates associated with the lower left corner of Fig. 3, seedling survival 0·000001 and seed viability of 0·001, when developing a non-invasive fertile M. × giganteus crop. This is not, however, a guarantee that any specific rates will prevent invasive population dynamics over the long term.
We acknowledge that results from this model, like all population models, are most valuable for the heuristic insights they provide (Crone et al. 2011), identifying specific demographic parameters, and approximate magnitudes, that if targeted could reduce the invasive potential of M. × giganteus. A specific limitation of our study is omission of rhizome fragment dispersal. Rhizome systems in large grasses can be fragmented by disturbance and dispersed potentially long distances. For example, fragmentation and dispersal of A. donax rhizomes in California riparian habitats can occur by machinery and flooding (Boland 2008). We suggest that the importance of rhizome dispersal for the spatial population dynamics be assed in three steps. A dialogue between agronomist and invasion biologist could identify the most likely and important (greatest distance) rhizome dispersal pathways of M. × giganteus (e.g. small watercourses, large streams/rivers, machinery movement between production field, machinery movement between production regions). Empirical field and laboratory studies could estimate dispersal kernel parameters in each pathway, although there is considerable difficulty in determining long-distance dispersal in waterways. Lastly, a modelling simulation approach could be used that focussed on determining the influence that varying the portion of rhizome fragments entering each dispersal pathway has on rates of population spread.
Our approach considers only the demography and population dynamics of hypothetical feral populations of M. × giganteus outside of cultivation. The strength and spatial scale of propagule pressure originating from bioenergy production was not considered in our modelling scenarios. Metapopulation modelling efforts should be conducted in the future with the goal of understanding source (bioenergy production fields) and sink (adjacent unmanaged lands) dynamics. The relatively slow spread rates we estimated for sterile M. × giganteus would not apply in scenarios where rhizomes were dispersed long distance by machinery or through watercourses.
Future efforts in the United States are needed to evaluate the invasive potential of the most common M. × giganteus varieties both in terms of empirical field estimates demographic performance across a range of environments and modelling simulations to project population dynamics over time. To our knowledge, information on the demography of M. × giganteus outside of cultivation is limited in time-scale, habitats considered, and variety of M. × giganteus. Only two published studies have evaluated the demography of M. × giganteus outside of cultivation and both considered the ‘Illinois clone’ variety (Barney et al. 2012; Matlaga, Schutte & Davis 2012). The demographic performance of the ‘Freedom clone’ variety of M. × giganteus, which constitutes a minority of the overall commercial market in the United States but is specifically targeted for the Southeast United States (B. Baldwin, personal communication), has not been considered outside of cultivation. Rhizome fragment establishment of the Illinois clone has been studied in riparian and dryland habitats of Central California over 2 years (Barney et al. 2012). The demography of adult plants and rhizome establishment has been studied in early successional habits of Illinois over 3 years (Matlaga, Schutte & Davis 2012). Future studies are needed that consider a wider range of both managed and unmanaged habitats, longer time-scales and additional varieties of M. × giganteus. Perhaps most crucial, estimates are needed of sexual reproductive parameters (e.g. seed survival, seed germination, seedling survival) for fertile M. × giganteus varieties within habitats adjacent to production fields. Additionally, studies are needed to determine whether non-reproductive demographic rates (survival, growth and lateral expansion) are identical between fertile and sterile varieties of M. × giganteus. Collaborations are needed between agronomists developing fertile varieties of bioenergy crops and ecologist tasked with evaluating invasive potential. When deciding which cultivars should receive approval for large-scale production, economic benefits of crop production must be weighed against invasion scenarios based on local empirical data.
While our study focussed on the Midwest United States, it must be recognized that efforts are underway to jumpstart bioenergy crop development and deployment on a global scale (Dornburg et al. 2010). Due to intense international interest in growing perennial grasses (i.e. Panicum virgatum, A. donax, Phalaris arundinaceae and M. × giganteus) for bioenergy (Lewandowski et al. 2003; Jorgensen 2011), a need exists to assess the risk of invasion for these species across many countries. Despite this global need, we argue that because demographic rates are inherently site and region specific (e.g. Shea et al. 2005) and play an important role in determining invasion success risk should be evaluated on a local scale. Our perspective is in contrast to the WRA approach that has been carried out to evaluate invasion risk of bioenergy crops for locations, such as U.S. state of Florida (Gordon et al. 2011). In Europe, a mixture of varieties of M. × giganteus have been considered for biomass production, the most common being the Hornum variety (Heaton et al. 2010), as well as the fertile seed producing M. sinensis (Lewandowski et al. 2003). To our knowledge no attempts have been made to evaluate the invasive potential of these varieties in Europe or elsewhere. However, observations that single M. × giganteus plants have established in Germany from garden rubbish (Brennenstuhl 2008) suggest that rhizome establishment is a viable recruitment pathway in this region. Additionally, seedlings of M. sinensis have been found outside of cultivation in Europe (see 'Introduction').
Our study considers the potential population trajectory of M. × giganteus outside of cultivation in Illinois, United States but does not provide insight into the potential impact of an expanding population. It is known that invasive plants can alter population, community and ecosystem processes through various mechanisms (Levine et al. 2003). It is not known whether traits that typify an ideal bioenergy crop [e.g. rapid growth, competitive with weeds, drought tolerant (Heaton, Voigt & Long 2004)] would predispose these species to have a significant impact in the case of escape and invasion of natural areas. However, it remains difficult to assess the risk associated with bioenergy crops without data on their potential impacts on natural ecosystems.