A preintroduction evaluation can be performed in tandem with typical agronomic yield trials. This approach can both quantify ecological risk and assess the suitability and economic performance of the species within a particular region. To assess the invasive potential of any proposed bio-fuel species, including switch grass in California, we propose a seven-step evaluation protocol . These evaluations should be performed for all candidate genotypes and cultivars as well as for transformed genotypes of native species because ecological interactions can vary widely within a species. Science-based information generated from risk assessments, bio-fuel crop ecological studies, niche modeling and other evaluations can guide risk mitigation decisions at appropriate points within bio-fuel research and development, crop selection and production, harvest and transportation, storage site selection and conversion/refinery practices .Risk assessment tools have been used in Australia and New Zealand as an aid in decision making for the proposed introduction of novel species for horticultural, agronomic and other purposes. For potential bio-fuel species, risk assessment should serve as a basic first step in evaluating their invasive potential, whether the species are exotic, native, 4×8 grow tray novel constructs or genetically modified. We performed a risk assessment for switch grass in California using the Australian model .
Our analysis produced an inconclusive result, with an “evaluate further” classification . The first question in the risk assessment asks whether the species is domesticated. A “yes” response favors acceptance , under the assumption that domestication generally reduces the inherent weediness of wild types, which are wild plants not selected for production traits . As previously discussed, this is true for most agronomic and horticultural species, but the opposite is true for bio-fuel crops because selection for favorable bio-fuel crop characteristics generally enhances “weedy” characteristics . When we answered the first question differently, the outcome changed from “evaluate further” to “reject.” We further evaluated switch grass as a hypothetically sterile cultivar. In this case, the weed risk assessment yielded an acceptably low risk that it would become invasive. This suggests that seed production may be key to the potential invasiveness of switch grass. However, a lack of seed production does not guarantee a low risk of invasion, considering that the giant reed , which is sterile, is highly invasive in California. The invasiveness of giant reed is due to its ability to regenerate from stem nodes after the stem is detached from the rhizome as a result of flooding or control efforts. Despite the invasive potential that our analysis shows for switch grass in California, there are no documented cases of the species escaping in agricultural or natural systems. This, however, may be a function of the limited number of opportunities for introduction of switch grass propagules outside of intentional planting areas. It is also important, therefore, to conduct studies that will quantify switch grass performance in various ecological settings in order to mitigate the risk of propagule escape and establishment.The natural distribution of a species is largely controlled by climate factors, with precipitation and temperature playing the dominant roles .
Bioclimatic envelopes, or climate matches, can provide both an estimate of range suitability for the bio-fuel crop species outside cultivation and the agronomic potential of the bio-fuel crop in the targetregion . There are numerous methods for estimating the bio-climatic envelope, including CLIMEX, Maxent, GARP, BIOCLIM, classification and regression tree, and simple logistic regression. CLIMEX has been used to model the distribution of bio-control agents , poikilothermic animals and many invasive plant species . The strength of CLIMEX for invasive species applications is that the model can be based on the historical range and supplemented with empirically derived biological and physiological data . We performed a CLIMEX analysis of switch grass using the plant’s native range as a basis in building the model and then supplementing it with environmental tolerance data from greenhouse studies . In a global model of potential suitability, the potential cultivatable range of switch grass was very broad, both with and without irrigation inputs . Subsequent analysis of potential suitable habitat in the western United States indicated that much of the region is unsuitable for switch grass, likely because of the very dry summers of arid and Mediterranean climatic regions . However, when adequate yearlong soil moisture was available , the suitable range of switch grass increased dramatically throughout much of the western United States . This could indicate that the successful cultivation of switch grass would depend upon summer irrigation, while any escape from cultivation and invasion into natural areas would likely be confined to riparian or wetland areas with a permanent water source. Riparian systems are the most heavily invaded habitats in the Central Valley of California, as they possess the primary limiting resource of soil moisture . Furthermore, riparian areas often border production fields, and traversing them would be unavoidable during bio-fuel biomass transport.
The CLIMEX model does not forecast yield potential, but it does demonstrate that some regions of California are suitable for establishment and persistence of switch grass. In support of our suitability prediction in California, Pedroso et al. evaluated the agronomic potential of switch grass in four regions of the state. This evaluation showed high productivity in both study locations in the Central Valley, which was considered a highly suitable region based on our bio-climatic index . Switchgrass grown in the Imperial Valley , near the margin of highly suitable climatic conditions in our analysis, also produced high yields. In contrast, yields and survival of switch grass were lowest in the most northern, cooler, mountainous region of Tulelake, near the Oregon border in northeastern California. Our model also predicted that this region of the state would be poorly suited to switch grass establishment, even with irrigation.Each bio-fuel species should be evaluated for various physiological and environmental tolerances. This information can be used to identify the ecosystems that are most susceptible to invasion and can also be integrated into risk analysis and bio-climatic and agronomic models to estimate, and subsequently mitigate, the likelihood of invasion . Based on results from our CLIMEX analysis of switch grass in the western United States, water availability should be the major limiting factor for switch grass naturalization. To test this, we conducted a greenhouse study to evaluate switch grass’s tolerance of soil moisture stress at various levels of water availability, ranging from moisture deficit to flooded, and we also assessed the germination, establishment, performance and reproductive potential of four common ecotypes, both upland and lowland . Our results showed that cultivars of switch grass performed well in both well watered control and flooded conditions. Although switch grass survived extended periods without water, individual plants in drought treatments were shorter, with lower measurements for leaf area and specific leaf area, and they produced fewer tillers and less biomass . As expected, lowland types outperformed upland types in the flood treatment and also displayed higher fitness under most conditions, which likely explains why they are the target of germplasm improvement for bio-fuel cultivation . We concluded that switch grass, particularly lowland ecotypes, 2×4 flood tray has the ability to germinate, establish and flower in low moisture and even more so in flooded conditions. The evidence further supports the climate-matching data and indicates that soil moisture is the limiting factor in the establishment and growth of switch grass in regions of the western United States. While tolerance to a range of soil moisture conditions may increase the cultivatable range of switch grass, it also suggests that the species is not likely to be very competitive in natural areas exposed to prolonged drought, as is common in much of California. In another study, we grew switch grass in outdoor mesocosms under irrigated and rainfed conditions and assessed the spatial distribution and abundance of roots using minirhizotron images and whole root-system sampling . Although plants survived extended periods of drought, their shoot and root biomass, root length density, numbers of culms and culm height were greatly reduced under dry conditions. These data support the results of the greenhouse study .
The rainfed treatment reduced switch grass whole-plant biomass by 83%, culm production by 67% and root length density by 67% from the levels of irrigated plants. However, switch grass grew roots continuously into regions of available soil moisture as surface soil layers grew increasingly dry . A deep-rooting habit and continuous root growth from regions of water depletion to moister regions are strategies used for drought avoidance by plants exposed to periodic water stress . It is important to note that while switch grass survived dry, rainfed conditions, its performance was significantly reduced. This level of performance would be unacceptable for agronomic production and would also reduce the ability of switch grass to establish and compete with resident vegetation in drier natural areas.The results of our climate-matching analyses as well as the biological and physiological studies allowed us to identify habitats that were most susceptible to invasion by switch grass. From our previous work, we knew that riparian corridors and perhaps even rice production fields are the regions most likely to be susceptible to switch grass invasion in California. In subsequent work , we confirmed these findings by introducing switch grass propagules into a riparian habitat under controlled conditions and evaluating their colonization, survival and establishment potential under varying levels of soil moisture availability and competition. The results supported our greenhouse and mesocosm studies, again demonstrating that while switch grass can survive under drought conditions, its performance on upland sites away from streams was very poor compared to that of switch grass plants adjacent to the stream. This confirms our conclusion that riparian regions of the state are the areas most potentially susceptible to switch grass invasion, while dryland regions of California have very low susceptibility to invasion. Of equal importance, switch grass grown without competition in the first year in the wet habitat produced about six times more tillers than switch grass growing in an intact resident plant community with competition, and the tillers were twice as tall and yielded eight times the above ground biomass . This further indicates that, even in a suitable habitat, switch grass is not highly competitive with other vegetation.The probability of establishment of an invasive population is directly proportional to the propagule pressure from outside sources . In the case of switch grass, outside sources will be production fields, harvest and transportation equipment and biomass storage sites. Our initial risk assessment for the invasive potential of switch grass in California determined that seed production and dispersal were the means of the greatest threat that it would become invasive . To aid in their efficient conversion into energy, cellulosic bio-fuel species are typically harvested after senescence in the field, usually in late fall. In our seed biology experiments, we showed that switch grass germinates and survives under conditions that range from 10% soil moisture to submersion in water . From these experiments, we estimate that an average switch grass field would produce between 300 and 900 million seeds per hectare. Using a conservative estimate of 300 million seeds per hectare and 60% dormancy, approximately 120 million seeds per hectare would be capable of germinating, given adequate soil moisture conditions . This tells us that mitigation practices will be needed to reduce the risk of seeds spreading to sensitive ecosystems. Mitigation practices could include the planting of sterile cultivars, cleaning equipment before moving it to other areas and using closed transport systems and storage facilities.As with genetically modified food and feed crops, screening for possible cross hybridization with related and desirable species should be obligatory to reduce the chance of genetic contamination or creation of novel hybrids . In California, there are five native species and five introduced species within the genus Panicum . To date, there is no evidence of hybridization between switch grass and any other Panicum species, regardless of its native origin. Thus, the likelihood that switch grass would either contaminate the gene pool of native Panicum species or enhance the weediness of nonnative Panicum species through hybrid vigor seems very small. Mitigation recommendations In August 2009, the U.S. Invasive Species Advisory Committee , a group of non-federal experts and stakeholders chartered under the Federal Advisory Committee Act of 1972, adopted nine recommendations for the federal government’s bio-fuel programs .