Assessing habitat susceptibility and resistance to invasion by the bioenergy crops switchgrass and Miscanthus × giganteus in California
Highlights
► Switchgrass and miscanthus performance was greatest in the moist lowland habitat. ► Both species had low survival in the dry upland habitat. ► Switchgrass seed establishment was low in both habitats. ► Riparian areas of Central California may support escaped switchgrass.
Introduction
Bioenergy crops hold great promise in lowering dependence on non-domestic sources of energy, reducing carbon emissions, and propping up rural economies [1]. The bioeconomy is anticipated to integrate a multitude of crops across the United States (US) that include grains, oil crops, perennial grasses, fast-growing trees, and possibly algae [2]. An area comparable in size to that currently in maize and soybean (∼60 million hectares) is estimated to be necessary to meet federal mandates of 136 hm3 of renewable liquid fuels in the US by 2022 [3]. Agricultural intensification of this magnitude will have many direct and indirect impacts on landscape composition, nutrient dynamics, greenhouse gas budgets, wildlife habitat, and native biodiversity [3].
One possible consequence of the bioeconomy is the utilization of currently, or potentially, invasive species as bioenergy crops [4]. Due to logistics and economics, bioenergy crops will be required to be highly productive with limited inputs of pesticides, fertilizers, and irrigation while being tolerant of annual aboveground biomass removal [5]. This will require that crops harbor few pests and be highly competitive with other plant species. These traits describe the invasive ‘ideotype’ [6], and typify many of our worst invasive species [7], most of which were intentionally introduced [8]. It would be injudicious to assume bioenergy crops are inherently safe, or risky, based on their selected traits and goals of introduction. Bioenergy crops present a unique opportunity to study fundamental principles of invasion ecology, while assisting a nascent industry achieve sustainability goals of protecting natural capital and ecosystem function [9]. Such studies need to be conducted at the early stages in the development of this industry, as the bioeconomy may bring a level of agricultural intensification previously unseen in history.
The invasion process comprises several phases punctuated by barriers that are typically protracted over many decades before an invasion manifests [10]. The introduction and large-scale dissemination of bioenergy crops to vast swaths of the US may dramatically contract the invasion process and bypass the early stages of invasion (introduction and colonization). Additionally, Richardson and Blanchard [11] argue that bioenergy crops will forgo the three primary barriers to invasion—geographical (climate), environmental, and reproductive—as a result of crop selection and breeding, deliberate introduction and dispersal, and cultivation. Bioenergy crops will be chosen, based on climate matching, to be agroecoregionally appropriate and maximally productive [12]. When combined with intentional cultivation, harvest and transport, these factors greatly increase the probability of successful escape from production fields and establishment outside cultivation [13], [14].
However, not all receiving habitats will be equally susceptible to invasion by bioenergy crops. The location of production fields, storage sites, and conversion facilities in a diverse landscape matrix, that can include existing production agriculture (row crops, pastures, orchards, etc), riparian areas, forests, rangeland, wetlands, and protected areas, can complicate bioenergy crop invasion evaluation and mitigation [9]. Additionally, harvesting and transporting of bioenergy feedstocks may serve as a major introduction pathway for bioenergy crops, with habitats along transportation corridors having a higher probability of propagule invasion than habitats deeper in the landscape matrix.
It is well established that the probability of invasion is related to propagule pressure in that environment [15], and that understanding propagule dispersal will be extremely important [16]. Each habitat will vary in receiving propagule load, and each propagule has some non-zero probability of colonizing, surviving, establishing, and naturalizing in each habitat type in the landscape matrix of the bioeconomy. However, the invasive potential of most candidate bioenergy crops, and the invasibility (susceptibility to invasion) of most habitat types to bioenergy crop species are entirely unknown for those species not already well established. Therefore, there is an acute need to begin evaluating the susceptibility of likely receiving habitats to many bioenergy crop introductions. These data will inform risk analyses and policy decisions that would allow, prohibit, or require mitigation practices for specific bioenergy crop production sites, harvest protocols, transport corridors and equipment, and storage locations [9].
Despite the likely diversity of crops that will comprise the bioeconomy, it is anticipated that the majority of US bioenergy crops will be perennial grasses [17], of which switchgrass (Panicum virgatum L.) and giant miscanthus (Miscanthus × giganteus) are the two leading contenders [18]. Switchgrass is native to most of North America east of the Rocky Mountains, but not California. Giant miscanthus (M. × giganteus) is a sterile hybrid occurring in Japan as a cross between Miscanthus sinensis Anderson and M. sacchariflorus (Maxim.) Franch., both of which are native to Southeast Asia. These taxa have been the subject of three decades of research in the US and Europe for biomass production [17], though much of their basic ecology remains unknown. The widespread planting of giant miscanthus has lead to some documented cases of escape in Europe [19], though their provenance is unknown.
Our objective is to begin evaluating the susceptibility of likely receiving habitats to switchgrass and giant miscanthus introduction in California. Neither species is native to California, and switchgrass was briefly on the California Department of Food and Agriculture's Noxious Weed List [20] due to potential invasiveness concerns. Our previous work has shown that most of the western US is climatically unsuitable for switchgrass and giant miscanthus due to the prolonged summer dry season [12], [21]. However, further analysis demonstrated that if adequate yearlong soil moisture was available (e.g., riparian areas), the western climate, especially that of the California Central Valley, is highly suitable to both species. Riparian systems are the most heavily invaded habitats in the Central Valley, as they possess the primary limiting resource of soil moisture [22], and are the subject of extensive preservation efforts [23]. Tens of thousands of kilometers of streams, irrigation canals, and sloughs exist in the Central Valley. These often border production fields and would be unavoidably traversed during feedstock transport. Therefore, we targeted riparian habitats of the Central Valley, which we contrasted with a typical California dryland habitat in close proximity to a riparian site. The objective was to empirically evaluate the susceptibility and resistance of these two habitats to switchgrass and giant miscanthus introduction. Using two complementary experiments covering propagule introduction through colonization, survival, and establishment we aimed to answer the following questions: 1) Can switchgrass and giant miscanthus establish in riparian and dryland habitats?; 2) What role do available soil moisture and the competitive environment play?; and 3) Is establishment influenced by propagule introduction timing?
Section snippets
Site description
To evaluate the establishment and performance of switchgrass and giant miscanthus we initiated two complementary experiments in adjacent, but contrasting habitats: lowland (high soil moisture availability with associated riparian vegetation) and upland (low soil moisture availability and associated dryland vegetation). Both habitats occurred along the dam-controlled Putah Creek in Davis, CA (38.52N, −121.74W), which experiences high flows in winter and spring, and lower flows in summer. Putah
Lowland
During all three phases, survival in the lowland habitat varied as a function of the species*competitive environment interaction (Table 1). Giant miscanthus survival was ∼70% in both competitive environments at the end of the Colonization phase, but was dramatically reduced to 5% and 18% in the –Comp and +Comp plots, respectively, by the Establishment phase (Fig. 1a). Switchgrass survival was always higher without competition, and was 61% (-Comp) and 34% (+Comp) by the end of the Establishment
Propagule introduction
Switchgrass seed germinated in both upland and lowland habitats, but failed to establish in all instances. The pollen bags that surrounded each mesocosm became opaque after the creek rose during the wet season, which deposited 2–5 cm of silt and mud in the lowland system. Therefore, the switchgrass that germinated in these bags became thin and etiolated, suggesting a lack of light, and eventually died. The switchgrass that germinated in the upland typically established to the 2-3 leaf stage,
Acknowledgements
We would like to thank Andrew Fulks for providing access to the field site. This research was funded by a University of California Discovery Grant.
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Renewable energy and biodiversity: Implications for transitioning to a Green Economy
2017, Renewable and Sustainable Energy ReviewsCitation Excerpt :Finally, certain feedstocks, especially perennial grasses such as miscanthus and switchgrass, might be invasive [302–306]. Riparian habitats can be particularly susceptible [226,307–309], while there are fears that non-sterile strands of miscanthus will be difficult (or even impossible) to be contained [310]. Jatropha is the main 1st generation feedstock linked to invasive behavior and has been banned from cultivation in parts of Australia and South Africa (pre-emptively) [311].
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2015, Ecological ModellingCitation Excerpt :Using other empirical data, we then estimated relative M. × giganteus performance for each of the other cover types. In riparian areas both the growth (Barney et al., 2012) and survival rates M. giganteus were measured at ∼20% of grasslands. Miscanthus × giganteus performance in forested areas are ∼1.5 times as strong as they are in riparian areas (from Blacksburg, VA site in Smith and Barney (2014) which had all cover types), and thus ∼30% of grasslands, leading us to use a population growth rate for deciduous forests of rd = 0.10.
Suitability of Miscanthus species for managing inorganic and organic contaminated land and restoring ecosystem services. A review
2014, Journal of Environmental ManagementCitation Excerpt :However, these positive effects on biodiversity may diminish with crop age and canopy closure (Bellamy et al., 2009), probably due to species specialization. The most cultivated Miscanthus species, M. × giganteus, has a lower probability of becoming invasive (Gordon et al., 2012) than its partner species including M. sinensis (Quinn et al., 2010; Barney et al., 2012). M. sinensis and M. sacchariflorus produce viable seeds, suggesting that intra- and interspecific variability exists, and that their invasion potential is unpredictable.
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2014, Biomass and BioenergyAided phytostabilization using Miscanthus sinensis×giganteus on heavy metal-contaminated soils
2014, Science of the Total EnvironmentCitation Excerpt :Wanat et al., 2013 have shown that M. sinensis × giganteus can be grown on highly polluted soils without substantial accumulation of As, Pb and Sb. A potential problem of using Miscanthus spp. might be its invasiveness, although some studies indicate that this risk is rather low (e.g., USDA-NRCS, United States Department of Agriculture — National Resources Conservation Service, 2011; Barney et al., 2012). Since the demand for biofuels to replace fossil fuels is likely to increase in the future, more land will also be needed to fulfill these increasing demands.