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The Relative Risk of Invasion: Evaluation of Miscanthus × giganteus Seed Establishment

Abstract The sterile hybrid, giant miscanthus, has emerged as a promising cellulosic bioenergy crop because of its rapid growth rate, high biomass yields, and tolerance to poor growing conditions; these are traits that are desirable for cultivation, but also have caused concern for their contribution to invasiveness. New seed-bearing lines of giant miscanthus would decrease establishment costs for growers, yet this previously unresearched propagule source increases fears of escape from cultivation. To evaluate the consequences of seed escape, we compared seedling establishment among seven habitats: no-till agricultural field, agricultural field edge, forest understory, forest edge, riparian, pasture and roadside; these were replicated in Virginia (Blacksburg and Virginia Beach) and Georgia (Tifton), USA. We use a novel head-to-head comparison of giant miscanthus against five invasive and three noninvasive species, thus generating relative comparisons. Overall seed germination was low, with no single species achieving germination rates >37%, in all habitats and geographies. However, habitats with available bare ground and low resident plant competition, such as the agricultural field and forest understory, were more invasible by all species. Giant miscanthus seeds emerged in the roadside and forest edge habitats at all sites. Early in the growing season, we observed significantly more seedlings of giant miscanthus than the invasive and noninvasive species in the agricultural field. Interestingly, overall seedling mortality of giant miscanthus was 99.9%, with only a single 4 cm (1.58 in) tall giant miscanthus seedling surviving at the conclusion of the 6-mo study. The ability to make relative comparisons, by using multiple control species, was necessary for our conclusions in which both giant miscanthus and the noninvasive control species survival (≤1%) contrasted with that of our well-documented invasive species (≤10%). Considering the low overall emergence, increased propagule pressure may be necessary to increase the possibility of giant miscanthus escape. Knowledge gained from our results may help prepare for widespread commercialization, while helping to identify susceptible habitats to seedling establishment and aiding in the development of management protocols. Nomenclature: Giant miscanthus, Miscanthus × giganteus J. M. Greef and Deuter ex Hodk. and Renvoize. Management Implications: There is tremendous concern about exotic bioenergy crops escaping cultivation and becoming invasive species. One such crop, the sterile hybrid Miscanthus × giganteus, possesses many desirable agronomic traits, but is expensive to plant. Newly developed fertile lines add a previously under-researched source of wind-dispersed propagules, increasing the chance for establishment outside cultivated fields. Contrary to previous studies of M. × giganteus drought tolerance using vegetative propagules, we found much lower survival of seedlings in dry environments. Of the seven habitats we examined, those with more bare ground and low competition from resident vegetation were more susceptible to invasion. Low competition environments and adequate seed to soil contact is important for seed germination. While M. × giganteus did not exhibit the same ability to establish (one of 16,000 introduced seeds survived 6 mo) as other well-known invasive species we examined, the ability of this species to produce as many as 2.5 billion spikelets ha−1 yr−1, increases the chance of successful establishment. Based on our findings, M. × giganteus is less likely to be problematic in conventional agricultural fields subject to tillage or herbicide applications. Scouting areas near production fields or along transport routes, especially those with low resident plant competition, may help detect young plants when management is relatively easy. Low light environments did not deter seedling emergence of M. × giganteus, and therefore should not be overlooked. Young seedlings of M. × giganteus appear to have high seedling mortality rates, especially when subjected to environmental stress. However, identification of even small populations, at an early age, could be critical for effective eradication as once plants become established and develop extensive rhizomes, management is likely to be much more difficult.