Why are tall-statured energy grasses of polyploid species complexes potentially invasive? A review of their genetic variation patterns and evolutionary plasticity

Perennial tall-statured grasses are regarded as a sustainable source of renewable energy for their high yields of lignocellulosic biomass, low resource input, wide ecological tolerance and capacity for storing large amounts of atmospheric CO2 in their perennial underground rhizome systems. These same traits, that make such crops agronomically attractive and sustainable, make these species highly competitive and potentially invasive. Several perennial energy crop grasses are outbreeding species that belong to cosmopolitan polyploid species complexes, i.e. groups of interbreeding species with ploidy variation. The cultivation of such highly productive and genetically diverse crops can have unwanted consequences through the evolution of invasive species. The goal of this review is to provide the scientific community, including agronomists, breeders, users and nature managers, with an introduction to the genetic dynamics occurring within the polyploid species complexes of the emerging energy species Arundo donax, Miscanthus × giganteus, Panicum virgatum, Phalaris arundinacea and Phragmites australis, and the broad biogeographical extent of their gene flow impact. Such aspects are difficult to predict, and are not captured by invasion risk assessments and by the sustainability certifications of the bioenergy supply chain. The review integrates literature from the phylogenetic, cytology, population ecology and agronomic research and focuses on the evolutionary processes that shape invasiveness that can be activated post-introduction by the dispersal of pollen, seeds and plant fragments from the energy crops to the environment. Due to the high genetic diversity of the crops, the adverse effects that genetic drift and founder effect can have on the establishment of small populations are very unlikely. On the contrary the data collected suggests that the risk of fostering panmictic continental invasive populations is high. Agronomic measures, regulations and genetic improvements that can contain dispersal from crops are discussed, as well as urgent research needs