Phragmites australis is a cosmopolitan macrophyte that has colonized anthropogenic-impacted wetlands throughout North America. The populations that exhibit the most invasive traits all belong to a lineage that was introduced from Europe within the last two hundred years. The invasion of this P. australis lineage went undetected for years due to a native conspecific lineage, which over time were replaced by the introduced P. australis lineage. Until recently, only a few, small populations of introduced P. australis had become established in the wetland ecosystems of the American Southwest. Within the past 15 years, P. australis began to establish and rapidly expand along the Las Vegas Wash, its tributaries, and Lake Mead. Molecular analyses of samples taken in 2013 revealed hybridization between the native and introduced lineages and additional collections in 2014 confirmed that the rapidly growing P. australis populations were composed of hybrids. Due to the invasive traits of the hybrid populations, resource managers expressed concern about hybrid populations spreading over long distances by seed dispersal. We evaluated seed viability through germination with vernalization trials, and fecundity was determined by calculating the number of seeds per flower head and the average density of reproductive stems of each lineage.
Native populations had significantly higher germination rates than hybrid and introduced populations, but they exhibited lower overall fecundity due to a lower total seed production rate in our populations. Native lineages had the highest seed viability results with 22% germination without vernalization and 40.5% with cold treatments. Introduced lineages had lower germination rates, 11%, and showed no response to cold treatments. Hybrid populations had the lowest germination rates, 6%, but responded positively to cold treatments, resulting in 12.5% germination rates. Hybrid and introduced lineages had significantly more seeds per flower head than the native lineages. Given that hybrids are producing viable seed, it is likely that they will continue to spread throughout wetlands in the Las Vegas area and potentially to other wetlands in the Southwest. These areas are rare ecosystems that provide services such as habitat and breeding grounds for wildlife, nutrient cycling, and recreation areas. A better understanding of phenotypic traits of hybrid populations will be important to evaluate the risk of their continued expansion. These results have shown that hybrid populations exhibit traits intermediate to native and introduced lineages, but further studies will be needed to determine their potential to replace native lineages in the Southwest.