Greater flowering and response to flooding in Lythrum virgatum than L. salicaria (purple loosestrife)

<p>Newly introduced trait diversity can spur rapid evolution and facilitate local adaptation in the introduced plant <em>Lythrum</em> <em>salicaria</em>. The horticultural plant <em>L</em>. <em>virgatum</em> might further introduce meaningful trait variation by escaping into established L. salicaria populations or by hybridizing with <em>L</em>. <em>salicaria</em>. Although many experiments have focused on <em>L. salicaria</em> genotypes, relatively little is known about <em>L. virgatum</em> ecology. We used a greenhouse common garden to compare traits and flood response of <em>L. salicaria</em> and <em>L</em>. <em>virgatum</em> collected from two sources each in their native range. We tested the hypotheses that these two wetland taxa have comparable responses to flooding (inundation), and that flood tolerance correlated to higher fitness. Flooding produced stronger stress responses in <em>L. virgatum</em>. Compared to <em>L. salicaria</em>, <em>L. virgatum</em> shifted more aboveground allocation away from reproduction, decreased inflorescence biomass by 40% more, and produced 7% more stem aerenchymatous phellum, a specialized tissue that maintains aeration. Despite these more pronounced responses to flooding stress, <em>L. virgatum</em> had higher fitness (inflorescence biomass and reproductive allocation) than <em>L. salicaria</em>. Overall, <em>L. virgatum</em> differed from <em>L</em>. <em>salicaria</em> in functionally important ways. <em>Lythrum virgatum</em> persisted under flooding and produced more reproductive biomass than <em>L. salicaria</em> under both flooded and non-flooded conditions. However, inundation stressed <em>L. virgatum</em> more than <em>L. salicaria</em>. <em>Lythrum virgatum</em> is likely able to establish in the wetland habitats in which <em>L. salicaria</em> prevails but may possess broader habitat tolerances.</p>