Rapid growth and early flowering in an invasive plant, purple loosestrife ( Lythrum salicaria L.) during an El Niño spring.

Phenological shifts may play a role in the success of invasive species, especially in association with climatic variability. We studied the response of a North American population of the invasive plant, Lythrum salicaria L., to changes in local climate associated with the El Niño/Southern Oscillation Event (ENSO) of 1997-1998. For L. salicaria plants at two wetland sites near North Bay, Ontario, Canada, we made weekly observations of flowering phenology and monthly measurements of aboveground biomass during the 1997 and 1998 growing seasons (April-October). Reproductive output was measured as cumulative length and biomass of inflorescences at the end of the growing season. Temperature and precipitation during the 1997 growing season were typical for the region and provided good baseline data for comparison to the full effects of the ENSO event in 1998, which increased spring temperatures and reduced precipitation in the study area. In response to these conditions, populations of L. salicaria began to flower 14 days earlier (Julian day = 181 +/- 10) in 1998 than in 1997 (Julian day = 195 +/- 12), and accumulated more aboveground biomass early in the growing season (P < 0.05). However, by the end of the growing season, there were no significant differences between years in aboveground biomass or total inflorescence lengths, and senescence of plants occurred at similar times for both growing seasons. Advances in spring phenology during ENSO events offer several potential advantages to L. salicaria, and could have a significant impact on biological control programs initiated for this species in North America.