Early spring warming as one of the factors responsible for expansion of aquatic fern Salvinia natans (L.) All. in the Vistula delta (south Baltic Sea coast)

From the mid-19th century to the end of the 20th century, Salvinia natans (L.) All. occurred very rarely in the Vistula Delta (northern Poland), but from the beginning of the 21st century it was present in almost every watercourse and had formed very abundant populations. We examined the influence of temperature on the abundance of this plant and the efficiency of macrospore germination. Field work was carried out in 10 permanent plots every 14 days for 5 years. Macrospores germinate at water temperature of 12.4 ± 0.2°C or higher; at 20°C they develop more effectively than at 15°C. Usually, ice cover on the rivers melts in the second half of March. At this time, macro- and microspores emerge on the water surface and germinate in April. They occur in the water surface film at 15.1 ± 2.4°C and massively die during spring frost. After 1989, March and April mean temperature in the Vistula Delta rose by 1.6°C versus 1901–1988, and by 1.9°C versus 1851–1988. In 1951–1988, the mean temperature for March and April was +4.6°C and was characterized by considerable interannual variation (SD = 1.64), whereas in 1989–2009, it rose to +5.7°C and the variation range narrowed (SD = 1.24). We found that macrospores are active earlier during the warm and mild summers, germination is more effective, survival of young stages is higher, the growing season is longer, and the number of vegetative offsprings in a year is larger.