Phenology of the aquatic fern Salvinia natans (L.) All. in the Vistula Delta in the context of climate warming

We determined the phenology and architecture of Salvinia natans life stages by observing permanent plots in the Vistula Delta (Baltic Sea Region, N Poland) biweekly from 2006 to 2010. Germination of macro- and microspores was initiated at 12.4 ± 0.2 °C water temperature corresponding to early-April water temperature in the Vistula Delta. Early development of the female gametophyte took place at 14.2 ± 0.4 °C, and late development, with fertilization, at 18.3 ± 1.5 °C (April/May). Gametophyte development required about 35 days. During the study years the density of early gametophyte populations was 2522 ± 3327/0.25 m2, but only 437 ± 326/0.25 m2 for late-stage populations. The drop in density was due to ground frost in April. Due to climate warming in the Baltic Sea Region, ground frost occurs less frequently than previously, leading to the expansion of Salvinia natans in the Vistula Delta. Sporophyte development required about 170 days. The early juvenile sporophyte (Ja) consists of a floating leaf and a submerged leaf rudiment. Its development took about three weeks at 16.8 ± 1.2 °C. The late juvenile sporophyte (Jb) has a fully developed submerged leaf and lasts about four weeks at 18.4 ± 0.7 °C. The density of Ja populations was 432.7 ± 413.4/0.25 m2, but only 9.6 ± 12.9/0.25 m2 for Jb populations. The density drop in Jb populations was caused by spring ground frost. A mature individual consists of 2.1 ± 1.1 modules (structural units of a clone), 6.8 ± 4.5 modules for a spore-producing one. Senile individuals disintegrated into modules and died at 2.0 ± 1.1 °C. For about 160 days from autumn to early spring, Salvinia natans was found at the bottom of watercourses in the form of spores.