Seasonal growth dynamics and overwintering of the aquatic carnivorous plant Aldrovanda vesiculosa at experimental field sites

New potential sites for the critically-endangered, aquatic carnivorous plant Aldrovanda vesiculosa L. (Droseraceae) were selected in North and South Bohemia (Czech Republic) and both its seasonal growth dynamics and overwintering rate were investigated. Groups of either 5 or 10 plants were planted in 1 x 1 m nylon enclosures in selected shallow, dystrophic waters at the end of May. Plant growth characteristics and water chemistry were investigated at 2 to 5 week intervals over the 1994 growing season inside ten enclosures placed at six sites. Within seven enclosures at three sites, the seasonal growth was very fast and 38-141 turions developed from the initial five plants. Water at these sites was mesotrophic with a high concentration of CO2 1 and pH between 6.2 and 7.6. At the other three sites, plant growth was very poor. The above 0.1 mmol.l(-1) fastest plant propagation was found between late June and mid-August and corresponded with the warmest seasonal period. During this period, the doubling time of the total number of shoot apices was 16.4-34.9 days. Turions developed in mid-September and sank to the bottom by mid-October. In some enclosures, the turions overwintered on the wet bottom whereas they were submersed in the other ones. Though the turions were subject to frosts of up to -20 degrees C, none died due to the frosts. Grazing of turions by ducks or small rodents was found at some sites. The overwintering rate of turions at sites varied from 0 to 70% and was not related to seasonal growth rate. Most turions floated to the surface and germinated during late April-early May. It is suggested that considerable turion losses in stable, natural Aldrovanda stands are compensated for by fast seasonal shoot growth and branching which leads to the recovery of an abundant plant population.