THE INTRODUCTION OF THE AQUATIC CARNIVOROUS PLANT ALDROVANDA VESICULOSA TO NEW POTENTIAL SITES IN THE CZECH REPUBLIC: A FIVE-YEAR INVESTIGATION

Folia Geobotanica 34: 299-305 (1999)

Lubomír Adamec^l) & Jiří Lev²⁾

1) Institute of Botany, Academy of Sciences of the Czech Republic, Section of Plant Ecology, Dukelská 135, CZ-379 82 Třeboň, Czech Republic; fax +420 333 721136, E-mail adamec@butbn.cas.cz

2) University of South Bohemia, Faculty of Agriculture, Department of Ecology, Studentská 13, CZ-370 05 České Budějovice, Czech Republic

Keywords: Growth analysis, Reintroduction, Selection of new sites, Shallow dystrophic waters, South Bohemia, Water chemistry. 

Abstract: Five microsites in shallow dystrophic waters were selected for the introduction of a critically endangered aquatic carnivorous plant Aldrovanda vesiculosa in the Třeboň region in S Bohemia, the Czech Republic. The selected sites were fen pools close to two hypertrophic fishponds, Ptačí blato (four microsites) and Domanínský (one microsite). In June 1995, 30 plants from an allochthonous population in E Poland were introduced in each of the microsites. Water chemistry and plant growth dynamics were investigated at the microsites throughout the 1995-1996 seasons, and the maximum size of the subpopulations were estimated during the 1997-1999 seasons. The selected waters may be characterized as oligo-mesotrophic.

The warm and dry 1995 season resulted in a low water level (ca. 10 cm below average) and plant propagation was poor at all but one microsite. However, the rainy and colder 1996 season resulted in a high water level, and thus rapid plant propagation occurred at all four microsites at the Ptačí blato fishpond and between 841 and 2669 turions were formed. Here, the doubling time of the apices was between 13.5-19.3 days in the summer. At the Domanínský fishpond, however, the growth was relatively poor in 1996. The overwintering rate of turions (19-100%) was found as being high enough to keep the introduced plant populations stable. At each of the four microsites at Ptačí blato, the maximum numbers of shoot apices were estimated to be between 1,000-20,000 in the 1997-1999 seasons. Depth of free water appears to be the crucial factor for the rapid growth of Aldrovanda at some selected sites; water depth below 5 cm is unfavourable. Overall, Aldrovanda was successfully established in an intensively agricultural landscape in the Třeboň region.

INTRODUCTION

Aldrovanda vesiculosa L. is a critically endangered aquatic carnivorous plant, rapidly vanishing from Europe. The origin of its recent population in Europe is still unclear (BERTA 1961, ADAMEC & TICHÝ 1997). Moreover, its historical postglacial spread in Europe was highly irregular, variable in time and area, and probably dependent on the migratory routes of water birds (BERTA 1961, HUBER 1961). In Europe, it has been recorded at about 150 sites in the last two centuries. However, it has declined markedly in the last century, especially in the last 30 years, with only a few dozen sites remaining (ADAMEC 1995a, KAMIŃSKI et al. 1996).

In Europe, Aldrovanda propagates only by the apical branching of shoots (KAMIŃSKI 1987a, ADAMEC & TICHÝ 1997). Apical winter buds (turions) are formed in the autumn. Recently, ADAMEC (1999) selected several shallow dystrophic wetlands in S and N Bohemia (the Czech Republic) in order to investigate the plant growth dynamics of Aldrovanda at these sites within nylon enclosures. The study suggested that considerable turion losses in natural Aldrovanda stands could be compensated for by rapid seasonal shoot growth and branching, which thus maintains an abundant plant population.

The aims of the present study were to introduce Aldrovanda plants in five suitable microsites in two selected shallow dystrophic sites in the Třeboňsko Biosphere Reserve and Protected Landscape Area, S Bohemia, the Czech Republic, to investigate the growth dynamics of the introduced plants together with habitat factors over two seasons, and to estimate the maximum size of the subpopulations over the next three seasons. The selection of these sites and microsites was based on assessments of suitable water depth (0.15-0.5 m), water level fluctuations, dominant aquatic and emergent vegetation, the degree of shading by emergent vegetation, the character of bottom sediment, water transparency, and the CO₂ concentration in the water. The selection of the sites and microsites was optimized after considering the results of the above-mentioned Aldrovanda growth study in nylon enclosures (ADAMEC 1999).

Table 1. The characteristics of the selected microsites.

MATERIALS AND METHODS

Plants of A. vesiculosa were collected from Lake Długie in the Łęszna-Włodawa Lake District in E Poland (51°26'N, 23°06'E) in June 1993. The plants were cultivated outdoors in a plastic container (ADAMEC 1997a,b).

The selected sites, the Ptačí blato and the Domanínský fishponds, are described in detail by ADAMEC (1999). Important characteristics of the microsites are shown in Tab. 1. Microsites PB2, PB9, and D were selected at about the same places as in the above-mentioned growth study. Each microsite was labelled by a wooden rod. In mid-July 1995, the total coverage of macrophytes, plant dominants (> 40% of total coverage) and subdominants (10-40% of total coverage) were estimated in a 2 x 2 m area.

Field observations

On 12-13 June 1995, 30 adult Aldrovanda plants, with a total of 32-48 shoot apices, were introduced in a 0.3 x 0.3 m area in the centre of each of the microsites. The groups of introduced plants differed appreciably in their length and branching from each other (see Tab. 2). The growth characteristics of the introduced plants, water level, and basic water chemistry were measured according to ADAMEC (1999) several times throughout the 1995-1996 seasons.

Table 2. The development of introduced subpopulations A. vesiculosa in 1995-1999. The plants were counted in 1995-1996, while the total numbers of apices were estimated in 1997-1999. PL -number of plants (density.m^-2 in parentheses); AP - total number of apices; LE - mean shoot length (cm); ¹⁾ - sum of the values for PB1C and PB1T; ²⁾ - see PB1C.

At each observation, except in July and August 1995, all plants at a microsite and the number of first and second order branches on each plant were counted. Twenty plants were randomly selected from an area in a dense stand of Aldrovanda and the total length of the main shoot and the number of leaf whorls with mature traps were estimated. In September, the number of turions were counted. Only those overwintered turions and/or germinating plants which floated close to the water surface were counted in the spring. In all cases, the measured and counted plants were returned to the centre of the introduction area and scattered in a 1-4 m² area. The central parts of the microsites (2 x 2 m) were not disturbed. In the 1997-1999 seasons, the total numbers of apices were estimated at microsites between mid-August and early September. The estimates were based on plant density and area of the plant stands.

Between 10:00-17:00 local summer time the dissolved O₂ concentration, pH, and water temperature were measured in dense Aldrovanda stands ca. 2 cm below the water surface. Total alkalinity (TA; TA=[HCO₃^-]+2.[CO₃^2-]+[OH^-]-[H⁺]) was estimated by Gran titration and [CO₂] was calculated from the TA and pH values. In June and July 1995, filtrated water samples were analyzed for macro-nutrients (for all details see ADAMEC 1999). The concentration of humic acids in the filtrated water samples was estimated using a modification of a standard colorimetric method (PEKÁRKOVÁ & LISCHKE 1974). The modification is based on direct colorimetry at 420 nm of alkalized water sample (20 ml water + 0.5 ml 0.5 mol.l^-1 NaOH) in a 5 cm wide cuvette. Purified humic acids were used as calibration standards. It was found that the method was also very sensitive to tannin and thus, it records a sum of humic acids and tannins.

Table 3. Concentration of macronutrients and the sum of humic acids and tannins (HAT) in water in the stands of introduced A. vesiculosa in the Třeboň region, Czech Republic. Mean of two values from June and July 1995 is always shown.

RESULTS AND DISCUSSION

Summer growth and overwintering

In the 1995 season rapid growth of the introduced Aldrovanda plants occurred only at site PB1C where the total number of apices increased ten times (Tab. 2). The relatively short shoots and low branching found at site D in July 1995 reflect the introduction of much shorter and less-branched plants in site D but this could not limit the plant growth dynamics in August and later on. In the 1996 summer season, plant growth was vigorous at all microsites at Ptačí blato, but was rather poor at site D. In addition, about 250-300 other turions were found near microsite PB1T and 200-250 near PB9 in late September 1996. These plants had drifted out of the microsite area due to wind, and evidently were not counted. Assuming an exponential character of its propagation (ADAMEC 1999), doubling time of apices between 29-30 May to 16-17 July 1996 was 13.5 d at PB1C, 17.6 d PB1T, 14.8 d PB2, and 19.3 d at PB9. The same values (14.3-21.4 d) were found for Aldrovanda propagation in nylon enclosures at sites PB2 and PB9 in the very warm summer of 1994 (ADAMEC 1999). Since each introduced subpopulation consisted of both very short, freshly-separated branches (2-4 cm) and long adults plants, the standard deviation of the data has not been calculated.

The turion overwinteríng rate was sufficiently high at all microsites to maintain abundant stocks of Aldrovanda (Tab. 2). By 6 May 1996, 81-100% of the turions successfully survived at PB1T, PB2, and D, while only 19-22% at PB1C and PB9 survived (cf. ADAMEC 1995b). The data in Tab. 2 provide reliable information on the overwintering rate of turions. On 6 May 1996, the total number of plants corresponded to the total number of germinating turions, but this did not hold true later. The first turions released from the bottom in mid-April 1996. Between 40-64% of the total overwintering turions floated up at Ptačí blato as early as 20-21 April, but only about 13% at D. This delay at D could be caused by the deeper and colder water. Throughout the winter of 1995-1996, there was at least a 10 cm free water column at all microsites at Ptačí blato. Overall, turions overwintered much better under water at Ptačí blato in the present study (mean ca. 56%) than they did in the wet bottom throughout the winter of 1994-1995 (27%), when a portion of the turions was grazed by small rodents (ADAMEC 1999). The rapid seasonal growth of Aldrovanda at the microsites did not generally correspond with the turion overwintering rate (Tab. 2), as found also by ADAMEC (1999). The seasonal growth dynamics of Aldrovanda is markedly oscillatory and thus, even considerable turion losses at natural sites can be compensated for by rapid seasonal propagation. Furthermore, the high variability in turion overwintering rates, together with the dependence of the seasonal plant propagation rate on water level, could explain why the distribution of Aldrovanda at a site is irregular in time (ADAMEC 1995a).

Tab. 4. Water chemistry in stands of A. vesiculosa at the microsites over the 1995-1996 seasons. Mean and range of 8 measurements are shown; TA - total alkalinity.