Field growth characteristics of two aquatic carnivorous plants, Aldrovanda vesiculosa and Utricularia australis

Basic growth characteristics of two species of free-floating submerged carnivorous plants, the very rare and stenotopic Aldrovanda vesiculosa and the very common and eurytopic Utricularia australis, were investigated in a 10/11-day field growth experiment within three nylon enclosures at two artificial Aldrovanda sites in the Trebon region, S Bohemia, Czech Republic, at the peak of a growing season. Growth of Aldrovanda was best at a meso-eutrophic site (biomass doubling time, T-2, 8.4-10.7 days, mean growth of new leaf whorls 0.96 whorls days(-1), 1.6 developed branches per shoot) and slower at an oligo-mesotropbic site (T-2 17.2-21.5 days, growth of whorls 1.01 whorls days(-1), 0.1-0.5 branches per shoot). Growth of Utricularia was similar at both sites (T-2 19.8-33.2 days or 9.1-16.8 days, growth of whorls 3.1 or 2.7 whorls days(-1), 1.5-2.1 or 0.8-1.4 developed branches per shoot at the former or latter site, respectively). Throughout the experiment, both species at the meso-eutrophic site allocated relatively more biomass to the production and growth of branches, than to that of new whorls. The results show that Aldrovanda, although usually considered as competitively weaker, can grow faster during the growing season peak than Utricularia due to frequent branching and the subsequent rapid growth and separation of daughter shoots. Very rapid growth of rootless aquatic carnivorous plants in nutrient-poor habitats allows the consideration of ecophysiological adaptations that enable the plants to gain limiting mineral nutrients. These adaptations include carnivory, efficient nutrient reutilization from senescent shoots, and very high affinity for mineral nutrient uptake from water. Comparison of growth rates of rare and stenotopic A. vesiculosa and very common and eurytopic U. australis shows that differences in their rarity do not seem to be based on differences of growth rate.