A relict population of Isoëtes echinospora Durieu survived a thirty-year period of severe acidification and high concentrations of phytotoxic aluminium (Al) in Plešné Lake (Bohemian Forest, Czech Republic). The population consisted of only adult plants. Sporeling survival and age structure were examined during the population recovery in 2004–2008. Laboratory experiments were conducted to assess the effect of various pH values (4–8) and Al concentrations (0–1000 µg·l–1) on sporeling development. The responses of the sporelings to the experimental treatments were evaluated and compared with those observed in the lake. The experiments showed that an Al concentration higher than 300 µg·l–1, and high acidity (pH 4), inhibit sporeling growth, in particular resulted in a pronounced reduction in absorptive organs (macrogametophyte rhizoids, roots and root hairs). With increasing concentrations of Al and at pH 4, the ratio of the below-ground to above-ground sporeling biomass decreased to less than 1. The responses of the lake sporelings, rooting in the upper sediment layer, were similar to those exposed to 100–300 µg·l–1 of Al in the laboratory, and reflected the Al toxicity of the lake water. The quillworts at Plešné Lake survived because adult plants can tolerate these adverse conditions and are very long-lived. The population recovered when the pH of the water increased to over 5 and the Al concentration decreased to below 300 µg·l–1.
Isoëtes echinospora, a submerged aquatic quillwort, is native in northern latitudes and a rare glacial relict in mountain lakes in temperate Central Europe. A relic population of this quillwort in the Plešné jezero lake has recovered recently from a 30-year period of failure to reproduce caused by acidification. Early ontogenetic stages of the quillwort are considered to be the most vulnerable to environmental changes. Therefore, the objective of this study was to investigate the phenology of germination of I. echinospora. In a two-year experiment, we examined the time course of germination of micro- and macrospores and establishment of sporelings under (i) natural in situ conditions in the Plešné jezero lake and (ii) at various temperatures (6–17 °C) in the laboratory. We developed a mathematical model that describes the temperature-specific temporal changes in the early ontogeny of I. echinospora. Our experiments clearly show that spores do not germinate at once but gradually over time if exposed to favourable temperatures. Generally, percentage germination tended to increase during the course of a season under most temperature regimes but was inhibited at the lowest temperature. With increasing temperature, microspores germinated earlier and more successfully than macrospores, as described by the model. Sporelings also developed faster at the higher temperature. However, the highest temperature used in the experiments (17 °C) desynchronized the phenology of germination in I. echinospora as it resulted in the two types of spore not being available for fertilization at the same time. Thus, climate change might affect interactions between temperature and the phenology of quillwort reproduction and threaten the survival of this species in Central Europe.