Adaptation and growth performance of four endangered amphibious freshwater species

Small amphibious plants have decreased extensively in Europe over the last century. owing to eutrophication, formation of organic sediments and improvements for strong competitors. To deepen our understanding of this rarity we tested adaptation and growth of four species exposed in air and under water. All species saved costs by having the same leaf type in air and water, but Baldellia repens and Isoëtes echinospora maintained leaf porosity and cuticle gas resistance, while Pilularia globulifera reduced porosity and Crassula aquatica increased resistance in air. The relative growth rate in leaf number on shoots was higher for P. globulifera than I. echinospora and C. aquatica and higher in air than under water for C. aquatica and B. repens. The vegetative expansion by new lateral shoots was negligible for C. aquatica and I. echinospora, but high for B. repens and P. globulifera doubling areal cover within only 9-18 days. All species survived mild frost exposure for one day, but died after one week exposure to hard frost in air. Only B. repens and P. globulifera survived deep-frozen in ice at −9 °C. We conclude that ability to grow in air and water and tolerate mild, not hard, frost is a common ability of the small amphibious species. Slow growth rate is not a general feature. Low stature represents the main common threat to their survival in competition with tall, overtopping plants benefitting from nutrient enrichment and less physical disturbance that reduce the formation of exposed soils suitable for small amphibious species.