Being evergreen in an habitat with atenuated seasonal contrasts – a major competitive advantage ?

This study experimentally compares growth of four aquatic macrophytes species (Sparganium emersum Rehm., Hippuris vulgaris L., Groenlandia densa (L.) Fourr., Luronium natans (L.) Raf.) with different modes of winter persistence in a thermally buffered habitat. The question is whether the above-ground persistence of photosynthetic structures during winter can compensate for low growth rates of evergreens when they compete with fast growing, non-evergreen species. The experiment was carried out in experimental boxes installed in a former river channel fed by stenothermic ground-water. Species were planted pairwise in experimental boxes in autumn, monitoring was carried out in the following year: degree of evergreenness was evaluated through number of ramets present prior to the main growing season, competitive ability of species was assessed during the growing season through cover measurements at three dates. Degree of evergreenness varied considerably between species, ranging from increase of ramet numbers during winter in Luronium over preservation of ramet numbers in Groenlandia and Hippuris to nearly complete winter die-back in Sparganium. Luronium presented the highest cover during the entire growing season. Only by the end of summer did Sparganium attain a similar cover through a high growth rate. Despite large differences in cover, growth appeared largely unaffected by the kind of treatment partner a species was confronted with. Nevertheless, this study demonstrates that being evergreen can compensate for a low growth rate during the main growing season when it is associated to extensive growth outside this main growing season. Mots clef: aquatic macrophytes; competition; phenology; plant strategies; seasonal cycle