Environmental control of clonal growth in Carex nigra: What can be masked under the name Carex nigra subsp. juncella in the Czech Republic?

Carex nigra plants forming elevated dense tussocks are often named C. nigra subsp. juncella, as opposed to rhizomatous C. nigra subsp. nigra. It is uncertain, however, whether the cespitose growth form is a hereditary trait useful for definition of the distinct taxon or a site modification of little taxonomic value. We used vegetation analyses (phytosociological relevés) to reveal main patterns in ecological demands of the cespitose C. nigra plants in the Czech Republic, and three cultivation experiments to assess changes in clonal growth of C. nigra under various environmental conditions. In the field the cespitose C. nigra plants were typically found in abandoned wet meadows near open water, whereas the rhizomatous morphotypes frequently occurred also in regularly mown wet meadows and in peat bogs. The cespitose growth form disappeared in the cultivations, and the rhizome system responded plastically to immediate environmental stimuli. Number of rhizome branches and mean rhizome length decreased after defoliation of aboveground parts and denudation of belowground parts, whereas increased due to inundation. In the population from a cold site in high altitude (Modrava, Šumava Mts.), however, the originally cespitose plants repeatedly produced shorter and less numerous rhizome branches than the rhizomatous plants cultivated in the same conditions. This suggests ecotypic (genetic) differentiation in some populations of C. nigra, driven by environmental selection for more compact growth form in climatically severe sites. The cespitose C. nigra plants thus arise polytopically, by different mechanisms. The growth form itself therefore cannot serve as the character reliably delimiting C. nigra subsp. juncella as the distinct taxon.