| Summary: | Carex buxbaumii is rare sedge of moist to wet meadows, marshes and fens. The population trends and detailed habitat requirements of this species remain unknown. In Poland, C. buxbaumii belongs to EN category of IUCN, while is even more endangered in other European countries. The knowledge on this species is limited, but probably it needs active protection for its maintenance. It is also known that plants respond morphologically to changes in the site conditions which may help to determine the ways of effective method of their conservation. Therefore, we examined the frequency and morphology of Carex buxbaumii growing in a sward of fen-grasslands i) abandoned for 15 years, ii) mowed, or iii) grazed by Konik horses. The description of the occurrence and of the morphological reaction to the differentiation in land–use type of C. buxbaumii is presented against the background of changes in vegetation cover that occurred in 2010–2016 in the examined fen-grassland.There was stated that C. buxbaumii grew on peat soils, of high content of organic matter and Ca+2. The type of land-use significantly differentiated both the frequency and morphology of C. buxbaumii. In the mowed site C. buxbaumii grew as an element of the Molinion vegetation and appeared there with 100% frequency. The individuals growing in the mowed site had the longest shoots, the longest inflorescence brackets, and the longest terminal spikes. In the site grazed by Koniks, C. buxbaumii was a component of Scheuchzerio–Caricetea nigrae vegetation, while its frequency of C. buxbaumii was 87%. The shoots there were the shortest, and in general, the morphology was the most differentiated. In the abandoned site C. buxbaumii abundance amounted 46%, and the individuals were smaller than those under the mowing regime but larger than those grazed by Koniks. In that site, the previous vegetation with high proportion of the Molinion and Phragmitetea species was replaced by Alnetea glutionosae vegetation, represented mainly by trees and shrubs. We have shown that the best conditions for the development of the C. buxbaumii population in the examined fen-grasslands are created under an extensive mowing regime. Keywords: Wet grassland management, Nature conservation, Endangered species, Fen-grassland restoration, Polish Konik horses
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