The vegetation of the high-Andean cushion-plant peatlands is mainly characterized by the cushion-forming Juncaceae of the genus Oxychloe, Distichia and Patosia, as well as Cyperaceae of the genus Zameioscirpus. This particularity of Southern Hemisphere cushion peatlands is unique and exclusive to the high-Andean regions of South America. The floristic composition and site factors of 340 peatlands were studied between 1995-2013 to investigate changes in the vegetation of the high Andean peatlands from its southernmost distribution in Argentina (39 degrees S, province of Neuquen) to the limit with Bolivia (22 degrees S, province of Jujuy). Site factors were characterized based on water samples. Data for precipitation and temperature conditions was obtained from the WorldClim 2.0 dataset. Cushion-plant species and their accompanying flora clearly reflect the changing ecological conditions along the marked climatic gradient from south to north. A group of species is limited to the southern region of the study area (provinces of Neuquen and Mendoza), and another group is limited almost exclusively to the northern region (provinces of San Juan, La Rioja, Catamarca, Salta and Jujuy). Some Oxychloe species are distributed only in the transition zone between the winter and predominantly summer rainfall zone, however, many accompanying species show wide ecological amplitudes. The quality in the supply of minerals does not seem to play a decisive role in the differentiation of vegetation, except for the concentration of salts. Climatic conditions seem to be more relevant to the presence of the species. Our research shows that the survival of the plants is closely linked to the rainfall regime and seasonal temperature fluctuations. Similarly, the survival of plants and the potential formation of flowers and mature seeds depends on the presence of pollinators and species that disperse seeds (e.g. birds and camelids). Mobility by dispersion is also necessary for plant survival in the high Andean regions, which have experienced repeated climatic changes over the past centuries and millennia. This brought about the permanent displacement of the zoning of vegetation, and with it, the displacement of zones favorable for cushion-plant growth. The phylogenetic relationships and the spatial distribution suggest that the cushion-forming Juncaceae, depending on the evolution of climate, geomorphological processes and local site conditions, progressed step by step from the south to the north. Their current distribution is therefore largely understood as a result of short-term and long-term climatic changes.
