Ecological conditions of the altitudinal substitution of Petasites kablikianus and P. hybridus in the Polish Carpathians

Two species of the genus Petasites , P. kablikianus (occurs upstream) and P. hybridus (occurs downstream), form characteristic tall herb communities in two altitudinal zones of Carpathian streams and rivers. They are described as “altitudinal vicariants” due to their substitution with decreasing elevation on similar habitats along mountain streams. This would indicate that the phenomenon is conditioned by climatic factors. In this study, we investigated the factors that determine this substitution on streambanks, and whether P. hybridus competes with P. kablikianus and limits its spread from mountains to foothills. We analyzed 72 plant patches from the Western and Eastern Carpathians in terms of flora composition, and geomorphological features of the habitat. The content of organic matter, available Mg, P, K, C, total N, and pH were also analyzed for a representative group of patches. Phytocoenoses were analyzed based on the presence of one of the analyzed species or on the quantitative proportions, in the case of a co-occurrence. We found statistically significant differences in the soil structure (skeleton percentage), content of Mg, N, and C, and the values of the C:N ratio. These results revealed that P. kablikianus prefers skeletal soils with a lower clay content, that is rich in Mg, N, C, and organic matter. Petasites hybridus prefers finer soils, which are richer in pulverized and clay fractions, in a zone of decrease of soil richness and where a thick layer of humus is typical for its phytocoenoses. We found that the spread of P. kablikianus into lower locations is not limited by competition with P. hybridus , but rather by the quality and structure of the habitat. Therefore, both species should be treated as ecological rather than altitudinal vicariants.