Interpopulation variability and assessment of competitiveness of Erigeron canadensis L. in various biotopes of the secondary distribution range

Invasive plant species are an excellent model for studying rapid microevolutionary changes occurring in a secondary range. The model species, Erigeron canadensis L. (Asteraceae), began actively naturalizing in Russia at the beginning of the 19th century and is now one of the three most widespread invasive species. The aim of this study is to assess the inter-population variability of E. canadensis across a broad range of soil and climatic conditions and to identify the factors explaining the rapid evolution of this species' competitive abilities over at least two hundred generations. Forty six 1 × 1 m sample plots were established in different habitat types across Moscow, Moscow Region, and the Tula, Kursk, Oryol, Belgorod, Vladimir, Nizhny Novgorod, and Amur Regions, as well as the Perm and Stavropol Territories. For each plot, a list of all encountered species was compiled, and the tallest specimen of E. canadensis was selected. For this specimen, the length of the vegetative and generative parts of the shoot and the number of capitula were determined. Subsequently, the above-ground parts of all plants within a 0,5 × 0,5 m accounting plot around the tallest specimen were cut, and the air-dry biomass was assessed for: the tallest specimen of E. canadensis, all other specimens of E. canadensis, and all accompanying species growing within the plot. Very high inter-population variability of phenotypic traits was noted: the coefficient of variation for dry above-ground biomass exceeded 94 %, and for the number of capitula, it exceeded 95 %, indicating the high adaptive potential of the species. A strong positive correlation was revealed between the number of capitula and the plant's above-ground biomass. It was concluded that in its secondary range, E. canadensis has undergone strong directional selection and is capable of growing in diverse ecotopes, which explains its high phenotypic variability. Erigeron canadensis is an explerent with low competitive ability, and its presence affects natural phytocenoses only to a minor degree: some competitive effect was observed only on 13 % of the plots, where the biomass of E. canadensis was higher than the biomass of all other accompanying species. No stable plant groupings of E. canadensis with other species were found in any of the ecotopes it has colonized. Most species growing together with E. canadensis were encountered only sporadically, which contradicts the "Global competition" and "Invasional meltdown" hypotheses, which posit that initially established alien species "facilitate" the invasion of other alien species into the phytocenosis. A general conclusion was made that the global success of the E. canadensis invasion was facilitated by high seed productivity combined with phenotypic plasticity and a high rate of microevolution in the secondary range.

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