Ecological Differentiation between Invasive and Native Species of the Genus Epilobium in Riparian Ecosystems Is Associated with Plant Functional Traits

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The biomass allocation, leaf traits, and the CO2 assimilation rate in invasive Epilobium adenocaulon Hausskn and E. pseudorubescens A.K. Skvortsov and indigenous E. palustre L. were studied in the Middle Urals. E. adenocaulon had the maximum height, and E. palustre had the largest leaf canopy diameter. The most compact in linear sizes species E. pseudorubescens was characterized by the lowest biomass. The pattern of organ biomass allocation in invasive E. adenocaulon and E. pseudorubescens corresponded to plants with a ruderal strategy, which was expressed in a low proportion of roots and a large proportion of generative organs. Among the studied species, E. adenocaulon was characterized by the largest the stem mass ratio. The indigenous E. palustre had a large proportion of roots and leaves and a smaller proportion of generative organs, which indicated the stress-tolerant properties of this species. In addition to the pattern of biomass allocation, E. palustre differed from the invasive species in a large (30% on average) amount of chlorophyll and 1.2 times less carotenoid content, as well as low photosynthetic activity of chlorophyll. The rate of CO2 uptake per unit leaf area was the greatest in E. adenocaulon, which had thick leaves with a large leaf mass per area (LMA), exceeding that in species E. palustre and E. pseudorubescens with thin leaves with lower LMA by 1.8 and 1.6 times, respectively. A negative correlation between the CO2 assimilation rate per 1 g of dry weight and leaf density (LD) (r = –0.78, p < 0.001) predetermined the maximum value of the photosynthesis rate in E. pseudorubescens, which had the least dense leaves. The analysis of the general dataset of the functional traits indicated that the studied species of the Epilobium genus had different ecological strategies: E. palustre—S (stress tolerant), E. adenocaulon—CR (competitive-ruderal strategy), and E. pseudorubescens—R (ruderal species). It is concluded that invasive species do not have a direct negative effect on E. palustre, even being characterized by distinct ecological properties compared to native species. However, owing to the CR strategy, E. adenocaulon is able to invade riparian plant communities when they are affected by disturbing factors, which may lead to a decrease or to disappearance of the indigenous stress-tolerant species E. palustre and thus to its replacement in disturbed communities by the more competitive invasive species E. adenocaulon.