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The flora of the steppe Dnieper region is characterized by an abundance of naturalized alien species, some of which colonised over the last decade. Climate change, associated primarily with increasing temperature, became clearly manifested in this period. We tested the hypothesis that there is an association between climate change and the initiation of invasiveness of some alien plant species in the steppe Dnieper region. For this purpose, comparative studies of the distribution boundaries of naturalized alien trees, shrubs and herbaceous plants were conducted. Along the research route numerous 5–10-year-old broadleaf linden trees ( Tilia platyphyllos Scop.) were found in the man-made plantation communities of Dnipro city in areas with moist soil; seeded undergrowth was located at a significant distance from the adult linden plants. Numerous groups of young 7–10-year-old plants of the smoke trees ( Cotinus coggygria Scop.), which had a seed origin, were found in the shelterbelt and urban recreational plantations. Young 10–12-year-old virginal and generative plants of the black cherry ( Padus serotina Ehrh.) were found in large numbers in both the semi-natural and artificial plant communities at great distances from the adult trees. The alien plant species common hackberry ( Celtis occidentalis L.) showed the ability to form fairly sparse seminal seedlings, which was presented by the plants at the age of 4–7 years in both the natural and urban plant communities. The perennial herbaceous plant common milkweed ( Asclepias syriaca L.) was found in the course of the research in ruderal habitats, urban plant communities, and also in the agrocoenoses. The common milkweed reached its greatest abundance in fields of winter crops, where the spread of this species was accompanied by a sharp decrease in the number of other species of segetal plants. Our study results confirm that the extension of the distribution boundaries of alien species over the last decade was not related to the ground conditions of the steppe Dnieper region. At the same time, changes in climatic conditions were favourable for some naturalized alien species because they have created the opportunity for seed reproduction of species away from the maternal plants. Alien species C. coggigria , P. serotina and A. syriaca were also the most sensitive to the influence of the climate changes. Consequently, these species have the greatest potential for increasing their level of invasiveness and endangering the biodiversity in the steppe Dnieper region under conditions of climate change. We suggest that a simultaneous initiation of invasiveness of these several alien species leads to an increase in the degree of threat to the diversity of natural plants in the region. The study results confirm the urgent need for analysis and forecasting of the consequences of introduction of alien species, in order to prevent the undesirable effects that this would bring for the region’s native vegetation.

The problem of natural forests conservation is relevant in the Ukrainian steppe zone where the forest ecosystems develop in the unfavorable conditions of geographic mismatch; therefore, they suffer as a result of any environmental changes. The hypothesis of susceptibility of tree leaves’ metabolism even to slight changes of climatic parameters was tested. The study was conducted in more than 75 years-old natural mixed forest located on the right bank of the Samara river. The chlorophyll (Chl) content, and catalase (CAT), benzidine-peroxidase (BPOD) and guaiacol-peroxidase (GPOD) activities in leaves of autochthonous maple species ( Acer platanoides L.) and adventive acacia species ( Robinia pseudoacacia L.) were investigated on the lower (52 m above see level, a.s.l.), middle (74 m a.s.l.) and upper (96 m a.s.l.) coastal slope altitude. In maple leaves decreasing chlorophyll amount on the middle and upper plots (8.8% and 19.5% compared with the lower plot) together with Chla/Chlb ratio decrease (from 4.7 on the lower to 4.4 on the middle and 4.0 on the upper plots) was found. Chlorophyll content in acacia leaves decreased only on the upper plot (by 8.5% compared to the lower one), and Chla/Chlb ratio on the lower and upper plots reached 5.1. In maple leaves the increase in altitude was accompanied by the decrease in BPOD activity (by 26% and 63% on the middle and upper plots, accordingly), and GPOD and CAT (accordingly, by 31% and 37% on the upper plots compared to the lower plots). High correlation coefficients of light, temperature and humidity with activity of CAT (respectively, r = –0.98; r = –0.85; r = 0.83), BPOD (r = –0.96; r = –0.93; r = 0.90) and GPOD (r = –0.98; r = –0.82; r = 0.82) were estimated in A. platanoides leaves. Sharp decline in GPOD activity in R. pseu doacacia leaves was revealed on the upper plot (by 95% compared to the lower one), whereas BPOD activity increased by 47% on the middle plot but decreased by 74% on the upper one compared to the lower plot, and CAT activity on the middle and upper plots exceeded 4.3-fold and 1.8-fold the activity on the lower plot. In R. pseudoacacia leaves high correlation of light, temperature and humidity was found with GPOD activity (respectively, r = –0.99; r = –0.82; r = 0.82), whereas correlation with the light level was significant only for BPOD (r = –0.84), and no significant correlation was recorded for CAT activity. Results of the study confirm high sensibility of photosynthetic and antioxidant processes in leaves of both tree species to microclimate changes, which is defined as a conventional enhancement of aridity traits when moving upwards on the slope. It is suggested that the level of correlation between enzyme activity and microclimate and lighting parameters can serve as a marker of tree species adaptation to the slope altitude-associated changes of the local environmental factors in the natural forest.