The Effect of Population Pseudo-Rejuvenation in Adverse Changes in Living Conditions: A Case Study on the Highland Plant Species in the Ukrainian Carpathians

“Rejuvenation of population” is commonly considered as an increase in the proportion of young individuals in the population. In our opinion, the term “rejuvenation” has to be used in case of an increasing percentage of young individuals providing maintenance or increase of the entire population size. In return, an increasing percentage of young individuals while reducing the number of the entire population should be considered as “pseudo-rejuvenation.” A key feature of rejuvenation is the positive dynamics of the population. Instead, pseudo-rejuvenation is an indicator of negative exogenous or endogenous changes and is evidence of population degradation under adverse environment conditions. The article presents the most common examples of the processes of rejuvenation of populations in different plant species under the influence of anthropogenic and natural factors in the plant communities of the Ukrainian Carpathians. The main causes of rejuvenation and pseudo-rejuvenation of populations of the target plant species have been identified. Studies of population structure changes in situ were performed using both long-term monitoring transects laid 5–40 years ago and in new sites. Transects are located in the alpine, subalpine, and upper forest zones of the Ukrainian Carpathians in the height range of 1200–2000 m a.s.l. The study areas cover the most common alpine and chionophilic communities, some forest and subalpine phytocenoses.