Alien plants threaten native communities by altering their species composition and reducing native biodiversity. Forests are considered to be relatively stable ecosystems and resistant to plant invasions. Impatiens parviflora DC. is one of the most intensively spreading alien plant species in the forests of Lithuania. Phenotypic variations among I. parviflora populations in three different habitat types (with different species of dominant tree and the coverage of the first tree layer) in the environs of Vilnius (South East Lithuania) were analysed. The results of the investigation enabled to evaluate alien plant species I. parviflora invasion into different forest communities and estimate relationships between plants of forest communities and I. parviflora.

The study has shown that this invasive species is spreading not only in disturbed areas, but also in places where other species do not grow due to the lack of the light. However, the number of individuals is increasing in proportion while the coverage of trees and shrubs is decreasing. The rise of the level of illumination increases not only the number of individuals in the populations, but also the above-ground biomass of I. parviflora: plants are larger, produce more side shoots. The correlation between the coverage of I. parviflora and native species statistically is significantly negative (r = -0.78) in the disturbed places.