Small balsam Impatiens parviflora DC. (Balsaminaceae) is the invasive species that colonises natural European forest. The morphological and physiological traits of the species under different natural conditions were analysed. The studies were carried out in the forest communities in the National Park (Ojców National Park - Southern Poland) with the known history of the plot: P1 - bottom of the valley, Alno-Ulmion Br.-Bl. et R.Tx. 1943, P2 - terrace of the valley, Tilio-Carpinetum Tracz. 1962 stachyetosum, P3 - south slope of the valley, Tilio-Carpinetum Tracz. 1962 typicum. The plots differed with the soil parameters and microclimatic conditions as well as with species richness, species composition and cover of I. parviflora. The significant statistical differences in the length of the aboveground parts of collected small balsam specimens on the studied plots were showed. The longest shoots among specimens growing in plot P1, and the shortest in plot P3 were observed. In the underground part of plants, the highest values of water content among the specimens on plot P1 and the lowest on plot P3 were revealed. The highest percentage of electrolytes leakage among the specimens from the plot P2 and the lowest of the plot P1 were observed. The significant differences of the chlorophyll a fluorescence of I. parviflora leaves on the plot P2 were observed, compared to specimens from the two remaining plots. The results pointed that disturbance light availability in dense forest canopy could influence on abundance the local population of small balsam.