The investigations of trait variability in Arum alpinum were conducted in 2016–2017 in two Dentario glandulosae-Fagetum patches (Chełm Mountain Reserve, Western Carpathians). Patch I (510 m a.s.l.) was located on a sharp steep slope and dominated by plant species with oblong leaves, whereas Patch II (400 m a.s.l.) was situated at the bottom of a wide ravine densely overgrown by species forming broad leaves. The height of plants growing in vicinity of A. alpinum in both study sites was rather similar, whereas the soil reaction and humidity were much higher in Patch II. Despite the different habitat conditions, the abundance of both A. alpinum populations was similar. The length of leaf petioles, dimensions of leaf blades, as well as the number of leaves per rosette were significantly greater in Patch I. The dimensions of leaves and flowering stems of individuals growing in Patch I were greater in 2016 than in 2017. The length of infructescences and number of fruits in both populations did not show either spatial or temporal variability. Moreover, the positive correlations between length of the highest leaf petiole and the dimensions of leaf blade, as well as between the height of the flowering stems and number of fruits and length of infructescence were noticed. Our results providing interesting data about of the spatio-temporal variability in A. alpinum traits in Dentario glandulosae-Fagetum patches reveal the need for this type of research in other plant communities.