SUMMARYThe effect of altitude-associated environmental gradient on leaves metabolic features of Quercus robur L. (an oak) and Fraxinus excelsior L. (an ash) was investigated in the natural coastal forest at Bellegarde' International Biosphere Reserve in Steppe zone, Ukraine. Decrease in relative humidity contrary to increase in temperature and lighting under the forest canopy were observed on the river steep bank with altitudinal elevation from lower (52 m a.s.l.) to middle (74 m.), and upper (96 m). Responses of tree leaves photosynthetic and antioxidant systems to the environmental local changes were studied by measuring chlorophyll (Chl) content, as well as catalase and peroxidase activities. Decrease in Chl amount in the ash leaves at middle and upper altitudes (17 and 38% compared with lower), along with increase (8% and 13%, respectively) in the oak leaves was found out. Chl content was determined to correlate with light, temperature, and humidity in both leaves of ash (respectively, r = -0.94, r = -0.92, r = 0.90) and oak (r = 0.95, r = 0.93, r = -0.90). Catalase activity grew with increasing altitude in leaves of ash (2 and 2.2 fold compared to lower altitude) and oak (1.2 and 1.4 fold) as well. Contribution of catalase to the total antioxidant enzymes activity enhanced in leaves of both species with increasing altitude. The results confirmed high sensitivity of steppe forest trees even to slight altitude-associated environmental deviations. Data obtained can be used to assess the adaptive potential of woody species to the climate changes aiming towards greater aridity traits and select tree species for planted forest creation as well.