The effects of different climatic conditions on birch (Betula pendula Roth) and poplar (Populus) hybrids seedlings were studied. This study consists of two parts, firstly, an experiment with poplars and, secondly, the experiment with birch seedlings. The aim of this study was to evaluate the growth of birch and poplar seedlings under the current and modelled climatic conditions. Results showed that the simulation of short-term warmer weather conditions and higher CO2 concentration led to more intensive growth of silver birch saplings. Different climatic conditions did not significantly change photosynthetic and transpiration rate of one year old silver birch seedlings. Contrary the transpiration and photosynthesis rate of poplar seedlings significantly depended on drought and clone but not on hybrid. Drought altered the transpiration and photosynthesis intensity, water use efficiency, and correlation among most of the traits. The P. maximowiczii × P. trichocarpa, P. balsamifera × P. trichocarpa hybrids increased their water use efficiency under the drought conditions and were characterized by faster growth hybrids. The sensitivity of poplar hybrids to UV-B radiation varied and depended on the origin of their parental trees: poplar hybrids from the northern latitudes were more sensitive to the increased UV-B radiation than those of southern origin. The best growth after simulated warm winter was obtained for the P. deltoides × P. nigra and P. nigra × P. nigra hybrids, originating from the southern areas. The findings from this experiment are more applicable to young birch and poplar trees, but stress on young trees may be reflected in future tree growth.