Comparative analyses of gas exchange characteristics and chlorophyll fluorescence of three dominant tree species during the vegetation season in hemi-boreal zone, Lithuania

Current climate scenarios predict rising air temperature along with increasing frequency and intensity of summer drought in the Central and Eastern Europe. Severe drought episodes affect physiological processes in trees such as transpiration, photosynthesis and carbon allocation. Understanding gas exchange between plants and the atmosphere is important in woody plant research. The aim of this study was to evaluate differences in gas exchange characteristics and chlorophyll fluorescence of tree species prevailing in Lithuania (Scots pine, Norway spruce and Silver birch) and their physiological response to water stress. The study was conducted in Aukstaitija integrated monitoring station, Lithuania. Gas exchange parameters and chlorophyll fluorescence were measured during the vegetation season of 2016. Meteorological parameters were obtained from the monitoring station. Four weather periods with different meteorological conditions were identified. Under moderate drought conditions all investigated tree species demonstrated reduced photosynthetic rates, lower stomatal conduction transpiration rates, water use efficiency and instantaneous carboxylation efficiency. During moderate drought, intercellular CO ₂ concentration of Norway spruce was higher and this species demonstrated the highest decrease in instantaneous carboxylation efficiency. No significant changes of maximal chlorophyll fluorescence (Fv/Fm) among species were detected during different weather periods except Silver birch. The investigated tree species reacted differently to weather conditions. The Scots pine demonstrated the highest tolerance to different weather conditions. The study confirmed the sensitivity of Norway spruce to drought conditions. The Silver birch was the least sensitive to temperature and humidity conditions variation.