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Predicted climate changings can affect hemi-boreal forests especially for soil conditions with low water-holding capacity and susceptibility to soil water deficits. In our study, species-specific sap flux, transpiration and water use efficiency of growth WUE were investigated at a mixed hemi-boreal forest ecosystems, especially during temporary moderate dry periods in the main growing season. Therefore, two representative forest sites with different drought susceptibility were selected in Northwest-Lithuania. The aim of our investigations was to identify the responsible factors influencing transpiration and WUE from Scots pine Pinus sylvestris L. , Norway spruce Picea abies L. H. Karst. and birch silver birch: Betula pendula Roth. and downy birch: B. pubescens Ehrh. during temporary drought incidents at a water-limited, oligitrophic sand dominated forest site and at a water saturated, mesoeutrophic organic peat forest site for the whole vegetation period and two moderate short-term drought incidents in 2016. During the dry periods Norway spruce trees exhibited lowest sap flux compared to Scot pine and birch while similarly showing highest WUE. Up-scaled to a virtual pure stand none of the species were differing in species specific transpiration. Thus, we found no evidence for extended drought sensitivity of Norway spruce at the water limited site during the first investigation year. Sap flux of Scots pine trees was more or less constantly high during the main growing season and the dry periods at both plots, while WUE was quite low during the "dry periods". Thus, we assume that Scots pine trees are less affected by temporarily drought events than Norway spruce. Birch trees exhibited significantly highest sap flux during dry periods at the water limited site whereas quite low WUE was not differing between sites. Thus, birch may compete with Scots pine trees at the study sites in terms of water consumption and growth in future.

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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.