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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.

Sustainable forestry plays an important role in applying forest management measures. In many forests, management is oriented towards closer natural forest processes. Silvicultural systems that retain some part of the trees in the stand, such as shelter wood cuttings have been introduced. The aim of the study was to evaluate the early changes of ground vegetation after shelter wood cuttings in sites of different soil fertility gradient in pine dominated stands on sandy soils in the European hemiboreal zone. The Scots pine stands in which first step of shelter wood cuttings was applied were investigated. The stands were selected in fresh (normal humidity) sites according to soil fertility gradient: very poor, poor and medium fertility soils. All mosses, lichens, herbaceous vegetation and dwarf shrubs were recorded, and projection cover was estimated in June - August. Ellenberg’s indicator figures were used to evaluate site conditions. In shelter wood cuttings (5-6-year-old) the number of species was higher in medium fertile sites than in very poor sites. After shelter wood cuttings the abundance of herbaceous species increased in all sites, while the abundance of mosses and lichens decreased. Within the first year after shelter wood cuttings, the abundance of Ericaceae species decreased, and abundance of Rosaceae and Poaceae species increased. Average Ellenberg’s indicator values showed that site conditions changed after shelter wood cuttings. The changes were more intensive in medium fertile sites than in poor sites. After shelter wood cuttings the richness and abundance of ground vegetation changed. The intensity of change differed in sites of certain soil fertility and was caused by alteration of site conditions, which were more homogenous in sites of the certain fertility in uncut stands than in cuttings. Key words: herbs, mosses, non-clear cuttings, vegetation

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