The paper presents relationship between the Standardised Precipitation Index (SPI) and physiological responses of individual trees in a beech stand using an example of an experimental plot in Bienska valley (Zvolen, Slovakia). SPI is a widely used tool for monitoring both short-term and long-term droughts, and for the assessments of drought impacts on agriculture. Due to the complex ecosystem bonds, monitoring of drought in forests often requires a sophisticated technological approach. The aim of the paper was to correlate the SPI on the physiological responses of trees that were recorded during the performed physiological research (sap flow, and stem circumference increment) at the site in the growing seasons (May to September) of the years 2012-2014. The results revealed a relationship between the index and the physiological responses, although the problem with the impact of other environmental factors has also come up. The secondary correlation, in which soil water potential that significantly affects physiological responses of forest tree species was used as a dependent variable, showed a tighter relationship with the SPI. We found the highest correlation between the soil water potential and the values of SPI aggregated for five weeks. This indicates that the beech forest has a five week resistance to drought stress. The results also enable simple monitoring of the initiation of the drought stress by applying SPI for five weeks.

The paper focuses on the evaluation of transpiration as a physiological process, which is very sensitive to drought stress. Reactions of 25-year-old Norway spruce (Picea abies (L.) Karst.) trees to drought were examined during 2009 summer. Sap flow rate (SF), meteorological and soil characteristics were measured continually. Vapour pressure deficit of the air (VPD) and cumulative transpiration deficit (KTD) was calculated. During the second half of the vegetation period, the decrease in soil water content was observed and irrigation was applied to a group of spruce trees, while the second group was treated under natural soil drought. On the days, when the differences in transpiration between irrigated (IR) and non-irrigated (NIR) trees were significant (21 days), transpiration of NIR trees was only 23% of the transpiration of IR trees. We found significant differences in transpiration when the soil water content (SWC) of NIR variant at a depth of 5-15 cm ranged from 10.4 to 13.7%. Under both regimes of water availability, daily transpiration significantly responded to atmospheric conditions. However, the influence of all assessed meteorological parameters on SF of NIR trees was significantly lower than on IR tree. The dependency of transpiration on evaporative demands of atmosphere decreased with the decreasing soil moisture. Cumulative transpiration deficit of the stand during the entire evaluated period was 50.9 mm. The difference between the transpiration of the mean NIR tree and of the mean IR tree was 278.8 L over the assessed period of 47 days (5.9 L per day). The transpiration of NIR trees was 40.3% from the transpiration of IR trees during this period.

We monitored seasonal dynamics of stem water status of four coniferous species (Abies alba, Larix decidua, Picea abies and Pinus sylvestris) planted at the Borová hora Arboretum (300 m a.s.l., Zvolen valley, Central Slovakia) beyond their ecological and production optima, in the region with warmer and drier climate compared to the sites of their origin. Species-specific stem water deficit and maximum daily shrinkage were extracted from diurnal band dendrometer records of stem circumference recorded by digital band dendrometers DRL26 installed on five trees per species, and correlations with environmental variables were analysed. The seasonal stem circumference increment of all tree species was higher in 2017 than in the drier and hotter year of 2018. The greatest seasonal stem circumference increment in the observed periods of 2017 and 2018 was observed for A. alba and P. sylvestris, respectively. The highest and lowest values of daily and seasonal stem water deficit were observed for L. decidua and A. alba, respectively. The analysis of trees' short-term response to extreme climate events seems to be the promising and suitable method for detecting tree species tolerance towards drought.

Physiological response of European beech under soil and atmospheric drought conditions was investigated in this study. A group of six beech trees was irrigated during the growing season 2012, while the second group of non-irrigated (control) beech trees was treated under natural soil drought. During the experiment, we observed more than 45-day long period when no precipitation fell on the soil surface. The relationship of P N (CO 2 assimilation rate) to g S (stomatal conductance) was very tight in both groups, which indicates that stomatal opening was the main factor limiting P N . The statistically significant differences in g S between the groups of trees were revealed only on the last measuring day. The significant differences in P N were confirmed on the days when the differences in soil water potential (Ψ S ) appeared. On these measurement days, the P N values of irrigated individuals were approximately 1.9 or 3.3 times greater than the values of non-irrigated individuals. At the level of primary photosynthetic processes (chlorophyll fluorescence parameters) we did not observe lower values of the control individuals in comparison with the irrigated trees in any of the evaluated parameters. Long-term soil water deficit caused strong decrease of leaf water potential (Ψ L ) in the control trees, but Ψ L values of the irrigated trees were also rather low due to diurnal dynamics in higher parts of crown. Close relationship between Ψ L and g S was confirmed for the control (non-irrigated) trees, but could not be confirmed for the irrigated trees. We revealed significant influence of VPD (vapour pressure deficit of the air) on g S only in the control group. On the days when Ψ S decreased, the stomata of the non-irrigated trees were closed in spite of the low VPD values. Almost complete stomatal closure in both groups of trees was caused by the increase of VPD to 1.2 kPa. Keywords: Fagus sylvatica; water deficit; vapour pressure deficit; leaf water potential; stomatal closure Abstrakt Príspevok sa zaoberá skúmaním fyziologických reakcií buka na atmosférické a pôdne sucho. Skupina šiestich bukov bola zavlažovaná počas vegetačnej sezóny 2012, pričom druhá skupina bukov (kontrola) bola ponechaná v prirodzených pôdnych vlahových podmienkach. Počas experimentu bola pozorovaná viac než 45 dní trvajúca perióda, kedy sa zrážková voda nedostala na povrch pôdy. Potvrdili sme silnú závislosť rýchlosti asimilácie (P N ) na prieduchovej vodivosti (g S ), čo dokazuje, že otvorenosť prieduchov bola hlavným limitujúcim faktorom priebehu asimilácie CO 2 . Štatisticky významné rozdiely g S medzi skupinami boli potvrdené len počas posledného merania. Významné rozdiely v P N sa potvrdili pre dni, v ktorých sme zistili výrazné rozdiely vo vodnom potenciáli pôdy (Ψ S ). Počas týchto dní boli priemerné hodnoty P N pre zalievané stromy 1,9, resp. 3,3-krát vyššie v porovnaní s nezavlažovanými jedincami. Na úrovni primárnych fotosyntetických procesov (parametre fluorescencie chlorofylu) sme nezistili nižšie hodnoty kontrolných jedin...

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