Introduction

Ongoing climate change poses many new and unknown challenges for ecosystems. One of the greatest threats are changes in the water supply caused by changes in precipitation distribution during the season. Such water deficiency can occur over the short- to long- term and at local, regional or continent-wide scales, such as summer drought wave from the west to central Europe in 2003 (Ciais et al. 2005; Bréda et al. 2006; Rouault et al. 2006). The effects of drought can be either direct, such as tree mortality due to cavitation or drying up (Ciais et al. 2005; McDowell 2015), or indirect—high temperatures and drought have effect on every plant living processes, such as tree physiology, biochemistry or phenology. In turn, this can affect plant–insect relationships through nutritional quality of wood, foliage or sap and thus plants mechanical and chemical defenses against pests attacks. High air temperatures and drought also enhance risk of forest fires (Jankovsky and Palovcikova 2003; Breshears et al. 2005; Hlasny et al. 2011). Moreover, over the longer time scale, repeated drought can impact an ecosystems’ plants/trees species composition, as previously dominant plants may diminish in number and plants hitherto outnumbered by others may win the competition because of their different water management strategies (McDowell 2015). Thus, biodiversity, and especially species diversity, may be strongly affected. It is more evident, that at present, drought is more and more frequently observed in areas previously not considered to be affected by lack of water (Allen et al. 2010). Extreme consequences of more frequent and severe drought can lead to the extinction of forests in some localities (McDowell 2015). The impact of drought differs within the time of the growing season, namely affecting seasonality of plant growth on longer time scale (Vicente-Serrano et al. 2013). Drought has different impacts at different elevations and latitudes. While drought is very likely to become the key limiting factor for forest stands at lower altitudes (Hlásný et al. 2011), at higher altitudes the threat is not as critical yet due to higher precipitation rates and lower temperatures; however, conditions are changing (Dai 2013).

Norway spruce (Picea abies (L.) H. Karst) and European beech (Fagus sylvatica L.) are the most important economic species in forestry operations within the Czech Republic and within Europe (Úradníček and Maděra 2001; Mäkinen and Isomäki 2004; Kenderes et al. 2008). These two species often are planted under conditions which are not optimal. Within the European region, 6–7 million ha of pure spruce monocultures have been planted outside the area of their original natural habitat, which means mostly at sites originally occupied by deciduous or mixed forests (Teuffel et al. 2004). The broadleaf forest area has thus decreased from its natural 66% extent down to its present 33% (Kenk and Guehne 2001). Under such conditions, an ecosystem can easily be disrupted such that damages are irreparable and losses in forest yields follow. Norway spruce is sensitive to drought because its root system is shallow (Nadezhdina et al. 2014). European beech was proven to be more drought-resistant than Norway spruce (Pretzsch et al. 2013). With its heart-shaped root system (Ghestem et al. 2011), beech is able to obtain water from deeper soil layers (1 m and deeper; Kodrík and Kodrík 2002) and has greater resilience against drought stress than does spruce. Nevertheless, beech is also predicted to lose its area of natural distribution (Bolte et al. 2007) as well as its ability to compete and regenerate naturally due to possible water shortages (Geßler et al. 2007). The ability of European beech to adapt to changing environmental conditions is not well-documented however, and therefore studies are needed to address this issue (Nguyen 2016).

Trees, as other plants do, regulate carbon dioxide uptake and water losses through stomatal responses, which make it possible to establish some equilibrium between the risk of lethal water losses and cavitation, and the decrease of assimilation caused by carbon dioxide transport limitation.

Two basic strategies for balancing water relations are known: (1) isohydric plants try to protect their organs against desiccation by closing their stomata and thus reduce carbon uptake. Norway spruce belongs to this group of plants (Lyr et al. 1992); and (2) anisohydric plants leave their stomata open, maintain carbon uptake at a constant level and risk losses of water and xylem embolism. European beech belongs to this group (Leuschner 2009).

Daily courses of the light response curves of the photosynthetic activity of isohydric plants typically shows hysteresis. Although light response hysteresis has previously been shown in agricultural plants (e.g. Pingintha et al. 2010), knowledge about these reactions remains lacking in forests of various types like spruce and beech, and it will be useful to investigate response of both mentioned tree species.

The eddy covariance method (EC, Aubinet et al. 2012) as a modern tool for observation of daily, monthly and annual courses of carbon flux between the forest stand and atmosphere is a valuable approach for assimilation hysteresis observation. It provides continuous measurements of carbon dioxide and water vapor exchanges between an ecosystem and the atmosphere for long periods of time, i.e. seasons or years.

For the purpose of presented research on the possible differences in drought-stress response strategies between Norway spruce and European beech, the extraction of specific days from the climate data set was done to identify days with lowered water availability occurring at the ecosystem.

The objectives of this study were to compare dry conditions and conditions without drought stress with regard to gross primary production (GPP) and net ecosystem production (NEP) for spruce and beech stands. Differences in the daily courses of GPP and NEP of these species were evaluated with regard to an expected decrease in CO2 uptake during dry days. The possible occurrence of CO2 uptake hysteresis in daily production was investigated in relation to the two tree species.

Materials and methods

Site description

Data from two experimental ecosystem sites were used: Bílý Kříž, which is part of the Integrated Carbon Observation System (ICOS) research infrastructure and Štítná nad Vláří, which is part of the Czech Carbon Observation System (CzeCOS) research infrastructure. The Bílý Kříž site is within an evenly aged Norway spruce (Picea abies) stand situated in the Moravian–Silesian Beskydy mountains (49°30′08′′N, 18°32′13′′E) with a mean elevation of 875 m a.s.l. The forest was 35 years old in 2013. Mean annual air temperature (1998–2012) at the site was 6.8 °C and mean annual precipitation (1998–2012) 1250 mm. The soil type is haptic podzols with the densest root layer in the range of 5–15 cm. The total soil depth varies between 60 and 70 cm. Hereinafter, this site is referred to as the “spruce forest.”

The Štítná site is an even-aged beech forest (Fagus sylvatica) situated in the Bílé Karpaty hills (49°02′09′′N, 17°58′12′′E). The mean elevation is 540 m a.s.l. The forest was 112 years old in 2013. Mean annual air temperature (2010–2012) at the site was 8.4 °C and mean annual precipitation (2010–2012) 770 mm. The soil type is cambisol, and the densest root layer is around 50 cm depth. Total soil depth reaches 90 cm. Hereinafter, this site is referred to as the “beech forest.”

Both the beech and spruce forests are situated in the cold and wet region of the Czech Republic’s eastern mountainous border.

Data measured

Ecosystem data collected over a three-year period (2011–2013) were used. Eddy covariance systems were installed for flux measurements at both investigated sites. The systems use an ultrasonic anemometer (R3 or HS-50, Gill Instruments, Hampshire, UK), a Li-7000 (beech forest) or Li-7200 (spruce forest) fast-response infrared gas analyzer (Li-COR, Lincoln, NE, USA), and Windows Interface Software for the Li-7200 (Li-COR, Lincoln, NE, USA). Raw data were post-processed using open-source software EddyPro® (Li-COR; EddyPro is a registered trademark of Li-COR Biosciences in the United States and other countries). Half-hourly averages of fluxes are available for further analysis.

Air temperature was measured by EMS33R (EMS Brno, Czech Rep.), incident photosynthetically active radiation (RPA) was measured by Quantum Sensor, EMS 12 (EMS Brno, Czech Rep.). Global radiation (Rg) was measured by CM6B sensor (Kipp & Zonen, Netherlands) at the spruce forest site and by CNR 1 sensor (Kipp & Zonen, Netherlands) at the beech forest.

Computations

The available water resource (AWR) was computed using the SoilClim semi-empirical model (Hlavinka et al. 2011) interpolated to 500 m grids for two soil depth profiles: 0–40 cm and 40–100 cm. AWR refers to a percentage of soil field capacity. The threshold for a lowered availability of soil water was considered to be 50% (Larcher 2003).

The vapor pressure deficit (D) was computed using EddyPro® software as the difference between actual water vapor pressure (calculated from the ideal gas law) and its saturation value according to Campbell and Norman (1998).

Canopy resistance (rs; m s−1) was estimated using inversion of the Penman–Monteith equation. This assumes that heat storage change can be ignored (i.e., the total energy available for evapotranspiration is that of net radiation):

$$ r_{s} = \frac{{sr_{a} \left( {R_{n} - {\lambda E}} \right) +\uprho_{a} c_{p} D}}{{{\lambda E\gamma }}} - r_{a} $$
(1)

where ra (s m−1) is aerodynamic resistance computed according to Launiainen (2010), Rn (W m−2) net radiation, λE (Wm−2) latent heat flux, D (Pa) vapor pressure deficit, cp (J g−1K−1) heat capacity of air at constant pressure, s (Pa K−1) the slope of the saturation vapor pressure curve, ρa (g m−3) air density, and γ (Pa K−1) the psychrometric constant.Canopy conductance (Gc, m s−1) was estimated as the inverse of rs:

$$ G_{c} = \frac{1}{{r_{s} }} $$
(2)

Clearness index (Kt; dimensionless), used to indicate clear days, is defined as:

$$ K_{t} = \frac{{R_{g} }}{{R_{ext} }} $$
(3)

where Rg is global radiation (W m−2) and Rext is extraterrestrial radiation (W m−2) (Spitters et al. 1986).

Analyses

Measured fluxes of CO2 represent the net ecosystem exchange (NEE; μmol m−2 s−1). To estimate gross primary production (GPP; μmol m−2 s−1), a partitioning algorithm was used (Reichstein et al. 2005) for the beech forest and an Arrhenius type of ecosystem respiration (Reco; μmol m−2 s−1) model (Johnson and Thornley 1985; Hikosaka 1997) was used for the spruce forest. Due to the complicated site terrain and a catabatic flow of CO2 moving downslope at the spruce forest, the mentioned partitioning algorithm could not be used. GPP was then computed as NEE minus Reco. For the analysis of half-hourly GPP light responses (GPP × RPA dependency) only good-quality daytime data (quality code 0 and 1 according to the CarboEurope flagging system, Mauder and Foken 2006) were used. Unreliable data of low quality (quality code 2) were not used.

Daily sums of net ecosystem production (NEP; kg(C) ha−1 day−1) were analyzed also. To see the response to drought, daily NEP sums were computed from the gap-filled eddy covariance half-hourly CO2 fluxes (NEE). Gaps in NEE data were observed most frequently during mornings and evenings. Therefore, only days with not more than 20% of gaps were used (minimum 38 measured half hours per day). Spruce forest data were gap-filled using functional relationships similar to Falge et al. (2001). Light response of NEE was fitted by non-rectangular hyperbolic curve (Rabinowitch 1951). The Arrhenius type function (van´t Hoff 1898) was used to model temperature response of Reco. For the beech stand, the gap filling technique according to Reichstein et al. (2005) was used.

Data selection

Only daytime data (defined as RPA > 60 µmol m−2 s−1) during the growing season (May–October) were considered. Typical days considered to be under short-term drought stress (DS) were chosen according to the following rules:

  • No rain for at least the previous 3 days;

  • Clear day (smooth radiation course, no clouds, Kt > 0.7);

  • D not less than 1000 Pa (Körner 1995; Lasslop et al. 2010).

  • Days without drought stress (NS) were selected:

  • Clear days shortly after rain (less than 3 days);

  • D less than 1000 Pa.

All selected days (DS and NS) were characterized by similar radiation conditions (no statistically significant difference in sum of daily radiation: p = 1 at both sites between DS and NS days). D thresholds were validated by cross-checking against the available water resource (AWR).

Only days during July–September matched the rules for DS days, and therefore NS days were chosen from the same period.

Data were evaluated using light response of the GPP to RPA fitted by the Michaelis–Menten equation (Michaelis and Menten 1913):

$$ {\text{GPP}} = \frac{{\alpha \cdot R_{PA} \cdot G_{\hbox{max} } }}{{\alpha \cdot R_{PA} + G_{\hbox{max} } }} $$
(4)

where α is the initial slope of the light response curve and Gmax the saturation value of GPP at the infinite light level. For purposes of comparison and testing of differences in the light response of GPP, light intensity was divided into categories with thresholds at each 300 μmol m−2 s−1. The same intervals were used to test differences between morning and afternoon light responses in GPP. The interval length was chosen as the shortest possible with respect to the amount of data left in the categories.

Final numbers of samples used for analyses were:

  • Spruce forest drought stress (DS) = 12 days.

  • Spruce forest no stress (NS) = 14 days.

  • Beech forest DS = 10 days.

  • Beech forest NS = 11 days.

Results

Variation of meteorological factors

Incoming RPA (half-hourly averages) did not differ statistically between DS and NS days (Student’s t test, p = 0.56 for spruce, p = 0.24 for beech).

Average half-hourly air temperature at the top of the canopy differed statistically between DS and NS days (t test, p = 0). DS days had 5.6 °C and 4.8 °C higher temperatures than NS days in spruce and beech forest sites, respectively. The differences may have been caused by lower evapotranspiration rates and thus lower cooling effect during DS days.

The selection of days according to D was proven by AWR at two soil depths (Table 1). Deeper soil levels during DS days tended to have lower variability than did higher levels. During NS days, the situation was the opposite at the spruce forest while at the beech forest the variability was the same. Mean AWR values showed large differences between DS and NS days at both levels at both sites.

Table 1 Comparison of average daily mean of available water resource for two soil profiles (0–40 and 40–100 cm) for spruce and beech forests
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Comparison of GPP

The Michaelis–Menten light response curve was fitted separately to the data points from NS and DS days for both sites (Fig. 1a, b; parameters of the fit listed in the Table 2). All fits were statistically significant (p < 0.001). We have observed higher differences in GPP values between DS and NS days in the spruce than in beech forest (Fig. 1). During DS days the distinctively lower assimilation capacity in comparison to the NS days was observed.

Fig. 1
figure 1

Light responses during the period with drought stress (DS) and without (NS) in spruce a and beech b forests. Fitted light response curves represent Michaelis–Menten dependency with parameters listed in the Table 2. Half hour GPP estimates were used

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Table 2 Parameters of the fitted Michaelis–Menten light response curve GPP
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As computed from the light response curve of GPP fit, at the spruce forest, the GPP curve, on DS days reached less than 70% of the GPP curve during NS days in the morning and evening (RPA < 300 μmol m−2 s−1). At midday hours (RPA > 1200 μmol m−2 s−1), that proportion was less than 50%. At the beech forest, zero or small differences in GPP at low irradiation (mornings and evenings) was observed. At midday hours (RPA > 1500 μmol m−2 s−1), the proportion was 77%.

Daily courses of GPP and G c

To understand the physiological reasons for differences in GPP between NS and DS days in the spruce and beech forests, diurnal courses of GPP were analyzed. In spruce forest, most of the DS days (8 out of 11) showed hysteresis type of behavior in the GPP light response curve. Until approximately 10:30 a.m. GPP was increasing with increasing RPA. Then, in spite of RPA still increasing, GPP started to decrease and continued to do so until the end of the day. On NS days, such behavior was less apparent or entirely absent. In the beech forest, the afternoon GPP during DS days exceeded the morning production (8 times out of 10). On NS days in the beech forest, such behavior was less apparent.

Daily courses of the canopy conductance were also analyzed (Fig. 2a, b). In the spruce forest, morning values of all DS days exceeded afternoon values. During NS days, such behavior was still apparent in most of the cases (data not shown). In the beech forest, asymmetry of Gc courses was absent for both DS and NS days.

Fig. 2
figure 2

Daily courses of canopy conductance during a day with drought stress at spruce (11 September 2012, a) and beech (29 July 2013, b) forests

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Figure 3 shows the differences and statistical significance (Student’s t test) of the assimilation activity (GPP) within light intensity groups between morning and afternoon hours. Hysteresis was clearly visible in the spruce forest during DS days (Fig. 3a). All differences were statistically significant except for the early morning and evening period (RPA: 60–300 μmol m−2 s−1). When light intensity reached approximately 600 μmol m−2 s−1, the rise in GPP slowed even though RPA continued to increase, GPP then decreased.

Fig. 3
figure 3

Comparison of light responses of gross primary production (GPP) of spruce (a, b) and beech (c, d) forests during the period with (DS; a, c) and without (NS; b, d) drought stress. Data points in each category were tested for differences in GPP response according to the time of day. Statistically significant differences are noted by stars in the upper parts of the graphs. Whiskers represent minimum and maximum, boxes stand for quartiles and horizontal line shows median

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During NS days in the spruce forest (Fig. 3b), the differences between morning and afternoon hours were significant above the RPA value approximately 300 μmol m−2 s−1. GPP reached its morning level after RPA dropped below 300 μmol m−2 s−1. In contrast to the DS days, during the NS days GPP continued to rise with increasing light intensity.

In the case of the beech forest (Fig. 3c, d), the behavior observed on DS days and NS days was similar. There were no significant differences during midday hours (approximately 8:30 a.m.–3:30 p.m.). Afternoon GPP was either higher or lower than the morning GPP. Instead, significant differences occurred in the late afternoon and evening hours (RPA below 900 μmol m−2 s−1), at which time GPP exceeded the morning values. The only exception was the interval 300–600 μmol m−2 s−1 during NS days, where the difference was insignificant due to the small number of observations.

Comparison of NEP

The average daily net ecosystem production (NEP) in the spruce forest was 50 kg(C) ha−1 day−1 during NS days and 31 kg(C) ha−1 day−1 during DS days (Fig. 4a). During DS days, therefore, the spruce forest stored up to 38% less carbon. In the beech forest, the average daily NEP was 46 kg(C) ha−1 day−1 during NS days and 36 kg(C) ha−1 day−1 during DS days, respectively (Fig. 4b), which was approximately 21% less. In the spruce forest, overall NEP during the study showed statistically significant differences between DS days and NS days (p = 0.009). In the beech forest, these differences were not significant at the level α = 0.05 (p = 0.077).

Fig. 4
figure 4

Comparison of net ecosystem production of days with drought stress and without stress in spruce (a) and beech (b) forests. Whiskers represent minimum and maximum, boxes stand for quartiles and horizontal line shows median

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Lower assimilation activity at both sites cannot be attributed to enhanced ecosystem respiration (Reco) rates. Reco was lower during DS days as compared to NS days in both stands. Photosynthesis still plays a major role in changes in carbon uptake. Average Reco during DS days was 5.34 (± 0.81) µmol m−2 s−1 and 3.70 (± 0.54) µmol m−2 s−1 for spruce and beech stands, respectively, while during NS days it was 6.83 (± 1.6) µmol m−2 s−1 and 4.19 (± 1.18) µmol m−2 s−1 for spruce and beech stands, respectively.

Comparison of canopy conductance

The comparison of half-hourly canopy conductance values (Fig. 5) shows statistically significant differences (t test, p < 0.001) reached during DS and NS days. In both stands, lower values of Gc during DS days compared to NS days were apparent and statistically significant. Maximum canopy conductance on DS days reached 39 and 52% of Gc values during NS days in spruce and beech stands, respectively.

Fig. 5
figure 5

Comparison of canopy conductance in spruce a and beech b stands during drought stress and no stress days. Whiskers represent minimum and maximum, boxes stand for quartiles and horizontal line shows median

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Table 3 shows the average and maximum half-hourly values of Gc during DS and NS days. For the spruce and beech forests, it is apparent that, under favorable conditions, forests are more photosynthetically active in comparison to the drought stress conditions (higher values of conductance on NS days).

Table 3 Average and maximum values of canopy conductance during selected drought stress days (DS) and no stress days (NS) in spruce and beech forests
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Discussion

The selection of days with and without drought stress was within the July–September period to represent the stable part of the growing season of needles and leaves, and thus a stable form of the assimilation activity. Beech leaves were already well developed but not yet approaching senescence (end of November; Hájková 2012). The selected days did not differ in the amount of incoming RPA but did differ in temperature and water availability both in the air and in the soil.

The presented results show that both sites were under reduced water availability conditions on DS days (AWR values below 50% of soil field capacity; Larcher 2003). During NS days, water conditions were favorable at both sites and in both soil levels (Table 1). According to the statistical analysis of AWR (Table 1), the beech stand was exposed to more intense drought conditions than the spruce stand. The daily sum of precipitation was on all investigated days, and all days were clear (Kt > 0.7). The root system of spruce species, which is very shallow (Ghestem et al. 2011), plays important role. According to site measurements, most of the fine roots are located within the 5–15 cm layer in the spruce stand and around 50 cm in the beech stand. Thus, ii may be assumed that the AWR statistics for the two soil profiles (0–40 and 40–100 cm) are representative for both sites. Although a beech species is able to root itself deeper than 100 cm, the volume of roots at the greater depths is negligible (Kodrík and Kodrík 2002).

Spruce exposed to reduce water availability showed high assimilation activity in the mornings and evenings when vapor pressure deficit was still low, while closing stomata during the day under unfavorable water conditions (Fig. 2a). Beech reacted to decreased available water resource by reducing assimilation activity throughout the day while leaving stomata open as much as possible (symmetrical daily course of Gc, Fig. 2b). During DS days compared to NS days, Reco was lower in both stands. Observed values of Reco during NS days are consistent with previously published results (Falge et al. 2002). Thus the differences in Reco were less than 1 µmol m−2 s−1 on average at both sites, while differences in Gmax reached 25 and 10 µmol m−2 s−1 in spruce and beech stands, respectively. This means that the sensitivity of assimilation to lower water supplies is mainly the result of stomata responses, especially for spruce, which is sensitive to water scarcity. This sensitivity is documented in every investigated aspect (half-hourly GPP responses to light, stomatal closure during the day connected with Gc courses, and daily NEP). The vascular system and tissues consist of tracheid elements in spruce, which are often the reason for water column to be easily broken and drying out (Panshin and de Zeeuw 1980). Spruce trees react to reduced water availability with an isohydric stomata strategy (Lyr et al. 1992), meaning immediate closure of the stomata and decrease in photosynthetic activity which protects the tree from desiccation. Immediate stomata closure leads to the decrease of production described by Ciais et al. (2005) during the extreme summer drought wave in Europe in 2003.

For the beech forest, half-hourly GPP responses to light, stomatal closure during the day connected with Gc courses, and daily NEP did not show a definitive response to water scarcity, however, there was still reduced assimilation activity on NS days. It is well-known that beech has roots in the shape of a heart reaching to deeper soil layers (Kodrík and Kodrík 2002). The vascular system and tissues of beech consist of vessel elements which cause the water column partly resist breaking due to water scarcity. Moreover, beech trees leave their stomata open longer and try to maintain photosynthetic activity as high as possible, thereby risking cavitation and death from xylem malfunction (Urban et al. 2012; Pretzsch et al. 2014). Geßler et al. (2007) states, that European beech may show reduced growth under limited water availability. Seedlings as well as adult trees may demonstrate xylem embolism and restricted nutrient uptake capacity, which may be lethal for individuals in some localities. These findings support our results.

The plant’s response to changing water supplies or values of D during the day is manifested by the daily GPP course, which reveals hysteresis. Pingintha et al. (2010) reported such behavior in peanut plants. Although light response hysteresis has previously been shown in agricultural crops (e.g., Pingintha et al. 2010), there remains a lack of knowledge about this reaction by various tree species.

Urban et al. (2012) reported afternoon stomatal closure in spruce stands which prevented water loss. During the morning, trees utilize water reserves stored in their trunks. Supplying water from the soil via roots is too slow for delivering sufficient water to leaves. Therefore, a water deficit appears in the afternoon (Köstner et al. 1998; Kanalas et al. 2010).

In the spruce stand, daily hysteresis in GPP response was found under both types of water supply conditions. This might have been caused by the correlation between D and RPA (all days from which data were used were clear) together with the strong reaction of Norway spruce to D increase by stomatal closure despite D remained below 1000 Pa on NS days (Figs. 2a, 3a, b).

In the beech stand, no clear hysteresis appeared. Granier et al. (2000) observed afternoon depression of photosynthesis during clear days in a young beech stand growing at low elevation while measuring water flux and sap flow. These findings differ from the results from an old beech stand which developed deep roots system wherein no afternoon depression of photosynthesis was observed. However, during DS days, photosynthetic carbon uptake was lower throughout the day in comparison to NS days. The late afternoon increase in GPP in the beech stand can be related both to its anisohydric strategy and to the obvious light penetrating into the deeper canopy due to its west–southwest slope. Thus, an involvement of lower leaf area into assimilation production was reached.

Zang et al. 2011 states that spruce is the most susceptible species in Europe to climate change, specifically at lower altitudes and on less fertile sites. Similar results were reported for the lower altitudes in the Alps (e.g. Elkin et al. 2013). Our findings suggest similar results. Zlatanov et al. (2017) reported that, according to the projection over the next 20–50 years, Silver fir is expected to replace spruce as a dominant species. They also predict a reduction of spruce productivity at lower elevations which is consistent with our results. A more difficult situation is with regards to the European beech. Lindner et al. (2010) states that beech is projected to face severe problems under increasing temperatures and milder winters may reduce winter hardening, increasing their vulnerability. European beech is an important species as a source of firewood. An economical alternative for this species, unlike spruce, however, does not exist (Zlatanov et al. 2017).

Pretzsch et al. (2014, 2015) show that mixing both species may be effective solution to many problems of monocultures. Total productivity of a mixed forest is higher than of monocultures. The advantages of mixed stands are hydraulic lift (redistribution of soil water) and improvement of soil fertility due to contrasting litter types (Pretzsch et al. 2015). A mixed forest is also expected to prosper at higher densities than monocultures (Pretzsch et al. 2015). The current state of forests within the Czech Republic includes over 50% of Norway spruce monocultures and almost 10% of European beech monocultures (Bělská et al. 2016). However, planting is slowly changing to mixed forests in some places. Considering speed of temperature shifts and redistribution of precipitatio, as several authors have (e.g. Bolte et al. 2009; Felton et al. 2016), we strongly suggest to shift forest management to mixed forest more intensively.

Conclusions

Presented results demonstrate the importance of lowered water availability for the carbon flux of both tree species. This response is rapid and a short term event of lower water supply could be responsible for significant reduction forest carbon uptake. Regarding the possible future development of climatic conditions in central Europe (Marková et al. 2016; Trnka et al. 2007), the investigation and evaluation of water supply deficiency response of carbon uptake for two representative tree species, i.e., coniferous spruce and broad-leaf beech, seems to be important.

The spruce forest reacted with a marked decrease in assimilation activity around mid-day and during afternoon hours which caused a hysteresis in the light response curve of GPP. The beech forest also reacted by lowering its assimilation activity, but during the entire daylight period the decrease in carbon uptake was not as dramatic. Despite the ability of beech to reach deeper soil levels, this ecosystem is still threatened by impending drought periods due to its inability to readily react to lowered water availability. The basic beech strategy is to avoid water deficit by developing deeper root system to obtain water from deeper parts of the soil.

The response of the forest stands representing the main tree species in central Europe, supports the assumption of the strong detrimental effect of changes in precipitation. Regarding the importance of forest carbon sink in global climate change mitigation, this is a serious finding. Moreover, these responses can affect forest management in large areas via more frequent episodes of drought, mainly at lower altitudes. It is evident that these changes in precipitation distribution are connected with future changes in forest soil carbon stock, which is extremely important for total carbon storage (Marková et al. 2016).