## 3 Results
### Impact of Drought on Ecosystem Carbon Fluxes at Monthly Timescale  
The monthly averages of daily sums of GPP and \(\mathrm{R_{eco}}\) were analysed separately to assess whether the observed net ecosystem productivity (NEP) reduction during drought was mainly a consequence of increased \(\mathrm{R_{eco}}\), reduced GPP, or contribution of both across the forest ecosystems (Figures 5-7). Figure 2: Response of gross primary productions to photosynthetically active radiation during the years with normal conditions (black) and affected by drought stress (red) in CZ-BK1 and CZ-RAJ. The half-hourly GPP values (points) were fitted using the logistic sigmoid light response curves (lines). Figure 4: Relationship between the light response curve (LRC) residuals of the light response curve of gross primary production and the soil volumetric water content (SVWC) at the spruce forest sites in CZ-BK1 and CZ-RAJ for the normal (NY) and dry years (DY). Red represents high VPD conditions and blue shows low VPD conditions. The grey dashed lines represent the piecewise regression model slope, whereas the black dashed lines show the breakpoint values. Figure 3: Relationship between the light response curve (LRC) residuals of the light response curve of gross primary production and the vapour pressure deficit (VPD) at the spruce forest sites in CZ-BK1 and CZ-RAJ for the normal (NY) and dry years (DY). Red represents low SVWC conditions and blue shows high SVWC conditions. The grey dashed lines represent the piecewise regression model slope, whereas the black dashed lines show the breakpoint values. It was observed that there was a significant decline in the total GPP by 14% and 6% during the main growing season period of the dry year for CZ-RAJ and CZ-BK1, respectively. There were also observed significant statistical differences (\(p<0.001\)) in the mean monthly GPP values recorded for July and August during the dry year as compared \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Variants** & \multicolumn{2}{c}{**CZ-BK1**} & \multicolumn{2}{c}{**CZ-RAJ**} \\ \hline  & Years with Normal & Dry Year (2015) & Years with Normal & Dry Year (2015) \\  & Conditions (2014 \& 2016) & Conditions (2014 \& 2016) & Dry Year (2015) \\ VPD [hPa] & 2.6 *** & 5.3 *** & 6.8 *** & 23.5 *** \\ Slope before breakpoint in VPD & \(1.12\pm 0.30\) & \(0.02\pm 0.12\) & \(0.08\pm 0.05\) & \(-0.55\pm 0.01\) \\ Slope after breakpoint in VPD & \(-0.39\pm 0.02\) & \(-0.70\pm 0.02\) & \(-0.54\pm 0.02\) & \(-0.22\pm 0.06\) \\ SVWC [m\({}^{3}\) m\({}^{-3}\)] & \(0.20\) *** & 0.16 *** & 0.18 *** & 0.19 *** \\ Slope before breakpoint in SVWC & \(-85.49\pm 68.37\) & \(305.61\pm 23.43\) & \(164.61\pm 25.14\) & 187.22 \(\pm\) 8.78 \\ Slope after breakpoint in SVWC & \(14.75\pm 2.60\) & \(5.03\pm 2.51\) & \(-1.27\pm 2.84\) & 52.16 \(\pm\) 6.24 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Breakpoints and Slopes from the piecewise regression of the light response curve residuals to vapour pressure deficit (VPD) and soil volumetric water content (SVWC) for the period of May–September. Figure 5: Monthly averages of daily sums of gross primary productivity (GPP) for May–September of the normal years (NY) and dry year (DY) in CZ-BK1 and CZ-RAJ. The tables within the figure represent the mean monthly vapour pressure deficit (VPD) and soil volumetric water content (SVWC) values from May to September for each forest station. Figure 6: Monthly averages of daily sums of ecosystem respiration (R\({}_{\text{eco}}\)) for May–September of the normal years (NY) and dry year (DY) in CZ-BK1 and CZ-RAJ. The tables within the figure represent the mean monthly air temperature (Tair) and soil volumetric water content (SVWC) values from May to September for each forest station. Figure 7: Monthly averages of daily sums of NEP for May–September of the normal years (NY) and dry year (DY) in CZ-BK1 and CZ-RAJ. to the years characterized by normal conditions at both forest sites, especially for CZ-RAJ (Figure 5).