## 4 Discussion
Therefore, our results obtained for the five pine stands, 331 \(\pm\) 19 mm in the moist year 2017, 242 \(\pm\) 20 mm in the drought year 2018, and 294 \(\pm\) 6 mm on average in the period 1991-2019, are in the range of [72; 73; 74] when recognizing ground vegetation. In the beech site, stand examined the transpiration of ground vegetation is neglectable so that a direct comparison with results obtained from sap flow measurements is possible. There, we obtained annual transpiration of 356 mm in the moist year 2017 and 214 mm in the very dry year 2018. The average for the period 1991-2019 amounted to 287 \(\pm\) 7 mm. Again, based on sap flow measurements, beech canopy transpiration within the growing season ranged between 250 mm and 350 mm [76; 77; 78; 79; 80]. Our model approach revealed that the rise of temperature during the last decades is accompanied by an increase in transpiration. In fact, seasonal variations of the terrestrial evapotranspiration are positively correlated with temperature in Europe [81] and showed an increasing trend at a global scale within the last decades [82]. However, actual canopy transpiration is obviously limited by soil moisture and the hydraulic conductance from bulk soil to leaf [83]. In our simulations, we could show that the temperature increase comes along with drier conditions as shown by the increasing number of dry days per year indicated the transpiration index (Figure 4). In particular, the last two drought years 2018 and 2019 pushed forest damages in the study region. With the year 2020, we now observed the third consecutive year of drought. As obtained by the large-scale forest condition monitoring of the ICP Forests network (Level I), defoliation and tree mortality has increased in almost all tree species in recent years [84; 85]. In the study region, pines showed the highest defoliation since the beginning of the 1990s, accompanied by biotic damages, leading to the death of individual trees. Beech trees showed the highest records of defoliation in the year 2019, and especially old trees obtained dieback of the crown starting from the top. Our study revealed that 2018 was the most severe drought year within the whole period 1961-2019. In 2019 seepage was distinctly lower than in 2018, while soil water content kept on a low level comparable with 2018. As shown by the climate water balance (Figure 3), the year 2018 ended up with the highest water deficit observed on all sites (with exception of site 1203) within the whole period. The lack of winter precipitation and consequential refilling of the soil water body most likely reinforced the impact of drought conditions in 2019, especially at the beginning of the growing season. Furthermore, the depletion of the internal water storage during subsequent drought years obviously affects the whole-tree water relationship [86]. For example, [PERSON] et al. [87] observed an increase in the contribution of whole-plant exchangeable water to daily transpiration of a young beech stand during the drought of 2003 (with a moist year in front) to 67% in maximum. Hysteresis effects are not considered in the model approach that applied to why actual transpiration might be even overestimated in periods of prolonged drought. The strong decrease of the modeled seepage water in 2019, furthermore, indicates the impact of the subsequent drought years and the decline in plant-available soil water. This effect, however, might be more important at sites of high water-holding capacity compared to sandy soils. [PERSON] et al. [88] showed that in pine stands canopy, transpiration was sensitive to decreasing soil moisture at an earlier point of relative extractable water in loamy than sandy soils. The increase of dry days (TI; Figure 3) of pine stands in the last two years accompanies the strong increase in defoliation of pine trees in the study region in 2019. Thus, we argue that pine stands are vulnerable to subsequent drought years due to the depletion of both soil and whole-tree water reservoirs, which might increase in the future (Figure 4).