## 3 Results and Discussion
### The Response of NEE and LE Fluxes to the Extreme Temperature Anomalies
#### 3.1.1 Variation in NEE and LE Flux in Warm Season  
In all considered forest types and forest ecosystems during the warm season, extremely hot conditions (temperature anomalies exceeding the 95% quantile) were mostly associated with increased CO\({}_{2}\) release into the atmosphere (positive NEE anomalies exceeding STD are presented by red bars in Figures 7-10; positive trends in NEE anomalies with increasing temperature are also shown in Figure S4.1). There was also a tendency for the NEE anomaly to decrease as the temperature anomaly increased for most stations (Figure S5.1). The highest percentage of hot days accompanied by a positive NEE anomaly exceeding STD (up to half of all hot days) was observed in the evergreen coniferous forests (Figure 7a). Extremely high temperatures stressed photosynthesis and reduced the GPP rate of forest ecosystems. As a result, more CO\({}_{2}\) was released into the atmosphere. It is noteworthy that the negative NEE anomalies (blue bars) also occurred during the hot periods (Figure 7a), but the percentage of days with them was much lower (6-15%). The increase in temperature anomaly was accompanied by a prevailing decrease in CO\({}_{2}\) uptake or an increase in CO\({}_{2}\) release (Figure S5.2). Exceptions are two stations located in Finland (FI-Hyy) and the United States (US-NR1), where the probability of positive and negative NEE anomalies was almost equal. The Niwot Ridge experimental site (US-NR1) is located in a subalpine forest ecosystem at an elevation of 3050 m [59]. The Hyytiala site (FI-Hyy) is located in a homogeneous 60-year-old pine (_Pinus sylvestris_ L.) forest stand [60]. Sufficient soil moisture conditions due to uniform precipitation distribution throughout the year, including the period of strong positive temperature anomalies, may be a key reason for the equal probability of positive and negative NEE anomalies observed during extremely high temperatures. The inverse response of NEE to high temperature was found at the evergreen coniferous forest station Billy Kriz in the Czech Republic (CZ-BK1). The dominant tree species in the forest is the 40-year-old Norway spruce (_Picea abies_ (L.) [PERSON]) [61]. The results show that in the forest, higher CO\({}_{2}\) uptake (blue bar) occurred more often during the hot periods (Figure 7a). The Billy Kriz forest site is located in a hilly region (elevation about 900 m above sea level) and is characterized by a relatively high amount of annual precipitation (>1300 mm), which provides sufficient soil moisture conditions and may ensure low sensitivity of young forest stand to positive temperature anomalies [62]. In mixed forests and evergreen broadleaf forests (Figure 8a), as well as in deciduous broadleaf and needleleaf forests (Figure 9a), positive NEE flux anomalies above STD, corresponding to high CO\({}_{2}\) release, also dominated under extremely high temperatures, but the percentage of days was lower. The lower effect of high temperatures on plant photosynthesis and CO\({}_{2}\) uptake in these forests compared to the evergreen needleleaf forests may be due to the higher sensitivity and vulnerability of dark coniferous forest stands to higher temperatures and positive temperature anomalies [63; 64; 65; 66; 9]. Thus, it can be expected that the mixed and broadleaf tree species had higher drought and heat tolerance and were less sensitive to heat anomalies. This may be the reason why the NEE anomalies at the DK-Sor [67] and It-Col (deciduous broadleaf forest) [68] sites were evenly distributed and a positive temperature anomaly led to both increased CO\({}_{2}\) uptake (blue bars) and increased CO\({}_{2}\) release (red bars) (Figures 8a and 9a). Figure 7: The percentage of days when NEE and LE anomalies greater than 1 STD occurred simultaneously with extremely high (**a**,**c**) and low (**b**,**d**) temperatures during the warm (**a**,**b**) and cold (**c**,**d**) seasons in evergreen needleleaf forests (see text for details).