## 3 Results and Discussion
### The Response of NEE and LE Fluxes to Extreme Precipitation
#### 3.2.1 Variation in NEE and LE Flux in Warm Season
The number of cases where the anomalies of API and NEE/LE were observed on the same day is shown above the bars. The evergreen needleleaf forests had high CO\({}_{2}\) uptake during the warm season for 9-28% of the days with low API (blue bars in Figure 13b). Positive NEE anomalies (red bars) occurred more frequently than negative ones during the dry periods in only two evergreen needleleaf forests in North America: a black spruce forest in Alaska (US-Uaf) [79] and an over-mature Douglas fir/western hemlock forest (US-Wrc) [80]. In both ecosystem types, the high sensitivity of confers to a lack of precipitation can be explained by the low adaptive capacity of woody plants to soil drought, e.g., due to the shallow root system. Such an effect may also be due to the low water storage capacity of the soil horizons or a number of other reasons. In the black spruce forest, this phenomenon was due to permafrost, whereas in the Douglas fir/western hemlock forest, it can be attributed to hilly topography and high runoff. High CO\({}_{2}\) uptake during dry periods was observed in all evergreen, deciduous, and mixed forests studied (blue bars in Figures 14b and 15b), except for coniferous broadleaf mixed forests in the northernmost part of Japan. This was apparently due to the higher resistance of mixed forest stands to drought events compared to monospecific forests [81; 82]. In the larch (needleleaf deciduous) forest, the lack of precipitation resulted in higher CO\({}_{2}\) emissions (positive NEE anomaly) (red bars in Figure 15b), indicating stressed photosynthesis caused by a lack of precipitation and reduced soil wetness. In most of the analyzed temperate and boreal forests, heavy precipitation was associated with strong negative LE flux anomalies (yellow bars in Figures 11-16), indicating suppressed evaporation due to the high water vapor pressure in the air during and after precipitation, as well as reduced incoming solar radiation due to cloudy weather (Figures 11a and 12a,b). The observed positive LE anomalies were associated with increased evaporation after precipitation events due to both increased plant transpiration and interception evaporation (Figure S6.3) [83]. The effect of an extremely high API index on LE was almost opposite to the effect of extremely high precipitation on LE, indicating the strong difference between the evaporation response to the instantaneous and prolonged effect of soil wetting. On days with  
Figure 16: The percentage of days when NEE and LE anomalies greater than 1 STD occurred simultaneously with extremely high (**a**) and low (**b**) API during the cold season in mixed evergreen broadleaf, deciduous broadleaf, and needleleaf forests (see text for details). The number of cases where the anomalies of API and NEE/LE were observed on the same day is shown above the bars. heavy precipitation, the air is usually highly saturated with water vapor, and clouds reduce incoming solar radiation, which also significantly limits the rate of evapotranspiration. On the other hand, after periods of heavy precipitation, transpiration and evaporation rates can be relatively high due to sufficient soil moisture, especially under warm and sunny weather conditions [84]. In the evergreen needleleaf forests at 6 (Ca-Ca3, Ca-LP1, FI-Sod, IT-Ren, US-NR1, US-Uaf) of 11 flux stations, the increase in evapotranspiration was more frequent during wet periods--up to 29% of the days in which API > STD were characterized by positive anomalies of LE fluxes (green bars in Figure 13a), and the dry periods resulted in weaker evaporation (yellow bars in Figure 13b) in only 4 of 6 flux stations (Ca-LP1, FI-Sod, US-NR1, US-Uaf). Obviously, the soil moisture completely controls the intensity of LE fluxes in the atmosphere. However, in the Douglas fir forest in Canada (Ca-Ca3) [85] and in the mountainous spruce forest in Italy (IT-Ren) [86], positive LE anomalies (green bars) dominated on days with both extremely low and high API. The Italian spruce forest was unevenly aged, with a high proportion of old trees.