## 3 Results and Discussion
### The Response of NEE and LE Fluxes to Extreme Precipitation
#### 3.2.2 Variation in NEE and LE Flux in Cold Season  
In contrast to the warm season, when precipitation has both direct and indirect effects on CO\({}_{2}\) and H\({}_{2}\)O fluxes through changes in soil moisture content that affect GPP and RE, plant transpiration, and soil evaporation, during the cold season in boreal and temperate forests under negative temperatures and permanent snow cover, the physical and biological mechanisms of both direct and indirect effects of precipitation on CO\({}_{2}\) and H\({}_{2}\)O fluxes are completely absent. During the cold season, the response of CO\({}_{2}\) and H\({}_{2}\)O fluxes to anomalous precipitation and API can only be expected at the beginning and the end of the cold season, before and after the establishment of permanent snow cover and while the ground is not frozen. Such effects can also be observed during prolonged thaw periods without snow cover. A stronger influence of precipitation on atmospheric fluxes is expected in forests in more southerly regions, where positive temperatures prevail during the winter months and there is no stable snow cover (beech forests in Denmark (DK-Sor) and Italy (IT-Co)) [90]. Prolonged thaw periods with a complete absence of snow cover are very common in mid-latitude regions with maritime climates. In the same areas and during the same time periods under positive air and soil temperatures, the effects of the lack of precipitation and the soil moisture deficiency (low API) on CO\({}_{2}\) and H\({}_{2}\)O fluxes can also be expected. In our study, we paid more attention to mid-latitude forest ecosystems under warmer climatic conditions (with high winter thaw frequency, including stations without permanent snow cover), where the direct and indirect influence of precipitation on CO\({}_{2}\) and H\({}_{2}\)O fluxes could be revealed. The Ru-SkP, Ru-Fyo, US-Uag, US-Pfa, Fi-Hyy, Fi-Sod, Ca-Oas, Ca-Gro, and Ca-Sbo stations were excluded from the analysis of the precipitation effect on CO\({}_{2}\) and H\({}_{2}\)O fluxes during the cold season. In the beech forest in Denmark (Dk-Sor) (forest site with no permanent snow cover in winter) [67], the percentage of strongly negative NEE anomalies (blue bars) exceeded strongly positive anomalies during extremely high precipitation days (Figure 12c). At other stations located in mixed, evergreen, and deciduous broadleaf forests, the difference between the probabilities of strong negative or positive anomalies was negligible. The NEE response to the cumulative strong precipitation forcing (API > STD) was not significantly different from the simultaneous response in evergreen needleleaf forests (Figure 13c): Increased CO\({}_{2}\) emissions (red bars) were observed at most stations. However, the difference between negative and positive anomalies was smaller compared to precipitation, and in the mountainous spruce forest of Italy (IT-Ren) [86], the CO\({}_{2}\) uptake (blue bar) even exceeded the CO\({}_{2}\) release (red bar). The high CO\({}_{2}\) uptake associated with the wet periods was also observed in old beech forests in Heinrich, Germany (DE-Hai) [91] and in Denmark (Dk-Sor), whereas in evergreen broadleaf forests the strong emission occurred more frequently (Figure 16a). The dry periods occurring during the cold season (low API) led to intense CO\({}_{2}\) uptake at most stations (blue bars in Figures 13d and 16b), similar to the warm season. The maximum percentage of days with strong negative NEE anomalies (blue bar) that were associated with low API (about 40%) was observed in the mixed forest of Switzerland (CH-Lee) [92]. The relationship between LE fluxes and precipitation during the cold season varied across ecosystems, as well as within a single biome. Heavy precipitation resulted in increased evaporation at several stations located in the evergreen needleleaf forests (Figure 11b).