## 1 Introduction  
Boreal forests constitute about one-third of the mean global terrestrial carbon sink and are the major contributors to the global terrestrial carbon sink trend [1]. Climate warming, as well as anthropogenic and natural disturbances, alter two main components of the ecosystem carbon balance--gross primary production (GPP) and total ecosystem respiration (TER)--as well as CO2 net ecosystem exchange (NEE) with the atmosphere, leading to changes in the terrestrial carbon stock and atmospheric CO2 concentration [2, 3, 4, 5, 6]. GPP is an ecosystem-level photosynthetic gain of CO2 and TER is an ecosystem (plant, animal, and microbial) respiratory loss of CO2-C [7, 8]. Recent observational and modeling studies have demonstrated a high sensitivity of GPP and TER in boreal forest ecosystems to changes in environmental variables such as air and soil temperature, precipitation, soil moisture, and vapor pressure (VPD) deficits, snow cover dynamics, and growing season length (GSL) [9, 10, 11, 12, 13]. Previous studies have reported that the climate warming occurring in high latitudes extends the GSL, leading to the increased GPP and net ecosystem productivity (NEP = GPP-TER) of boreal ecosystems [14, 15, 16]. The response of the CO2 exchange in boreal forests to positive temperature anomalies has seasonal variations. Increased air temperatures in the winter to spring transition led to the shifting of the start of the growing season to earlier dates in many coniferous forest ecosystems, while positive temperature anomalies in the summer to autumn transition provide high decomposition rates and can increase an ecosystem's carbon loss [17; 18; 19]. GPP and TER in boreal forests are generally temperature- and radiation-limited; thus, air temperature anomalies are important factors of the interannual CO\({}_{2}\) exchange variability in boreal forests [20; 21]. The weakening of the temperature dependence of the CO\({}_{2}\) uptake among northern regions during the last few decades has been reported [22]. Additionally, an increasing frequency of heatwaves and droughts creates a water limitation in the growing season's CO\({}_{2}\) uptake in many boreal forest ecosystems [23; 24; 25; 26], thus increasing the role of the local moistening conditions as a factor controlling the ecosystem's CO\({}_{2}\) flux variability. Climate extremes and anomalies influence ecophysiological processes in forest ecosystems in a non-linear manner [27]. Thus, both positive and negative anomalies in air temperature and precipitation can potentially change environmental conditions from the optimal range for ecosystem productivity and the decomposition of organic matter. Moreover, the effect of extreme events on CO\({}_{2}\) uptake and release processes is dependent on the timing and duration of the event [27]. Increased air temperatures in late winter and spring provide an early start of the growing season in many boreal forest ecosystems and consequently provide GPP growth. However, the increased water consumption in spring associated with primary production can lead to water stress in summer [28]. This effect is especially strong when a warm spring is followed by a dry summer [29]. Heatwaves and droughts occurring in the growing season are often associated with an increased number of clear-sky days, positive air temperature anomalies, as well as negative precipitation anomalies. While increased global radiation (Rg) in boreal forests is favorable for photosynthesis, decreased precipitation can lead to an insufficient soil moisture content (SWC) as well as increased vapor pressure deficit (VPD), consequently triggering the stomatal closure and inhibiting GPP [30; 31]. Extremely high temperatures can also damage leaves and affect biochemical processes that are sensitive to temperature changes [32; 33]. TER increases along with temperature, but an extremely low or high SWC may limit TER through both autotrophic and heterotrophic components [34].