## 1 Introduction  
Terrestrial ecosystems are major carbon sinks in the global carbon cycle. Arid and semiarid ecosystems cover approximately 40% of the Earth's land surface [1, 2], and play a key role in terrestrial carbon dynamics [3, 4]. Forests constitute approximately one-third of the Earth's land surface and store about 90% of terrestrial vegetation carbon [5]. The average annual carbon sequestration of forest ecosystems was about 3.5 \(\pm\) 1.0 Pg C from 2008 to 2017 [6], suggesting that they are the main carbon pool in terrestrial ecosystems [7]. Moreover, forest ecosystems play an important role in global carbon and water cycles [8, 9]. At present, considerable efforts have been spent on global carbon and water fluxes of tropical, temperate, and rigid forests, and fruitful research results have also been achieved [10, 11]. Previous studies showed that the growth season of coniferous forests in the middle-high latitudes of the northern hemisphere was prolonged, and their carbon sequestration was significantly enhanced with the climate warming [10; 12; 13]. However, some experts predicted that with global warming, ecosystem respiration (RECO) would be stimulated, especially in coniferous forests of the northern hemisphere at high latitudes (mainly in permafrost), which would release more CO\({}_{2}\) and may switch from a carbon sink to a carbon source at the end of the 21 st century [14; 15]. However, there are few studies on the carbon and water fluxes of coniferous forest ecosystems in drylands. Therefore, clarifying the dynamic changes of carbon and water fluxes of coniferous forest ecosystems in drylands will help us better understand and predict the change trends of its carbon budget and water balance in the context of climate change. The responses of carbon exchange and evapotranspiration (ET) to environmental factors in different forest ecosystems are different due to their physiological differences, which may lead to differences in the overall forest carbon budget and water cycle [16]. Understanding the patterns and drivers of CO\({}_{2}\) and H\({}_{2}\)O fluxes in different forest ecosystems is of great significance in the context of global climate change. The ET is an important component of the hydrological cycle and energy balance [17; 18; 19]. Similar to ET, the exchange of CO\({}_{2}\) between vegetation and the atmosphere is closely related to the water cycle, which is affected by aboveground and underground biophysical processes [20; 21]. Net ecosystem exchange (NEE) is the difference between CO\({}_{2}\) absorbed by the ecosystem through plant photosynthesis and the CO\({}_{2}\) released by the respiration of plants and soil microbes [20]. Gross primary productivity (GPP) refers to the total amount of carbon fixed by plants through photosynthesis [22; 23]. Ecosystem respiration (RECO) can consume >50% of GPP and plays a key role in the global terrestrial net carbon balance [24; 25]. The ET, NEE, GPP, and RECO variability are mainly affected by environmental factors (e.g., temperature, moisture, radiation, vapor pressure deficit (VPD), etc.) and biological factors (e.g., leaf photosynthesis capacity, plant growth rhythm, microbial activity, etc.) [24; 26; 27; 28]. Therefore, understanding the responses of ET, NEE, GPP, and RECO to environments in coniferous forest ecosystems of drylands is important for assessing the water cycle dynamics and carbon sink potential of forest ecosystems under climate change [29; 30]. The carbon and water cycle processes and their couplings are core to determining the carbon budget and water balance of forest ecosystems in semimid regions [21]. Water use efficiency (WUE) is the link between the carbon and water cycle in the ecosystem, and it also reveals the interaction between carbon sequestration and water consumption in terrestrial ecosystems [31; 32; 33] because WUE is defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET) [32; 34; 35].