# Impact of Shifts in Vegetation Phenology on the Carbon Balance of a Semiarid Sagebrush Ecosystem
## 4 Discussion  
Here, we characterize and discuss the phenology of a semiarid sagebrush-grass ecosystem using MODIS EVI, a high-temporal resolution of GCC derived from PhenoCam, and measured carbon fluxes as well as the impacts of phenology shifts on the carbon balance. We observed contrasting seasonal variations in both PhenoCam-derived GCC and carbon fluxes in 2019 and 2020 (Figure 6 and Table 1). In general, seasonal variations in PhenoCam-derived GCC were in phase with those of carbon fluxes (Figure 7), consistent with the previous studies in other ecosystems (e.g., forests and grasses) [24; 29]. We argue that the contrasting seasonal variations in GCC and carbon fluxes in 2019 and 2020 are primarily driven by the woody species at our study site [8]. The US-Hn1 site is characterized by relatively sparse shrubs (10%) and a large fraction of grasses [15]. Shrubs can utilize their vast rooting system to access groundwater deep within the soil [12; 47], while grasses with dense shallow roots tend to use water in the topsoil layer [9]. In 2019, due to the presence of snowpack, the time of the GCC and carbon flux peaks were postponed, and the growing season for the shrubs was extended with available water from snowmelt, while in 2020, with warm and dry winter and early spring, the vegetation activity and carbon uptake peaked a couple of weeks earlier than in 2019 [48], and then decreased dramatically due to limited available water for both shrubs and grasses (Figure 4).  
Environmental drivers play an important role in regulating seasonal variations of GCC and carbon fluxes (Figures 4 and 6), though the role and importance of temperature and water availability vary across different phenological stages, especially for dryland ecosystems [8; 9; 29]. In winter and early spring, temperature is an important factor limiting ecosystem activities (e.g., photosynthesis and respiration) [48] (Figure 9). With a warm winter in 2020, the magnitudes of GPP and NEE in February were relatively larger than 2019 (Figure 6). In the late spring, summer, and autumn months, plant-available water is a limiting factor [8; 9]. Due to the increase in solar radiation and air temperature as well as the associated increase in VPD, the increase in ET caused a rapid decrease in soil water availability in the topsoil layer (i.e., SWC) (Figure 4), and further resulted in the senescence of grasses and thus a reduction in carbon uptake [9], especially in 2020 (Figures 6). In 2019, we believe that plant-available water in a deep soil layer was replenished by snowmelt and, thus, shrubs maintained functioning during the dry months [12] (Figure 2). Note that the US-Hn1 site is an upland site with a deep vadose zone where the lateral groundwater-river water exchange is not a resource of plant-available water [49].  
In summary, our results suggest that the presence of winter snowpack can be a critical driver for the annual carbon uptake of semiarid sagebrush ecosystems in snowy regions. A cold and wet winter that delays snow melt can result in shifts in the phenology of such ecosystems and further enhance the annual carbon uptake. However, global warming has been more prominent in winter than in other seasons, as indicated by the observed and modeled declining trends in both snow depth and snow cover duration in many regions worldwide [17; 18; 50]. How future climate change will affect the carbon balance in snowy semiarid regions still needs to be studied further due to the complex interactions among temperature, rainfall, snowpack, and evaporation [50].