# Impact of Shifts in Vegetation Phenology on the Carbon Balance of a Semiarid Sagebrush Ecosystem
## 1 Introduction  
Dryland ecosystems constitute approximately 41% of the Earth's land surface; thus, they have remarkable impacts on global carbon and water cycling [1, 2]. Recent studies have illustrated that dryland ecosystem productivity plays an important role in regulating the trends and interannual variabilities of global terrestrial carbon sequestration [2, 3, 4]. These ecosystems, mainly including grasslands, shrublands, and savannas, are quite fragile and rather sensitive to changes in weather and climate conditions [5, 6]. Enhanced warming and severe droughs have the potential to significantly alter the capacity of dryland ecosystems to sequester carbon [6, 7]. Understanding how these ecosystems respond to weather and environmental variability is critical for improving our projections of global carbon sink-source dynamics. Site studies and models have shown that water availability, indicated by soil moisture and precipitation, as well as evapotranspiration (ET), plays an important role in maintaining the productivity of dryland ecosystems (regarding changes that can significantly influencethe structure and functioning of dryland ecosystems) [8; 9; 10]. In dryland regions, the annual net ecosystem exchange of CO2 (NEE) often switches between negative (carbon sink) and positive (carbon source) from year to year, mainly due to annual total precipitation crossing a threshold, which varies between different dryland ecosystems [9]. Vegetation structures have also been shown to influence soil water distribution [11; 12]. Shrub species with rooting depths of several meters can vertically transport water from wetter to drier soil layers and maintain function during dry months [12]. Hence, woody species, such as shrubs in dryland ecosystems, usually respond slower to changes in environmental drivers than non-woody species, such as grasses, which rely heavily on shallow water availability [9]. The seasonal snowpack stores winter precipitation, which would replenish soil water storage by snow melting in the spring [13]. The snowpack also reduces soil water loss due to decreased evapotranspiration, resulting from an increase in the reflection of solar radiation by snow cover [14]. In the inland Pacific Northwest (iPNW), seasonal precipitation is the primary source of soil water availability for natural ecosystems with limited groundwater access [15], approximately 37% of which fall as snow [13]. However, the snowpack has shown strong interannual variability [16]. Given the widespread declining trends in both snow depth and snow cover duration in many regions of the world [17; 18], examining the potential impacts of snowpack variability on dryland ecosystems is urgently needed for accurate projections of global carbon balance under future climate change. In addition to the variabilities in NEE and gross primary production (GPP), variations in ecosystem respiration (Reco) have also been associated with changes in soil water availability [19]. The temperature sensitivity of respiration in drylands is strongly controlled by soil moisture [19]. Dryland ecosystems commonly experience large pulses in Reco in response to heavy precipitation events, which account for a substantial portion of the ecosystem's carbon balance [20; 21]. In wet years, increases in GPP are often offset by a large Reco [9]. However, it is not yet clear how Reco responds to changes in soil water availability due to snow melt in semiarid ecosystems. Phenology controls the seasonality of vegetation functioning and ecosystem feedback to changes in environmental drivers [22; 23]. The increasing number of eddy covariance (EC) sites co-located with digital cameras (i.e., PhenoCam) for phenology monitoring have contributed to improving our understanding of the relationship between ecosystem productivity and phenology [24; 25; 26; 27; 28; 29]. The green chromatic coordinate (GCC) extracted from PhenoCam images is a plausible vegetation index used to track plant development throughout the season [25; 26].