# Impact of Shifts in Vegetation Phenology on the Carbon Balance of a Semiarid Sagebrush Ecosystem
## 3 Results  
### Weather and Environmental Conditions  
We used long-term climatology (1989-2018) and vegetation phenology (2000-2018) from the PRISM climate group and MODIS data product, respectively, as a reference to characterize the precipitation, air temperature, and vegetation activity in 2019 and 2020 (Figure 3). Annual precipitation in 2019 (215 mm) was 19 mm higher than the climatological mean annual total precipitation (196 \(\pm\) 48 mm), while 2020 (150 mm) was 46 mm lower than the average. The annual mean air temperature in 2019 (11.1 \({}^{\circ}\)C) was 0.7 \({}^{\circ}\)C lower than the long-term average (11.8 \({}^{\circ}\)C), while 2020 (12.8 \({}^{\circ}\)C) was 1.0 \({}^{\circ}\)C warmer than the average. Compared to the 30-year average climatological conditions, 2019 was characterized by a cold and wet winter (Figure 3a,b). February and March of 2019 were 7.0 \({}^{\circ}\)C and 5.2 \({}^{\circ}\)C colder than the long-term averages of 3.5 \(\pm\) 2.3 \({}^{\circ}\)C and 7.8 \(\pm\) 1.3 \({}^{\circ}\)C, respectively. January-April 2019 were wetter than their long-term averages, with unusually high precipitation in February (64 mm vs. the climatological mean of 18 \(\pm\) 11 mm). Based on the records from the HMS Monitoring Network and PhenoCam images, the area around the tower was covered by snow from 6 February to 15 March 2019 with an accumulated snow depth of approximately 0.8 m. The year 2020 was characterized by a warm and dry winter. The monthly mean air temperatures in January and February were 2.9 \({}^{\circ}\)C and 1.9 \({}^{\circ}\)C warmer than their climatological averages, respectively. The monthly precipitations in February through April were lower than their climatological averages with only a few centimeters of accumulated snow. The long-term (2000-2018) phenology (Figure 3c) illustrated a well-defined annual cycle of greening and senescence of the sagebrush ecosystem around the tower location. Generally, the growing season was from February to June, with peak vegetation activity in April. Compared to long-term averages, the years 2019 and 2020 represented contrasting characteristics of vegetation phenology. In 2019, the vegetation greenup date was delayed, with vegetation activity peaking in May, and the monthly mean EVI in 2019 remained well above its averages from May through October. However, in 2020, the vegetation greenup date was slightly earlier than its average, with the peak vegetation activity occurring in March and the growing season was shorter compared to 2019. Meteorological and PhenoCam measurements collected from the flux tower confirmed that 2019 and 2020 experienced contrasting meteorological and phenological conditions (Figure 4), especially in winter and early spring. In February and March of 2019, the snowpack provided a thermally stable and humid layer for the sagebrush ecosystem (i.e., approximately 0 \({}^{\circ}\)C of T\({}_{\text{soil}}\) and 12% of SWC at 5 cm, respectively), even though T\({}_{\text{air}}\) was well below 0 \({}^{\circ}\)C. The G\({}_{\text{CC}}\) value increased dramatically after snow melting and reached its maximum value at the end of April when the SWC decreased to approximately 8%. The grasses then faded and died in May, while the shrubs remained active until September. In 2020, with higher VPD and no snow cover, soil water loss continued from January through April. The G\({}_{\text{CC}}\) increased slightly in February and March with no clear peaks. In addition, there were a couple of rainfall events in the summer of 2020, resulting in a relatively wetter summer compared to the long-term average. Figure 4: Daily means of **(a)** global solar radiation (R\({}_{\rm g}\)), air temperature (T\({}_{\rm air}\)), **(b)** vapor pressure deficit (VPD), soil temperature (T\({}_{\rm soil}\)) at 5 cm, **(c)** cumulative daily precipitation (P), soil water content (SWC) at 5 cm, **(d)** camera green chromatic coordinate (G\({}_{\rm CC}\)), and MOSID EVI from 1 January 2019 to 31 December 2020 at the study site.