# Impact of Shifts in Vegetation Phenology on the Carbon Balance of a Semiarid Sagebrush Ecosystem
## 3 Results
### Comparison of Soil and Ecosystem Respiration  
As mentioned above, the small difference in the annual total ecosystem respiration despite distinct characteristics of meteorological conditions between both years inspired us to investigate the calculated R\({}_{\rm eco}\) using the measured R\({}_{\rm soil}\) as a reference. As shown in Figure 5d, the measured R\({}_{\rm soil}\) also exhibited unimodal-shaped diel variations similar to R\({}_{\rm eco}\) but with differences in the peak values. R\({}_{\rm soil}\) increased quickly in the morning, reached its  
Figure 7: MODIS EVI, daily sums of net ecosystem CO\({}_{2}\) exchange (NEE), gross primary production (GPP), and ecosystem respiration (R\({}_{\rm eco}\)) against camera green chromatic coordinate (G\({}_{\rm CC}\)) during the two-year period. The red solid and dashed lines indicate fitted relation and 95% confidence intervals, respectively.  
peak values at around noon, and then decreased gradually in the afternoon. The difference in the maximum ecosystem and soil respiration time indicates that there is still a need to improve the algorithms used for calculating ecosystem respiration, which is beyond the scope of the current study.  
We also compared the daily sums of \(\mathrm{R_{eco}}\) and \(\mathrm{R_{soil}}\) over two years (Figure 8). We divided the data into four groups according to the ranges of daily mean SWC. Each group consists of daily sums/averages with SWC located within a range of 0-3, 3-6, 6-10, and 10-15%, accounting for 26.6, 41.4, 14.5, and 17.5% of daily data, respectively. For different SWC groups, we fitted a linear regression between \(\mathrm{R_{eco}}\) and \(\mathrm{R_{soil}}\). Under very dry conditions (0-3%), \(\mathrm{R_{eco}}\) was generally larger than \(\mathrm{R_{soil}}\) by approximately 43%, in agreement with the theoretical hypothesis since \(\mathrm{R_{soil}}\) is a component of \(\mathrm{R_{eco}}\). For soil moisture conditions within the range of 3-6%, \(\mathrm{R_{eco}}\) was quite close to \(\mathrm{R_{soil}}\), whereas, under relatively better conditions (i.e., SWC within 6-10% and 10-15%), \(\mathrm{R_{eco}}\) was in general smaller than \(\mathrm{R_{soil}}\), suggesting that under these conditions, the calculated \(\mathrm{R_{eco}}\) is generally underestimated the true ecosystem respiration. Furthermore, the seasonal patterns of \(\mathrm{R_{eco}}\) were quite similar to those of \(\mathrm{R_{soil}}\) in both years, and the monthly totals of \(\mathrm{R_{soil}}\) were generally lower than those of \(\mathrm{R_{eco}}\), except for several months, e.g., May 2019 (Figure 6 and Table 1). In general, the annual total soil respiration was 154 and 127 \(\mathrm{gC}\) m\({}^{-2}\) in 2019 and 2020, respectively, and the difference in the annual total \(\mathrm{R_{soil}}\) between both years was comparable to the difference in the annual total \(\mathrm{R_{eco}}\).  
Figure 8: Daily sums of ecosystem respiration (\(\mathrm{R_{eco}}\)) versus daily sums of soil respiration (\(\mathrm{R_{soil}}\)) during the measurement period. Data are divided into four groups according to the daily average soil water content (SWC). The soil lines indicate the fitted relations for the divided groups. The bar plots show the frequency distribution of the divided groups.