3.1 Interannual variation of biophysical climate factors
We observed a non-significant increasing trend in temperature. The mean annual temperature over 13 years is 11.50 ± 0.69 ℃ (mean value ± standard deviation, coefficient of variation (CV)=12.86%), with 2012 being the coldest year (10.03 ℃) and 2021 being the warmest (12.48 ℃) (Figure 2b). Seasonal mean temperatures in spring, summer, and autumn showed a non-significant increasing trend over the 13 years, and the largest increase rate of 0.90% yr-1 occurred in spring (Figure 2a). Annual temperature also exhibited a non-significant increasing trend over the study period, with an increase rate of 0.78 % yr-1 (Figure 2b). Meteorological data from the Zhongwei meteorological station, approximately 20km, Shapotou during 1959–2017, show a significant increasing trend in air temperature with an annual increase rate of 0.38% yr-1 (Figure S2). Over our study period (2009–2021), the rate of increase in temperature exceeded the multi-year average rate.
The annual accumulative precipitation over the study period (2009–2021) was 180.12 ± 43.67 mm (CV = 24.25%) (Figure 2c). The maximum value of precipitation was 253.30 mm in 2018, which was nearly twice the minimum precipitation in 2013 (127.60 mm). The accumulative precipitation in spring, summer, and autumn is 19.7, 54, and 25.8% of the annual precipitation, respectively. Annual and seasonal accumulative precipitation showed no obvious trend during 2009–2021 (Figure 2c, d). Precipitation data from meteorological stations in Zhongwei from 1959–2017 showed an average annual precipitation of 180 ± 54.57 mm, which is almost consistent with our results. There was no clear trend in the annual precipitation (Figure S2).
The net carbon uptake period showed an increasing trend (Figure 3). SCUP in spring ranged from day of the year (DOY)75 to 97. ECUP in autumn ranged from DOY 289 to 321. DCUP ranged from 196 to 244 days. These three key indicators of the net carbon uptake period showed significant trends, with SCUP showing an early trend, ECUP demonstrating a delayed trend, and DCUP revealing a continuously increasing trend.