## 3 Results  
Seasonal and Interannual Variations in Climate, GPP, and Vegetation Indices at the Four Flux Tower Sites  
The climatic conditions differed dramatically between the temperate steppe sites (NM and XL) and the alpine meadow sites (GCT and SD) (Figure 4). The temperate steppe sites were characterized by a dry-warm climate, with a growing season precipitation and mean temperature of 219 mm and 14 \({}^{\circ}\)C, respectively, whereas the alpine meadow sites were dominated by a humid-cold climate, with precipitation and temperature of 465 mm and 6 \({}^{\circ}\)C, respectively. The drier climate in the temperate steppe compared with that in the alpine meadow can also be viewed in terms of soil water content (SWC) (0.12 vs. 0.26 m\({}^{3}\) m\({}^{-3}\), respectively) and vapor pressure deficit (VPD) (9.75 vs. 3.13 hPa, respectively). In addition, precipitation and SWC varied greatly by year in the temperate steppe sites. For example, the growing season precipitation in 2005 was less than half of that in 2004 (153.3 vs. 354.5 mm yr\({}^{-1}\), respectively) at the NM site. There were clearly more heavy rainfall events in the peak growing season (180-240 days of year) in 2007, 2008, 2010, and 2011 than in 2006 and 2009 for the XL site. The observed GPP, fPAR, EVI, NDVI, and LSWI were higher in the alpine meadow than in the temperate steppe during the growing season (Figure 5). For example, the annual GPP and mean EVI of the alpine meadows were 37.4% and 76.1% higher, respectively, than those of the temperate steppe (453.5 vs. 330 g C m\({}^{-2}\) yr\({}^{-1}\), respectively, for the annual GPP, and 0.34 vs. 0.19, respectively, for the mean EVI). Generally, the seasonal patterns of GPP were similar to those of the EVI, NDVI, LSWI, and MODIS-fPAR in all four sites. However, none of the indices captured the sharp reduction in GPP (Figure 5a,b) caused by the flash drought (almost no precipitation from late June to early July) and warm temperatures during the peak growing season in 2004 at the NM site and in 2007, 2008, and 2010 at the XL site. Inconstant patterns were also observed in the XL site during the drought years (Figure 5b). For example, the observed GPP peaked more than one month earlier than that shown by all the vegetation indices in 2006. Surprisingly, the alpine meadow sites showed clear dips in MODIS-fPAR during some years, possibly due to the poor quality of data from the growing season. Figure 4: Seasonal and interannual variations in precipitation (Pre), air temperature (Ta), vapor pressure deficit (VPD), photosynthetically active radiation (PAR), and soil water content (SWC) at the (**a**) NM, (**b**) XL, (**c**) GCT, and (**d**) SD flux tower sites. SWC was measured at depths of 0–20 cm at the NM and GCT sites and 0–10 cm at the XL site. SWC was not measured at the SD site due to the waterlogged soil surface. Figure 5: Seasonal and interannual variations in the observed gross primary production (GPP_tower), fraction of the absorbed photosynthetically active radiation (fPAR), normalized difference vegetation index (NDVI),  
enhanced vegetation index (EVI), and land-surface water index (LSWI) at the (**a**) NM, (**b**) XL, (**c**) GCT, and (**d**) SD flux tower sites. Interannually, GPP in the temperate steppe sites showed much larger variation throughout the year (Figure 5) and a closer relationship (\(p<0.01\)) with the vegetation indices (i.e., EVI, NDVI, and LSWI) (Figure 6) than that in the alpine meadow. However, GPP decreased more rapidly than the other variables during the severe drought years (i.e., 2005, 2006, and 2009) in the temperate steppe. For example, GPP in 2005 was 73.1% lower than that in 2004 when the EVI declined by only 21.6%. Interestingly, no significant relationships were observed between GPP and climate variables (e.g., Ta, Prec, VPD) in either of the grassland subtypes, possibly due to the limited number of observation years.