## 4 Discussion
An important finding in our study was that the recent lengthening of the growing season, as driven by warmer temperatures, can strongly enhance plant growth and increase wetland productivity at rates of 4.2 and 3.7 g\(\cdot\) C\(\cdot\) m\({}^{-2}\)\(\cdot\) year\({}^{-1}\) per day according to the LOS delineated by LSWI and GPP, respectively. Our results, however, are lower than the ORCHIDEE model average estimates of 5.8 g\(\cdot\) C\(\cdot\) m\({}^{-2}\)\(\cdot\) year\({}^{-1}\) across the Northern Hemisphere (\(>\)25\({}^{\circ}\)N) during 1980-2002 [37]. There is little doubt that a large number of environmental factors affect plant productivity; the real question is how much each of the main factors contributes to the observed signals [37]. Several studies have suggested that global warming is a key driving force for the observed increase in vegetation productivity at middle- and high-latitude ecosystems [42; 43; 44; 45]. Indeed, we showed that the annual dynamics of vegetation indices and GPP were mainly driven by inter-annual variation in air temperature in the Zoige wetland. Generally, the warming signal can influence productivity through two pathways [46]. First, rising temperatures have a direct impact on plant photosynthesis and growth. Second, the lengthening of the growing season [47], changing plant phenology [48], stimulation of plant growth [46], increasing nutrient mineralization rates [49], reductions in soil water content [50], and alterations in plant species composition and community structure [44; 51] have indirect impacts on productivity. In order to further evaluate the relative importance of changes in air temperature _versus_ phenology in controlling GPP, we ran multiple regressions combining air temperature and the growing season length, delineated by EVI, LSWI, and GPP. Results from these regressions indicated that the effect of phenological shifts on GPP were slightly weaker than those induced by warming (Table 2). Further studies on the links between wetland carbon uptake and emissions and the many environmental drivers are needed in the future. The net ecosystem C exchange is the balance between gross primary productivity and ecosystem respiration. In addition to GPP, ecosystem respiration is also sensitive to warming and changes in phenology [37; 52]. Warmer air and soil temperatures will have direct effects on plant autotrophic and soil heterotrophic respiration. Previous studies have shown that warmer soil temperatures generally stimulate soil microbial activity, resulting in increased ecosystem respiration [53]. Generally, the balance of GPP (C influx) and ecosystem respiration (C efflux) determines whether terrestrial ecosystems will act as a net C sink or source under global climate changes [15; 38]. In this study, however, we only focused on the effects of warming and the growing season extension on wetland C influx. As such, future studies on the C balance of the Zoige wetland should integrate continuous EC measurements and data from remote sensing over longer time periods with ecological modeling to evaluate whether warming will increase C emissions and if increased emissions will exceed the increases in C uptake. Although we showed that remote sensing satellite observation has the capability to retrieve vegetation with relatively high precision and the VPM model can accurately capture the U pattern of the plant growth driven by variation in temperature and PAR in the alpine wetland, there still exist model uncertainty and some differences between simulated and observed GPP. The discrepancies may be attributed to three influencing factors on the modeling results. One is the sensitivity of the VPM model to PAR and temperature. The second factor is the remaining effect of angular geometry on surface reflectance and vegetation indices. The third factor is the error (overestimation or underestimation) of the observed GPP. More details of model uncertainty are described by [PERSON] _et al._[PERSON] _et al._ (2013).