## 3 Results
### Correlations between GPP and Driving Factors
#### 3.4.2 Effects of Phenological Parameters and FPAR on GPP  
Based on the phenological parameters acquired as shown in Figure 7, we explored the relationship between the phenological parameters and GPP. In the GKM, an advance in SOG and delay in EOG leaded to an obvious extension of GSL over the last 34 years. This result was consistent with previous findings, demonstrating that climate warming causes an extension of GSL [46; 61]. According to many studies [59; 62; 63] that were conducted in other regions, the extension of GSL is an important factor for the increasing GPP. We also conducted correlation analysis between GPP and GSL per pixel along the temporal dimension and obtained the spatial distribution of the correlation coefficient; the result is displayed in Figure 8.  
Results (Figure 8) showed that in the central and southern GKM, there was a significant positive correlation between GSL and AGG, as was the case in abovementioned areas. The extension of GSL could explain the increase of AGG in some areas belonging to the GKM over the past 34 years, and this was consistent with the results of a previous study [59]. The majority of regions in the north and along the GKMR did not show a significant correlation between AGG and GSL, therefore some other elements should be considered as driving factors in those areas. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Station** & **SD** & **DAT** & **DMAT** & **DMIT** & **DP** \\ \hline \multirow{4}{*}{**Spring**} & 50,136 & 0.419 ** & 0.335 * & 0.398 ** & 0.257 & \(-\)0.153 \\  & 50,246 & 0.279 & 0.435 ** & 0.495 ** & 0.279 & \(-\)0.179 \\  & 50,349 & 0.397 * & 0.448 ** & 0.482 ** & 0.292 * & \(-\)0.310 * \\  & 50,353 & 0.390 * & 0.381 * & 0.493 ** & 0.200 & \(-\)0.150 \\  & 50,425 & 0.295 * & 0.445 ** & 0.419 ** & 0.407 ** & \(-\)0.070 \\ \hline \multirow{4}{*}{**Summer**} & 50,136 & 0.631 ** & 0.339 * & 0.459 ** & \(-\)0.144 & \(-\)0.324 * \\  & 50,246 & 0.614 ** & 0.669 ** & 0.811 ** & \(-\)0.155 & \(-\)0.580 ** \\  & 50,349 & 0.814 ** & 0.717 ** & 0.810 ** & \(-\)0.017 & \(-\)0.482 ** \\  & 50,353 & 0.835 ** & 0.665 ** & 0.753 ** & 0.247 & \(-\)0.640 ** \\  & 50,425 & 0.565 ** & 0.590 ** & 0.642 ** & 0.304 * & \(-\)0.470 ** \\ \hline \multirow{4}{*}{**Autumn**} & 50,136 & 0.474 ** & \(-\)0.041 & 0.349 * & \(-\)0.255 & \(-\)0.608 ** \\  & 50,246 & 0.491 ** & \(-\)0.026 & 0.410 ** & \(-\)0.328 * & \(-\)0.484 ** \\  & 50,349 & 0.677 ** & \(-\)0.041 & 0.314 * & \(-\)0.335 * & \(-\)0.441 ** \\  & 50,353 & 0.134 & 0.176 & 0.317 * & \(-\)0.051 & \(-\)0.429 ** \\  & 50,425 & 0.466 ** & 0.304 * & 0.333 * & 0.206 & \(-\)0.447 ** \\ \hline \multirow{4}{*}{**Growing Season**} & 50,136 & 0.620 ** & 0.385 * & 0.50,2 ** & \(-\)0.055 & \(-\)0.371 * \\  & 50,246 & 0.587 ** & 0.602 ** & 0.789 ** & \(-\)0.137 & \(-\)0.544 ** \\  & 50,349 & 0.753 ** & 0.626 ** & 0.773 ** & 0.005 & \(-\)0.513 ** \\  & 50,353 & 0.659 ** & 0.604 ** & 0.704 ** & 0.294 * & \(-\)0.623 ** \\  & 50,425 & 0.577 ** & 0.572 ** & 0.603 ** & 0.367 * & \(-\)0.470 ** \\ \hline \hline \end{tabular} SD refers to sunshine duration, DAT refers to daily average temperature, DMAT represents daily maximum temperature, DMIT represents daily minimum temperature, and DP represents daily precipitation; * \(p<0.05\), statistical significance of Pearson correlation.  
\end{table} 
Table 1: Correlation coefficients of the AG in three seasons and the growing season versus meteorological factors obtained from five different meteorological stations over the GKM from 1982 to 2015. Figure 8: Correlation coefficient between GSL and AGG in the GKM during 1982–2014. (Green indicates significant positive correlation (r \(>\) 0, \(p\) \(<\) 0.05), brown indicates significant negative correlation (r \(<\) 0, \(p\) \(<\) 0.05), and gray indicates non-significant correlation (\(p\) \(>\) 0.05)). Figure 7: Variation in phenological parameters in the GKM from 1982 to 2014 (SOG means starting day of growing season, EOG means ending day of growing season, and GSL means growing season length).