## 4 Results  
### Spatial-Temporal Variations in Vegetation in the ARB  
To detect the spatial-temporal variations of vegetation in the ARB basin, the Mann-Kendall test and Sen's slope were employed to examine the trends and abrupt changes in the LAI, FVC and GPP from 1982 to 2013 at the confidence level of 95%. Figure 3a-c presents the Mann-Kendall Z value and Sen's slope of the area-averaged annual vegetation indexes across the study region. During the study period, the annual FVC in the study region varied from a minimum of 0.327 in 1983 to a maximum of 0.352 in 2013 with a significant increase rate of about 0.0004/year (\(Z=3.57\), \(p<0.05\)). While, the changes in area-averaged annual LAI in the same duration were not significant (\(Z=-1.67\), \(p>0.05\)), these values varied from a minimum of 1.369 in 2003 to a maximum of 1.563 in 1999, with a slight decreasing trend of \(-0.0013\)/year. Overall, an increasing trend at 0.19 gC m\({}^{-2}\) yr\({}^{-2}\) (\(Z=0.21\), \(p>0.05\)) was found in annual GPP, however, large interannual fluctuations with a maximum value of 900.37 gC m\({}^{-2}\) yr\({}^{-1}\) (1988) and a minimum value of 784.05 gC m\({}^{-2}\) yr\({}^{-1}\) (2003) were observed. The period averages of LAI, FVC, and GPP for the four periods in Figure 3a-c indicated that the period averages of FVC gradually increased during the four periods, while the period averages of LAI and GPP changed most dramatically between Period 2 and Period 3 (before and after 2000).  
The sequential Mann-Kendall tests for the annual LAI with a forward-trend _UFk_ and backward-trend _UBk_, as illustrated in Figure 3d-f, indicated an obvious abrupt change in LAI in 2001. In addition, a significant abrupt change in GPP occurred in the study area around 2001. Several abrupt changes in FVC occurred from 1999 to 2008, but most of them were insignificant. These results suggested that all three types of indexes describing different vegetation statuses had undergone a rapid change around the year 2001.  
Figure 4a-i shows the spatial patterns of pixel-by-pixel Sen's slope and the Mann-Kendall Z value estimated over the ARB, as well as the significance of the trends computed at a 95% significance level for the time series LAI, FVC and GPP images. Apparent spatial heterogeneity for the changes of LAI from 1982 to 2013 over the ARB, as exhibited in Figure 4a-c, was carefully investigated. The results indicated that about 37.3% of the basin area showed an increasing tendency in terms of LAI, in which approximately 7.7% tended to increase significantly (\(p<0.05\)), where the most obvious increasing trend occurred  
over the Songnen Plain (an important food and oil production national base of China) in the southern part of the ARB. On the contrary, a decreasing trend in LAI was significant over about 18.9% of the ARB area, where the most significant reductions in LAI were mainly distributed in Mongolia and the Inner Mongolia Autonomous Region of China in the southwestern ARB area. Consistent with the area-averaged LAI trend, the LAI changes at the regional scale were insignificant over a large portion of the study area.  
Over past 32 years in the ARB, 84.3% of the total basin area has exhibited an increasing trend in FVC, and pixels with a significant increasing trend accounted for about 62.5% of this value (\(p<0.05\)), mainly being concentrated in the Chinese and Russian parts of the ARB. Only 2.0% of the total area showed a significantly decreasing trend, and pixels with a decreasing trend were mainly distributed in the southwestern margin of the ARB. For GPP, approximately 45.1% of the area in the ARB presented a decreasing trend, in which about 10.5% of the area was decreasing significantly (\(p<0.05\)). Furthermore, areas with an increasing tendency accounted for 54.9% of the area, among which, 16.3% showed a significant increasing trend over the ARB from 1982 to 2013 (_slope_\(>0\), \(p<0.05\)).