## 5 Results
### Spatio-Temporal Evolution of NPP in Northeast China
#### 5.1.2. Inter-Annual Variation of NPP of Different Forests Types  
Next, this study estimated the NPP of different kinds of forests to reveal the variation characteristics of these forests. The average value of five forests NPP in Northeast China from 2001 to 2019 were ranked from high to low as follows: temperate coniferous forest \(>\) mixed forest of temperate coniferous forest and deciduous broad-leaved mixed forest \(>\) temperate deciduous broad-leaved forest \(>\) cold temperate and temperate mountain coniferous forest \(>\) temperate deciduous shrub. Compared with other four forests, the NPP of cold temperate and temperate mountain coniferous forest showed a higher increasing trend, with a rate of 8.89 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\). The reason is likely that global warming has become an unstoppable global climate problem in the last 20 years, leading to a significant increase in NPP [59]. Similarly, the NPP of other forest types all showed an increasing trend of different amplitude. The remaining forest types with different increasing rate from high to low is Mixed forest of temperate coniferous and deciduous broadleaf, temperate coniferous forests, temperate deciduous broadleaved forest and temperate deciduous shrub, whose rate is 8.08, 7.41, 6.34 and 6.05 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\), respectively.  
From the NPP changes of different forests in different time periods (Figure 7), it was evident that the NPP of various forests showed a decreasing trend from 2001 to 2005, and then went up gradually and reached the highest point in 2007 and 2008. However, the NPP declined significantly again from 2009, mainly because of the severe summer drought. In recent years, that is from 2017 to 2019, the NPP of these forests increased significantly. Specifically, the temperate deciduous broad-leaved forest, which accounted for 49.14% of the study area, its average NPP reached 1067.91 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\). During the period of 2001 to 2019, there are two peaks for this forest type, which peaked at 942.75 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) in 2008 and 2067.91 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) in 2018, respectively. As for the cold and temperate mountain coniferous forest, mixed forest of temperate coniferous and deciduous broadleaf, their change trends of NPP were overall consistent with the one of temperate deciduous broad-leaved forest. Among them, the NPPs of the two types of forests decreased significantly during the period of 2004-2005 and 2008-2009. In the first stage, they varied from 803.66 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) to 753.08 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) and from 902.31 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) to 769.40 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\), respectively. Next, the NPP in the second phrase declined more seriously than previously, with a rate of 18.32% and 16.06%, respectively. The highest NPP values reached 951.19 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) and 1013.29 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) in 2008, respectively. However, the NPP inter-annual changes in temperate deciduous shrub were relatively stable compared with the other types of forests.  
Figure 6: Inter-annual variation of NPP, from 2001 to 2019, in Northeast China.