## 6 Discussion  
### Optimization about Station-Temporal Regularity Exploration  
In recent 19 years, although natural forests protection and other ecological restoration projects had been largely implemented in Northeast China, the ecological environment did not all improved persistently during the whole study period. Hence, the construction and ecological restoration of forests in Northeast China still require continuous attention of the government. This study displayed that the NPP in Northeast China from 2001 to 2019 generally exhibited a slow rising trend with certain fluctuations, especially in the period of 2005-2008 and 2017-2019. Similarly, a latest study in Northeast China based on climate simulation scenarios of RCP 4.5 and RCP 8.5, shows an increase from 2011 to 2100, with corresponding reference years (1981-2010) under the future climate change due to the drastic climate change [(60)]. As for the inter-annual NPP changes of different forests types, on the whole, the NPP of all forest types all showed an increasing trend, especially cold temperate and temperate mountain coniferous forest and mixed forest of temperate coniferous and deciduous broadleaf. [PERSON] et al. [(61)] also performed the NPP in China has a significant increase influenced by the temperature, precipitation and CO\({}_{2}\) concentration. Moreover, there is a larger increase in NPP in broad-leaf and needle-leaf mixed forest in Northeast China [(62)]. During the period of 2001 to 2019, they experienced three major fluctuations indicating its poor stability. Two reasons may be accounted for this change. On the one hand, large-scale drought events had likely an influence on the NPP. On the other hand, this sensitive area was susceptible to human activities and the transformation of different land use types [(37; 4; 63)]. Therefore, the policy of under-forest economy, that is the government firstly could take some intercropping measures between forests in vertical structures, should be introduced in areas with a good forests ecological environment to promote the common development of under-forest economy and forest environment protection. In addition, the government should then establish ecological restoration and comprehensive treatment plan to enhance the ecological service function. Figure 13: The inter-annual variation characteristics of NPP on daily accumulations, with an eight-day cycle at the end of the vegetation growth season (EOS), from 2001 to 2019. Summaries of and Sustainability Exploration of Forests and the Outlook for Future Ecological Environment  
As a traditional forest region of China and an ecological barrier to revitalize the old industrial base, the decrease of forests NPP in Northeast China will seriously affect the ecological environment and national ecological security pattern [64]. According to our data, the NPP in Northeast China generally showed a minimal increase trend from 2001 to 2019. However, we also found that 47.42% of the area still exhibited a decreasing trend in this study. Notably, the ecological environment still showed a trend of continuous degradation in a large proportion of regions. Therefore, the management of the continuously degraded area should be accentuated in the future ecological environment construction of northeast forests, in order to achieve the sustainable development of forests ecosystem. The study on the sustainability of NPP in Northeast China in the future indicates that the reverse direction feature of NPP change will be slightly stronger than the co-directional feature. It means that the degraded area will be slightly bigger than the improved areas. On the contrary, another study about Northeast China reported that the combined effects of climate change and CO\({}_{2}\) fertilization on the increase of NPP were estimated to be 10%-20% for the 2030s and 28%-37% in the 2090s, using the improved TRIPLEX model [4]. Differences in the future prediction are mainly attributed to the following reasons.