## 4 Discussion  
In terms of different regions, the level of CLP, the amplitude of the trend and the future sustainability were combined with the geographical characteristic to explore the driving forces that are contributing to guiding the CLP improvement projects. In the northwestern region of Jilin Province, the CLP level is low at level 2 (Figure 5b) and a significant increase was shown during 2000-2018, which will keep sustainability increasing in the future. The average annual precipitation in this area is 400-700 mm, which is less than the 700-900 mm in the southeast of Jilin. Most of the cultivated land was low-lying, which will degenerate to saline-alkali land and wind-blown land, where the ecological environment is fragile. It is also more vulnerable to natural disasters such as flooding, salinization and degradation, and the artificial irrigation facilities are poor in this area. These factors limited the regional CLP level. As shown in Figure 10, precipitation has greater regional differences compared to temperature in the northwest, than in the central part. Therefore, precipitation is more likely to affect regional productivity levels than temperature. In recent years, measures such as the construction of a cultivated land shelterbelt, high-standard cultivated land construction and water-saving irrigation projects have been implemented in the central and western regions to effectively control disasters and improve environmental conditions, which might be the reason for the significant increase in the region. The improvement of climatic conditions such as the increase of precipitation and the rise of temperature over the years is conducive to agricultural production, while the development of cultivated land regulation projects has gradually improved the water and fertilizer conservation capacity of cultivated land. At the same time, due to the continuous improvement in the technical production level, such as the cultivation of drought-resistant and high-yield seeds and complete cultivated land management technology, the CLP level in the northwest region presents a sustainable increasing trend in the future. In the central region of Jilin Province, the average annual productivity is level 3 and it showed a \begin{table}
\begin{tabular}{c c c} \hline
**SGPP** & **H** & **Variation Types** & **Percentage (\%)** \\ \hline \(>\)0.0005 & \(>\)0.5 & sustainability increased & 13.37 \\ \(<\)\(-\)0.0005 & \(>\)0.5 & sustainability decreased & 13.98 \\ \(>\)0.0005 & \(<\)0.5 & Anti-sustainability increased & 14.54 \\ \(<\)\(-\)0.0005 & \(<\)0.5 & Anti-sustainability decreased & 12.24 \\ \(-\)0.0005\(-\)0.0005 & \(>\)0.5 & Stable & 6.1 \\ — & \(=\)0.5 & uncertain & 39.77 \\ \hline \end{tabular}
\end{table} 
Table 3: Statistical result of the trend and the Hurst exponent. Figure 9: Spatial distribution of the future direction change. slight increase during 2000-2018, whereas unsustainability decreased and the future was found to be uncertain. The precipitation in the central region is higher than that in the northwest (shown in Figure 10). The artificial irrigation facilities are sufficient to meet the regional needs. Therefore, the productivity level has been higher in the central region for about twenty years. With the increase in precipitation and temperature in recent years [40], productivity is gradually increasing slightly. The average annual productivity level in the southeastern part of Jilin is at level 4. There were a slight increase and stable trend for 2000-2018, and a decrease in sustainability in the southeast region. The cultivated land in the southeast is mainly scattered sloping land. The soil humus is rich and the OMC is high. The precipitation is the highest in the whole region, and so the GPP is the largest. As the area with the highest CLP level in the whole region, with the increase of precipitation and temperature in the future, it may exceed the suitable temperature for crop growth.