## 3 Results
### Complex Correlation Analysis
#### 3.2.3 Impact of Human Activities on NPP  
Characteristics of Changes in NPP of Different Land Use Types  
By calculating the yearly mean value and overall quantity of NPP across different land use categories under different periods (Table 3), the multi-year mean NPP value, within descending order, was as follows: forest land (412.9 gC-m\({}^{-2}\)-a\({}^{-1}\)), grassland (369.5 gC-m\({}^{-2}\)-a\({}^{-1}\)), cropland (360.1 gC-m\({}^{-2}\)-a\({}^{-1}\)), built-up land (345.8 gC-m\({}^{-2}\)-a\({}^{-1}\)), unused land (300.1 gC-m\({}^{-2}\)-a\({}^{-1}\)), and water (270 gC-m\({}^{-2}\)-a\({}^{-1}\)). The annual total of NPP in descending order was as follows: arable agricultural land, forest areas, grasslands, urban territories, aquatic zones, and fallow lands. The yearly mean NPP value for the six types of land use demonstrated a rising pattern, and the annual total of NPP showed a decreasing trend only on unused land. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Land Use Type**} & \multicolumn{3}{c}{**Annual Average NPP (gC-m\({}^{-2}\)-a\({}^{-1}\))**} & \multicolumn{3}{c}{**Annual Total NPP (TgC)**} \\ \cline{2-7}  & **2001** & **2011** & **2021** & **Average Value** & **2001** & **2011** & **2021** \\ \hline Cultivated land & 289.5 & 362.6 & 428.2 & 360.1 & 11.6 & 14.4 & 17.1 \\ Forest land & 328.8 & 404.5 & 505.5 & 412.9 & 3.0 & 4.2 & 5.2 \\ Grassland & 285.7 & 365.2 & 457.5 & 369.5 & 2.3 & 2.4 & 2.6 \\ Water & 241.8 & 271.1 & 300.0 & 271.0 & 0.1 & 0.1 & 0.1 \\ Built-up land & 282.7 & 346.0 & 408.7 & 345.8 & 1.3 & 1.8 & 2.4 \\ Unused land & 235.6 & 299.5 & 365.0 & 300.1 & 0 & 0 & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The variation in annual average NPP and total NPP in northwest Liaoning Province in different periods. Figure 11: Interannual variation in annual average NPP in different soil type zones. Impact of Land Use Change on NPP  
We divided the 21-year period into two time periods, 2001-2011 and 2011-2021, to study the conversion between various categories of land use in northwest Liaoning and the affected changes in total NPP. We employed ArcGIS 10.8 software and Excel to create a map of land use changes (Figure 12) and a matrix of total NPP changes (Table 4). Between 2001 and 2011, the aggregate area of land use categories that changed in northwestern Liaoning was 6941 km\({}^{2}\), representing 11.2% of the overall area. The changes that occurred were mainly a growth in forested land area, watersheds, and built-up land, and a reduction in the area for the rest of the categories of land use. From 2011 to 2021, the overall area that changed was 6899.8 km\({}^{2}\), comprising 11.1% of the total area, and primarily, the change was due to a decrease in grassland area. Among the land use types, arable land and grassland were the types that changed the most frequently. Grassland mainly changed to arable and forest land, arable land mainly changed to forest land (941.8 km\({}^{2}\)), grassland (932.1 km\({}^{2}\)), and built-up land (946.3 km\({}^{2}\)) from 2001 to 2011, and mainly to built-up land (1088.8 km\({}^{2}\)) from 2011 to 2021. Owing to alterations in types of land use, the total amount of NPP also showed a certain degree of change, with an overall increase in the total amount of NPP under forest and built-up land, and an obvious decrease in the overall quantity of NPP influenced by the expansion in the area of cultivable land and grassland transferred out. The transformation of grassland into cropland and woodland changed the total NPP by 11.3 \(\times\) 10\({}^{-2}\) TgC and 11.2 \(\times\) 10\({}^{-2}\) TgC, respectively, from 2001 to 2011; the conversion of grassland to woodland changed the total NPP by 11.9 \(\times\) 10\({}^{-2}\) TgC from 2011 to 2021.