# An Analysis of Spatial Variation in Human Impact on Forest Ecological Functions
## 2 Materials and Methods
### Variables and Data
#### 2.2.2 Influencing Factors of Forest Ecological Function  
Accelerated economic development, urban expansion, and population booms can directly or indirectly damage the surface vegetation [22]. Population pressure stimulates the demand for agricultural and forestry products, which may initiate changes in land-use types, such as the conversion from woodland or grassland to farmland or other use, leading to a variation in vegetation cover and forest community structure [23; 33]. Ecological projects have contributed substantially to maintaining the forest ecological functions [34; 35]. Additionally, the long-term variations in FEF are interlinked with natural factors such as climate change that exert direct impacts on forest growth, cover and types [36; 37]. The influence of temperature and precipitation is also determined by altitude, which varies across the study area. Therefore, we selected eight indicators that effectively capture human activities and their impacts on FEF. We also added control variables to alleviate the problems of missing variables of natural factors (Table 2).  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**Variable** & **Symbol** & **Description** & **Unit** \\ \hline The level of economic development & EDS & GDP per capita & ten thousand yuan/km\({}^{2}\) \\ Development of the primary sector & AGRI & the share of primary sector in & \% \\  & & the GDP & \% \\ Development of the secondary sector & INDS & the share of secondary sector in & \% \\ Population density & PDS & the ratio of resident population to & thousand people/km\({}^{2}\) \\ Rate of urbanization & URB & the urbanization ratio of & \% \\ Production space & PDUC & the ratio of the area of productive & \% \\  & & land to county area & \% \\ Living space & LIVE & the proportion of the area for living & \% \\ Forest protection & PROT & level of forest land protection & - \\ Temperature & TEMP & annual average temperature & \({}^{\circ}\)C \\ Precipitation & PREC & annual average precipitation & mm \\ Elevation & ELEV & average elevation of the county & km \\ Slope & SLOP & average slope of the county & \({}^{\circ}\) \\ Soil & SOIL & average soil condition of the county & - \\ \hline \hline \end{tabular} Note: The land use is merged into three types, which are production space, living space, and ecological space. Among them, production space includes the land used for agricultural production as well as industrial and mine production, and living space includes urban and rural living land. The others are summarized as ecological space [38]. There are three levels of forest land protection in China. Grade 1 areas are specially protected and strictly regulated within critical ecological function zones, designated to safeguard biodiversity and unique natural landscapes; Grade II focuses on ecological restoration, management, and the construction of ecological barriers; and Grade III is dedicated to maintaining regional ecological balance and ensuring the supplies of forest products [32]. Evidently, Grade 1 represents the highest level of forest land protection. For clarity, this study employs the reciprocal of the protection grade for each sample plot to calculate the PROT index.  
\end{table}
Table 2: Description of variables.