2.3 Environmental factor measurement
Soil samples were collected along the diagonal (upper left, middle, and lower right) in each of the 30×30 m plots. A core of the top 20 cm of soil was taken at each point. Soil samples were mixed into a single sample and dried in the laboratory. After sieving, the physical and chemical indicators of the soil samples were measured in accordance with international standard methods. The indicators included soil organic matter (SOM, g·kg-1, potassium dichromate oxidation volumetric method, LY/T 1237-1999), soil total nitrogen (TN, g·kg-1, Kjeldahl boiling-diffusion method, LY/T 1228-1999), soil available phosphorus (AP, mg·kg-1, 0.05 mol·L-1 HCl-0.025 mol·L-1 1/2 H2SO4 extraction method, LY/T 1233-1999), soil available potassium (AK, mg·kg-1, 1 mol·L-1 ammonium acetate extraction flame photometry, LY/T 1236-1999) and soil pH (pH, H2O, 1:2.5, potentiometry, LY/T 1239-1999).
In each 30×30 m plot, elevation, latitude and longitude data were collected by a global positioning navigation system (GPS). In addition, stand characteristics (canopy and adult density) and other topographic factors (aspect, slope position and slope) were also recorded.