Fig. 4
Structural equation model of the influencing mechanism of stand density on Rh, Ra, and Rs. Stand density 2 is the square of the stand density; all lines are direct effects, the brown line is a negative effect, and the blue line is a positive effect. Note: according to previous results, it was found that the relationship between soil respiration and forest density was not a simple linear relationship. Therefore, the stand density square was introduced to construct a structural equation model, which made the model more meaningful