Response surface regression results and their F values at the four different elevations and the composite site.

The response surface was defined by a quadratic surface function: . Here, Z = Z score value of standardized tree ring index at different elevations and the whole site; X = Z score value of mean precipitation (previous July to current June); Y = Z score value of mean temperature (previous September to current March). The F value is calculated by , with notation as follows. : regression sum of squares, : residual sum of squares, p: degrees of freedom of the regression function, here p = 5, n: number of active data, here n = 55. If , the response surface regression is significant at the α level. **F value is significant at p = 0.01 level.