Table 6. Ordinary Least Squares (OLS) and Partial Least Squares (PLS) regressions to predict site index using the attributes of site and soil profile, and the attributes of soil physical and chemical properties in Quercus mongolica stands.

Attributes	Horizon	OLS regression	Adj. R2	P-value
Site and soil profile (S)	A	SI=15.7191+0.0025x(Altitude)+0.0473x(Slope)-0.4274x(Hue)	0.29	P<0.001
	B	SI=17.2327+0.0034x(Altitude)-0.5460x(Hue)	0.31	P<0.001
Soil properties (P)	A	SI=15.8084-0.0566x(C/N)+0.0081x(P205)	0.09	P=0.02
	B	SI=21.9871-1.2212x(pH)+18.0402x(K2+)	0.21	P<0.001
S+P	A	SI=17.4055+0.0449x(Soil depth)-0.7056x(Value)+0.0107x(P2O5)+0.8512x(Ca2+)-0.0854x(CEC)	0.27	P<0.001
	B	SI=20.0856-0.5391x(Hue)-2.2662x(Mg2+)-14.2228x(K+)0.28	P<0.001
PLS regression
Site and soil profile (S)	A	SI=16.0700+0.0021x(Altitude)+0.0459x(Slope)+0.01350x(Soil depth)-0.3661x(Hue)+0.0044x(Chroma)-0.2711x(Value)	0.25	P<0.001
	B	SI=17.7307+0.0032x(Altitude)-0.5023x(Hue)-0.1430x(Chroma)	0.32	P<0.001
Soil properties (P)	A	SI=15.4087-0.0303x(Silt)+0.04576x(Clay)+0.0547x(pH)+0.0837x(TN)-0.0409x(C/N)+0.0043x(P2O5)+1.2001x(Mg2+)-2.8150x(Na+)	0.05	P=0.14
	B	SI=18.7604-0.5537x(pH)+0.1602x(TN)-0.0108x(C/N)+0.0059x(P2O5)-1.9 033x(Mg2+)-8.0578x(K+)-12.5026x(Na+)	0.20	P<0.001
S+P	A	SI=12.9364-0.0021x(Altitude)+0.0331x(Slope)+0.0153x(Soil depth)-0.5629x(Hue)-0.0414x(Chroma)-0.3531x(Value)-0.0083x(Silt)-0.0024 x(Clay)+1.3659x(pH)	0.32	P<0.001
	B	SI=19.4395+0.0009x(Altitude)-0.0248x(Slope)+0.5197x(Hue)-0.0024x (Chroma)-0.0175x(Clay)-0.1249x(pH)+0.1825x(TN)-12.4536x(K+)	0.24	P<0.001