Wilting point could be substituted for field capacity in this and in alternative 

 models with little sacrifice in values. However, the addition of wilting point did 

 not improve any model containing field capacity. 



Available potassium (K) in the soil solum ranked second only to the field capacity 

 and wilting point variables in degree of correlation with understory yields, and pro- 

 duced an r2 of 0.73 when added to the "yield = f (time)" equation. The further addi- 

 tions of sand content of the A and B horizons raised the R^ to 0.81. However, equally 

 satisfactory results were obtained with the potassium and sand values of the B horizon 

 only : 



R^ 



yield = / (time) 0.38 

 yield = / (time + Kg) .70 

 yield = / (time + Kg + Sandg) .80 



as in the equation 



_ (T-llf 



Yield = - 228.051 + 588.614 e so +1.702 K 



H 



12.5 

 S and — 55 1 



+ 282.783 e i.aoo 



Holmes and Tackle (1962) found exchangeable potassium in the B horizons to be 

 noncorrelated with heights of dominant lodgepole pine trees (r = 0.055). In this study, 

 potassium in the B horizon was not significantly correlated with site index for lodge- 

 pole pine (r = 0.37), but potassium in the A horizon was significantly correlated 

 (P< .05) with site index (r = 0.46). A modest correlation with site index plus a 

 marked effect on understory yields suggests that fertilizing with potassium may be 

 beneficial both to understory production and to growth rates of lodgepole pine. 



The two above equations may serve as interim predictors of understory yield. The 

 response surfaces of these equations over the full ranges of time and sand content are 

 shown in figures 2 and 3 for the extremes of field capacity and of potassium, respec- 

 tively. In both instances, the change between surfaces is linear between the two 

 extremes shown for the third independent variable. Being additive, the models may not 

 fit too well near the left edge of the surfaces, since the expectation would be for 

 these to come closer together near time 0. Nonetheless, they do emphasize the substan- 

 tial effect of field capacity and of potassium on understory yields. 



7 



