TOPOGRAPHIC FACTORS 



As sole additive components to the "yield = / (time)" equation, slope accounted 

 for an additional 20 percent of the variation, elevation for 11 percent, and aspect for 

 less than 1 percent. 



This response to slope is interesting. One usually finds increasing slope steep- 

 ness associated with shallower and rockier soils and greater runoff, and therefore with 

 decreasing herbage production; but in this study, contrary to expectations, understory 

 production responded positively to slope. In addition, the data revealed a strong 

 coincidence of the higher slope values with the more moist sites whose expected influence 

 on yield is positive. This confounding of slope with other variables cast doubt on the 

 validity of using slope for predictive purposes in other areas; thus, it was eliminated 

 from further consideration. 



The remaining topographic variables, elevation and aspect, were added to the "yield 

 = / (time)" equation and this resulted in values as follows: 



yield = / (time) 0.38 



yield = / (time + elevation) .49 



yield = / (time + elevation + aspect) .51 



When added to more complex models, elevation did not contribute sufficiently to 

 values to warrant its retention. 



The effect of aspect was tested two ways: as cosine azimuth + 1, which places 

 minimum expected yields at due south; and as cosine azimuth + sine azimuth + 2, which 

 places minimum expected yields at southwest. Discounting the time effect, neither of 

 these components contributed materially (less than 2 percent) to accountable variation. 



SOIL FACTORS 



The influence of moisture on understory production was emphasized by the additive 

 contribution of field capacity of the A and B horizons to the "yield = / (time)" equa- 

 tion, with = 0.74. The further addition of sand content of the A horizon, another 

 moisture-related variable, produced the equation 



Yield = -251.4000 + 383.5379 e + 3.1646FC^ 



Sandj 



5 5r 



2.5 



+ 16.8020FCy + 291.7550 e 



1,200 



with R^ = 0.79. Contributions of other soil variables added to this model were 

 negligible . 



6 



