DISCUSSION 



Underground Accumulation 



If we interpolate root-mass values for layers not sampled in 1965 and 1966 (as 

 in fig. 2) , it becomes apparent that about half the roots (living and dead) are in the 

 top 30 cm. of the soil, about 30 percent in the second 30 cm., 15 percent in the third, 

 3 percent in the fourth, and 1 percent at depths below 120 cm. This profile differs 

 from that described by Pearson (1965) in the Artenrisia-gxass semidesert of eastern 

 Idaho (annual precipitation = 270 mm.), where 80 percent of the root mass was in the 

 top 20 cm., 18 percent in the second 20 cm., and 2 percent in the third, which rested 

 on bedrock. 



If we assume that the earlier sample underestimated total root mass in the fourth 

 and deeper layers to the same degree it did for layer III, then an approximation of the 

 profile of root distribution can be extrapolated from results of the later sample (the 

 curve in fig. 2). In layer III and below, coarse roots comprise a negligible fraction 

 of the total roots, and the relation of medium to fine roots appears to have reached a 

 constant ratio: 0.21 in layer II, 0.22 in layer III. Our estimate of total root mass 

 for the profile is summarized by 30-cm. increments in table 4. Total mass of living 

 and dead root material in the profile is calculated to be about 1,235 g./m.^, of which 

 75 percent is fine roots. Bjerregaard (1971) determined a root mass of 1,313 g./m.^ 

 in a community dominated by Atriplex con ferti folia about 350 km. north of our study sit 



An additional 297 g. of litter brings the total underground mass to about 1,532 

 g./m.2 (13,670 lb. /acre). For the calcic horizons below layer III, we assume half as 

 much litter relative to roots as was in layer III, because rodents, which transport 

 much of what we call underground litter from above, do not work those horizons. If our 

 guess is correct that 85 percent of the litter nearer the surface is derived from aeria 

 plant parts, only 135 g./m.^ of the underground litter originated beneath the surface. 



10 



