From all the aforementioned studies, a root area ratio in the 

 range of 0.05 to 0. 1 5 percent can be considered a reasonable or 

 preliminary lower bound estimate in a slope for purposes of 

 stability calculations in forested, granitic slopes of the Idaho 

 batholith. This topic will be considered further in a subsequent 

 section dealing with root reinforcement and its contribution to 

 slope stability. 



TREE STEM SURVEY 



Tree stem survey data from nearby experimental watersheds 

 in the adjacent Silver Creek drainage were used for calculating 

 slope surcharge from the weight of forest vegetation and for 

 estimating buttressing or soil restraint from tree trunks firmly 

 rooted to the underlying bedrock. 



A stem count and inventory for five cutting units on two 

 experimental watersheds in the Silver Creek drainage is 

 summarized in table 3. The inventory included only stems 12 

 inches (30.5 cm) and over in diameter at breast height. Average 

 tree spacings (S) were calculated assuming a simple cubic and 

 triangular array. Average size of openings (B) between "vertical 

 root cylinders" were calculated assuming the diameter of the 

 root cyclinder (Dr) was approximately five times the stem or 

 trunk diameter (Dj) at breast height. Field studies by Wu (1 976) 

 and Curtis (1 964) indicate that this diameter ratio is reasonable. 

 The various spacing relationships are illustrated schematically 

 in figure 19. 



Table 3. — Stem count and tree spacings for experimental watersheds, Silver Creek study area, Boise National Forest 



Number of Estimated 

 stems size of 









12 inches 







Estimated^ 



opening 







Total stem 



in diameter 



Average stem 



Average stem^ 



root zone 



between 



Cutting unit 



Total area 



area 



and over 



diameter 



spacing 



diameter 



root zones 





Acres 



Feet^ 







Feet 







Control Creek No.3 



96.6 



2,690 



960 



1.89 



66 



9.4 



57 



4 



51.5 



3,503 



1,567 



1.69 



38 



8.4 



30 



5 



19.5 



1,635 



778 



1.64 



33 



8.2 



25 



6 



21.1 



2,215 



880 



1.79 



32 



8.9 



23 



K-1 Creek 



36.7 



1,731 



620 



1.88 



51 



9.4 



41 



'Based on simple, cubic array where spacing (S) = [ (area cutting unit)/(number stems >12 in.) 

 ^Assume Dr = 5Dt (estimate based on data in Curtis [1964] and Wu [1976]). 



Dr 



LEGEND 



x-us n SOIL AND 



DECOMPOSED 

 GRANITICS 



DISINTEGRATED . 

 JOINTED GRANITIC 

 BEDROCK 



B 



S = Center to center spacing between trees 



Dt - Trunk diameter (of breast height) 



DR-Diometer of "vertical root cylinder" 



B = Spacing between root cylinders 



H = Thickness of soil and decomposed granitic mantle 



D. 



^^^^ 



Scale: 

 1 2 3 4 5 feet 



Figure 19. — Schematic diagram of spacing relationships between tree trunks and their "vertical root cylinders ' in granitic slopes. 



10 



